Structure Of The Atom - Class 9 Chemistry - Extra Questions

Draw the electronic configuration of the following element clearly stating the number of neutrons and protons in their nucleus.
Nitrogen.

Nitrogen has 7 protons and 7 neutrons.
1697556_1803499_ans_c7a64e8f36c04ee3b48ffe9f6c9b39dd.png


What is Mass number?

Number of protons and neutrons present in the nucleus of an atom is called as mass number.


On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom?

Based on Rutherford's model of an atom, which subatomic particle is present in the nucleus is a proton.


Which metal did Rutherford select for his $$\alpha-$$ particle scattering experiment and why?

Rutherford used gold for his scattering experiment because gold being the most malleable metal and can be hence beaten down to get very thin sheets.
The sheet of gold foil Rutherford used was only 1000 atoms thick. Gold's malleability made it possible for it to be beaten down to such a thin sheet.


State one major drawback of Rutherford's model.

The major drawback of Rutherford’s model of the atom is that it does not explain the stability of the atom.


In the figure given alongside:
A, B and C are orbits of electron in the atom.
Name $$X$$ and state the charge on it.
1809406_63a4f626bdff4346be03aa0e532b3a4b.png

X is a nucleus and it is positively charged.


Write the relationship between mass number and atomic number?

The relationship between mass number and atomic number is given below:
Number of neutrons = Mass number (A) - Atomic number (Z) 
                                   = A - Z


How do you represent an atom symbolically with atomic number and mass number ?

Symbolical representation of an atom is :$$^{A}_{Z}X$$ 
where, A = Mass number, X = atom, Z = Atomic number.


Give reason for the following:
Ernest Rutherford selected a gold foil for his experiments.

Ernest Rutherford selected a gold foil for his experiments because he wanted as thin a layer as possible, and gold is very highly malleable.


Explain the important features of Rutherford's Planetary model and write its defects.

Ernest Rutherford discovered the nucleus of an atom in 1911.
The main features of Rutherford's Planetary model are
(1) All the positive charges in the atom are concentrated in one tiny area, called the nucleus and the rest of the atom is mostly empty space.
(2) It explains the movement of electrons through the empty spaces around the nucleus like planets orbiting the sun.
Defects of Rutherford's Planetary model:
(1) It failed to explain the stability of electrons in a circular path around the nucleus. 
(2)  It failed to explain why individual atoms produce discrete line spectra 


An element X has a mass number 4 and atomic numberWrite the valency of this element?

Valency of the element is zero as K shell is completely filled with 2 electrons.


Name two particles found inside the nucleus. What are they collectively called?

Protons and neutrons are the two particles found inside the nucleus. They are collectively called nucleons.


Define the following term:

(i) Nucleons

The protons and neutrons are collectively known as nucleons.


The following data represent the distribution of electrons, protons and neutrons in atoms of elements $$P, Q, R$$ and $$S$$.
ElementProtonsNeutronsElectrons
$$P$$$$19$$$$22$$$$19$$
$$Q$$$$17$$$$18$$$$17$$
$$R$$$$17$$$$20$$$$17$$
$$S$$$$18$$$$22$$$$18$$
Which two elements form a pair of isobars?

Isobar are atoms of different elements with different atomic numbers but have the same mass number.
$$ Mass\ number= Number\ of\ protons + Number\ of\ neutrons $$

$$\bullet$$  For $$P\ element$$:
$$\text{Step 1: Finding the mass number of P element}$$
It has a number of protons and neutrons of $$19$$ and $$22$$ respectively
Therefore, $$ Mass\ number= 19+ 22 $$
                   $$ Mass\ number= 41$$
Hence, mass number of $$P\ element$$ is $$41$$

$$\bullet$$  For $$Q\ element$$:
$$\text{Step 1: Finding the mass number of Q element}$$
It has a number of protons and neutrons of $$17$$ and $$18$$ respectively
Therefore, $$ Mass\ number= 17+ 18 $$
                   $$ Mass\ number= 35$$
Hence, mass number of $$Q\ element$$ is $$35$$

$$\bullet$$  For $$R\ element$$:
$$\text{Step 1: Finding the mass number of R element}$$
It has a number of protons and neutrons of $$17$$ and $$20$$ respectively
Therefore, $$ Mass\ number= 17+ 20 $$
                   $$ Mass\ number= 37$$
Hence, mass number of $$R\ element$$ is $$37$$

$$\bullet$$  For $$S\ element$$:
$$\text{Step 1: Finding the mass number of S element}$$
It has a number of protons and neutrons of $$18$$ and $$22$$ respectively
Therefore, $$ Mass\ number= 18+ 22 $$
                   $$ Mass\ number= 40$$
Hence, mass number of $$S\ element$$ is $$40$$

$$\bullet$$  Since none of the masses of elements given in the question have the same number, therefore, none of the pairs of the elements are isobars.




The composition of two atomic particles is given below:
XY
Protons:88
Neutrons:89
Electrons:88
Which element/ elements do they represent? 

Given that:
The number of protons in X is 8.
The number of neutrons in X is 8.
The number of electrons in X is 8.
The number of protons in Y is 8.
The number of neutrons in Y is 9.
The number of electrons in Y is 8.

$$\bullet$$ Since the number of protons in the elements X and Y are the same, the elements must have the atomic number of 8 and thus, X and Y represent the Oxygen element.
$$\bullet$$ Since X and Y have the same number of protons but different numbers of neutrons. Thus, They are isotopes of oxygen (Oxygen-16 and Oxygen-17)


The composition of two atomic particles is given below:
XY
Protons:88
Neutrons:89
Electrons:88
What is the relation between X and Y ?

Given that:
The number of protons in X is 8.
The number of neutrons in X is 8.
The number of electrons in X is 8.
The number of protons in Y is 8.
The number of neutrons in Y is 9.
The number of electrons in Y is 8.

Since X and Y have the same number of protons but different numbers of neutrons. Thus, They are $$\text{isotopes}$$.

$$\textbf{Additional Information:}$$
The elements having the same number of protons but a different number of neutrons are known as "isotopes".


The composition of two atomic particles is given below:

XY
Protons:88
Neutrons:89
Electrons:88
What is the mass number of X ? 

Given that:
The number of protons in X is 8.
The number of neutrons in X is 8.
The number of electrons in X is 8.

The mass number of X can be calculated as shown below.
$$\begin{align*}{\rm{Mass}}\;{\rm{number}} &= {\rm{Number}}\;{\rm{of}}\;{\rm{protons}} + {\rm{Number}}\;{\rm{of}}\;{\rm{neutrons}}\\ &= {\rm{8}} + {\rm{8}}\\ &= 16\end{align*}$$

Therefore, the mass number of X is 16.


The following data represent the distribution of electrons, protons and neutrons in atoms of elements $$P, Q, R$$ and $$S$$.
ElementProtonsNeutronsElectrons
$$P$$$$19$$$$22$$$$19$$
$$Q$$$$17$$$$18$$$$17$$
$$R$$$$17$$$$20$$$$17$$
$$S$$$$18$$$$22$$$$18$$
Define isobars.

$$\text{Isobars:}$$ Isobars are atoms of different elements with different atomic numbers but have the same mass number.
For example, calcium and argon are isobar to each other, which can be shown as $$^{40}_{20}{Ca}$$ and $$^{40}_{18}{Ar}$$.



The atom of an element is made up of $$4$$ protons, $$5$$ neutrons and $$4$$ electrons. What is its atomic number and mass number ?

Protons $$= 4,$$ neutrons $$= 5,$$ electrons $$= 4$$
Atomic number = number of proton $$= 4$$
Mass number = number of proton + number of neutron $$= 4 + 5 = 9$$


$$\text{ With reference to period 3 of the periodic table-State}$$
The number of electrons in the penultimate shell of the element with valency -1.

$$\bullet$$ The shell which is present just before the valence shell is known as penultimate shell.
$$\bullet$$ The element with valency -1 is a halogen and in period 3 chlorine is the halogen that has valency -1.
$$\bullet$$ The electronic configuration of chlorine (17) is 2, 8, 7. 
$$\bullet$$ Thus, there are 8 electrons present in the penultimate shell of chlorine.


The composition of two atomic particles is given below:
XY
Protons:88
Neutrons:89
Electrons:88
What is the mass number of Y?  

Given that:
The number of protons in Y is 8.
The number of neutrons in Y is 9.
The number of electrons in Y is 8.

The mass number of Y can be calculated as shown below.
$$\begin{align*}{\rm{Mass}}\;{\rm{number}} &= {\rm{Number}}\;{\rm{of}}\;{\rm{protons}} + {\rm{Number}}\;{\rm{of}}\;{\rm{neutrons}}\\ &= {\rm{8}} + {\rm{9}}\\ &= 17\end{align*}$$

Therefore, the mass number of Y is 17.


Name the scientist who discovered that electrons are present in an atom in discrete orbits


$$\bullet$$ Name of the scientist who discovered that electrons are present in an atom in discreate orbits is Neil Bohr 

$$\bullet$$ Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another.


What is isobar?

Isobars are atoms of different elements having the same mass number but the different atomic number.
Example: Ar, K, and Ca are isobars as they have the same mass number viz. 40


Answer the following questions, which help you understand the difference between Thomson's model and Rutherford's model better.
(a) Is the average angle of deflection of $$\alpha$$-particles by a thin gold foil predicted by Thomson's model much less, about the same, or much greater than that predicted by Rutherford's model?

(b) Is the probability of backward scattering (i.e., scattering of $$\alpha$$-particles at angles greater than $$90^\circ$$) predicted by Thomson's model much less, about the same, or much greater than that predicted by Rutherford's model?

(c) Keeping other factors fixed, it is found experimentally that for small thickness $$t$$, the number of $$\alpha$$-particles scattered at moderate angles is proportional to $$t$$. What clue does this linear dependence on $$t$$ provide?

(d) In which model is it completely wrong to ignore multiple scattering for the calculation of average angle of scattering of $$\alpha$$-particles by a thin foil?

(a) The average angle of deflection of $$\alpha$$-particles by a thin gold foil predicted by Thomson's model is about the same than that predicted by Rutherford's mode

(b) The probability of backward scattering (i.e., scattering of $$\alpha$$-particles at angles greater than $$90^{o}$$) predicted by Thomson's model much less than that predicted by Rutherford's model.

(c) Scattering is mainly due to single collisions. The chances of a single collision increases linearly with the number of target atoms. Since, the number of target atoms increase with an increase in thickness, the collision probability depends linearly on the thickness of the target.

(d) Thomson’s model.
It is wrong to ignore multiple scattering in Thomson’s model for the calculation of average angle of scattering of $$\alpha$$-particles by a thin foil. This is because a single collision causes very little deflection in this model. Hence, the observed average scattering angle can be explained only by considering multiple scattering.


It is now believed that protons and neutrons (which constitute nuclei of ordinary matter) are themselves built out of more elementary units called quarks. A proton and a neutron consist of three quarks each. Two types of quarks, the so-called 'up' quark (denoted by u) of charge + (2/3) e, and the 'down' quark (denoted by d) of charge (-1/3) e, together with electrons build up ordinary matter. (Quarks of other types have also been found which give rise to different unusual varieties of matter.) Suggest a possible quark composition of a proton and neutron.

The net charge on proton is +e. This can be obtained by $$p=(\cfrac{2}{3}+\cfrac{2}{3}-\cfrac{1}{3})e=+e$$ 
Hence, proton has two 'up' quark and a 'down quark'.

The net charge on neutron is 0. This can be obtained by $$n=\left(\cfrac{2}{3}-\cfrac{1}{3}-\cfrac{1}{3}\right)=0$$
Hence, neutron has one 'up' quark and two 'down' quarks.


If $$Z=3$$, what would be the valency of the element? Also, name the element.



What are the limitations of Rutherford's model of the atom?

The major limitations of Rutherford’s model of the atom are:
  • It does not explain the stability of the atom. As we know now, when charged bodies move in a circular motion, they emit radiations. This means that the electrons revolving around the nucleus (as suggested by Rutherford) would lose energy and come closer and closer to the nucleus, and a stage will come when they would finally merge into the nucleus. This makes the atom unstable, which is clearly not the case. The electrons do not fall into the nucleus, atoms are very stable and do not collapse on their own.
  • It didn't explain the arrangement of an electron inside the atom.


State True or False.

In the $$\alpha$$-ray scattering experiment, some $$\alpha$$-rays get deflected due to the attraction of electrons.

In the Rutherford alpha ray scattering experiment, it is observed that some of the alpha rays deflected are due to the presence of the nucleus but not due the attraction of electrons.
So the given statement is false.



Rutherford's atomic model could not explain the stability of atom. How do you account for this?

Rutherford's atomic model did not has any scope for energy and it does not obey the Maxwell theory of electrodynamics. According to Maxwell's theory a small charged particle moving around an oppositely center should continue lose it's energy. 


A lead block used to keep the source in $$\alpha$$ - ray scattering experiment. What is its purpose?

Since lead has high density and $$\alpha -$$particles cannot penetrate through it, that's why lead block was used to keep the source in $$\alpha -$$ray scattering experiment. 


Where is the nucleus of an atom is situated ?

The nucleus of an atom is situated or located at the center of nucleus. It contains protons and neutrons.


Could aluminium foil serve the purpose in $$\alpha$$ - ray scattering experiment. Explain the reason behind it.

Atomic mass of Al is very low. In $$\alpha$$ ray scattering experiment when $$\alpha -$$rays were passed it should be deflected but if Al sheet was used $$\alpha $$ particle could push the nucleus and may not deflect back. This is because of the low atomic mass of Al.
Thus, Aluminium foil cannot be used in $$\alpha$$ ray scattering experiment.


Determine whether statements I and II are true or false.
I: Most of the fast-moving $$\alpha$$--particles passed straight through the gold foil.
II: Most of the space inside the atom is empty.

Most of the space inside the atom is empty because most of the $$\alpha$$--particles passed through the gold foil without getting deflected.
Statement I and II both are true.


Almost all the mass of an atom is concentrated in a small region of space called the _________.

The atom consists of protons, neutrons and electrons. The protons and neutrons are situated inside the nucleus. Electrons revolve around the nucleus and have negligible mass compared to protons and neutrons. Therefore, the mass of the atom is concentrated in a small region of space called $$\text{nucleus.}$$ 


Define the terms: (a) atomic number, and (b) mass number.

1. Atomic number: The number of protons present in the nucleus of an atom is known as the atomic number. It is denoted by ‘Z’.

2. Atomic mass number: The total number of protons and neutrons present in the nucleus of an atom is known as mass number. It is denoted by A.Generally atomic number and mass number of the atom of an element is represented by a symbol $$Z^{X}_A$$
'X’ is an element.


From the given examples, form the pair of isotopes and the pair of isobars:
$$_{18}Ar^{40}, _{17}Cl^{35}, _{20}Ca^{40}, _{17}Cl^{37}$$

$$I)$$ Isotopes have same atomic number but different mass number. $${ 17 }^{ { Cl }^{ 35 } }\quad { 17 }^{ { Cl }^{ 37 } }$$ are isotopes.
$$II)$$ Isobars have same mass number but different atomic number. $${ 18 }^{ Ar^{ 40 } }\quad \& \quad { 17 }^{ { Cl }^{ 37 } }$$ are isobars


Why the nucleus is positively charged?

Almost all of the mass in an atom is made up of the protons and neutrons in the nucleus. Protons are positively charged and neutrons have no charge on them. Because the nucleus is only made up of protons and neutrons it is positively charged.


Describe Rutherford's atomic model and also explain its drawbacks.

Rutherford's atomic model is the model which described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.

Drawback's of this model are:
  • It could not explain the line spectra of atoms.
      • It could not explain the stability of an atom because this model does not obey the Maxwell law of electrodynamics.
      • Rutherford’s model of the atom does not say anything about the arrangement of electrons in an atom.


Define Nucleus of an atom.

The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of the atom.


Define nucleus.

The nucleus is the center of an atom. Almost all of the mass in an atom is made up from the protons and neutrons in the nucleus with a very small contribution from the orbiting electrons. Neutrons have no charge and protons are positively charged. Nucleus is highly dense because of the whole mass of atom is concentrated in the nucleus, with electrons contributing negligible mass.


The ________ of an atom is very hard and dense.

The $$\text {nucleus}$$ of an atom is very hard and dense.
Atoms are composed of a massive, central nucleus surrounded by a swarm of fast-moving electrons. The nucleus is made up of protons and neutrons. Almost all of the mass (more than 99%) of an atom is contained in the dense nucleus. An atomic nucleus is much, much smaller than an atom.


On the basis of Rutherford's model of an atom, which subatomic particle is present in the nucleus of an atom? 

On the basis of Rutherford's model, proton is present inside the atom as a subatomic particle. It is a positively charged particle in the nucleus. 



What are the postulates of J.J Thomson? What are his drawbacks?

Postulates  of J.J Thomson:
(i) An atom consist of the positively charged sphere and electron are embedded in it.
(ii) The negative and positive charges are equal in magnitude so the atom as a whole is electrically neutral.

The drawback of J.J Thomson:
In the J.J. Thomson, plum pudding model no separation between electrons and protons as they are held together inside the atom which is not possible and led to instability of the atom but the atom exists stable.


Give the main drawback of Rutherford's model.

Rutherford proposed that electrons revolve at a high speed in circular orbits around the positively charged nucleus. Whenever a charged particle such as an electron is accelerated around another charged center ( nucleus ) which is under the force of attraction, there will be continuous radiation of energy. This loss of energy would slow down the speed of the electron. This would reduce the radius of the electron–orbit.


How will you find the valency of sodium, chlorine and argon?

The combining capacity of an atom is called its valency.

Now, sodium has only one electron in its outermost shell which it can lose very easy to gain an octet configuration. Hence, the valency of sodium is $$1$$.

Chlorine has seven electrons in its outermost shell and it could complete its octet by gaining one electron. Hence, the valency of chlorine is $$1$$.

Argon has a fully filled outermost shell with no valency available for an extra electron. Hence, its valency is $$0$$.


State True or False for following statement:

Rutherford's $$\alpha$$-particle scattering experiment gives the experimental evidence for deriving the conclusion that most of the space inside the atom is empty.

Most of the space inside the atom is empty because most of the $$\alpha$$-particles passed through the gold foil without getting deflected. Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
Hence, the given statement is true.


From Rutherford's $$\alpha-$$particle scattering experiment, give the experimental evidence for the conclusion that the nucleus of an atom is positively charged.

In Rutherford's alpha-particle scattering experiment, Rutherford derived the conclusion that nucleus of an atom is positively charged based on this experimental evidence:

When the alpha rays were made to fall on the foil, few of the rays were deflected by $${180}^{o}$$. This shows that the mass of the atom is concentrated at a very small space and is positively charged because it deflected the positively charged rays. This positively charged space was called the nucleus.


Write the important limitation of Rutherford's nuclear model of the atom.

According to one of the postulates of Rutheford's atomic model, The electrons revolve around the nucleus in circular paths.
But the revolution of the electron in a circular orbit is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know, as atoms are quite stable.


In the words of Rutherford "this result was almost as incredible as if you fire a $$15-inch$$ shell at a piece of tissue paper and it comes back and hits you"
Discuss the concerning observation of the $$\alpha-$$scattering experiment made by Rutherford. 

It is an experiment conducted by Rutherford that the gold foil is placed and $$\alpha-$$particles are bombarded on it. It bounces back with pressure.
Most of the fast-moving $$\alpha$$-particles passed straight through the gold foil. Some of the $$\alpha$$-particles were deflected by the foil by small angles. Surprisingly one out of every $$12,000$$ $$\alpha$$- particles appeared to rebound.


What is the number of protons, neutrons, and electrons present in the
 following elements?

(I) $$_{27}^{59}Co$$

(II) $${ _{ 47 }^{108}Ag }$$

$$_{27}^{59}Co$$
Number of protons = no. of electrons = $$27$$
Number of neutrons = $$32$$

$$_{47}^{108}Ag$$
Number of protons = no. of electrons = $$47$$
Number of neutrons = $$60$$


Rutherford's alpha-particle scattering experiment was responsible for the discovery of?

Answer: Atomic nucleus

Rutherford's experiment was the first to show that most of the mass of the atom is confined to a very small space at the center of the atom (the nucleus).


How was it shown that an atom has a lot of empty space within it?

Rutherford found that most of the alpha particles passed through the gold foil without any deflection and that shows most of the space in the atom is empty.


 Name the part of an atom where protons and neutrons are held together.

Nucleus is the central part of an atom in which protons and neutrons are held together.


(a) What is the nucleus of an atom and  what is the nature of charge on it?
(b) Name the scientist who discovered the nucleus of atom.

(a) Nucleus is a small positively charged part at the centre of an atom. Nucleus is positively charged.
(b) Rutherford discovered nucleus of an atom.


State the location of electrons, protons and neutrons in an atom.

Protons and neutrons are collectively present in the nucleus at the center of the atom while electrons revolve rapidly around the nucleus in fixed circular orbits called energy levels.


What important information is furnished by the alpha particle scattering experiment, performed by Rutherford, about the nucleus of an atom?

Rutherford’s alpha-particle scattering experiment furnished the following details about the nucleus of an atom :

(i) Nucleus of an atom is positively charged.
(ii) Nucleus of an atom is very hard and dense.
(iii) Nucleus of an atom is very small as compared to the size of an atom as a whole.


Give evidence for the existence of nucleus in an atom.

Rutherford's alpha scattering experiment gives evidence for the existence of atoms in nucleus.

When Rutherford bombarded a very thin gold foil with rays containing alpha particles, he made the following observations:-

1. Few rays got deviated from their path.
2. One out of 12000 rays was reflected by $$180 ^\circ $$ 

From these observations, he concluded that:-

1. Deviation of rays from their path shows some positive charge present inside the atom

2. The reflection of rays by $$180 ^\circ $$, shows that there is something solid in nature present inside the atom. These rays being one out of 100 shows that it's very small in size but has all of the atom's mass and positive charge concentrated.

Rutherford named it as the nucleus and hence is credited with its discovery.


Which part of an atom was discovered by Rutherfords alpha particle scattering experiment?

Nucleus was discovered by Rutherford in the alpha particle scattering experiment.


 What is neutron? State its relative mass and charge.

The neutron is a neutral particle found in the nucleus of an atom. Its relative mass is 1 u. It has no charge.


Draw the electronic configuration of the following element clearly stating the number of neutrons and protons in their nucleus.
Silicon

Silicon has 14 protons and 14 neutrons inside nucleus.
1697526_1803496_ans_ca3590d3d35f47eb95320fea1802f728.png


Give reasons for the following:
Ernest Rutherford got unexpected results from his experiment.

According to Thomson's atomic model, electrons were embedded in a sphere of a positive charge. This implies that according to Rutherford's gold foil experiment in which the alpha particles which are positively charged must be reflected or deflected in large numbers but after performing the experiment, it was being found out that most of the space inside an atom is empty which was contrary to that predicted in Thomson's model. Hence, Ernest Rutherford got unexpected results from his experiment.


What was the major drawback of Rutherford's model of an atom?

Rutherford suggested that The electrons revolve around the
the nucleus in circular paths. But the orbital revolution of electrons is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charge particles would radiate energy. Thus, the revolving electrons would lose energy and finally fall into the nucleus. If this were so the atom would be highly unstable and hence matter would not have existed but as we know the atoms are quite stable.


(a) Describe the Rutherford's model of an atom. State one drawback of Rutherford's model of the atom. 
(b) The mass number of an element is 23 and it contains 11 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?

(a) Rutherford conducted an experiment with a gold foil bombarded with fast-moving alpha particles.
1. A gold foil with 1000 atoms thick has been taken
2. Helium ions are used to bombard the gold foil.
3. It was expected that the alpha particles would be deflected by the subatomic particles in the gold atoms. Since the alpha particles were much heavier than the protons, he did not expect to see large deflections.
Alpha particle scattering experiment output was unexpected
Observations:
a) Most of the fast-moving alpha particles passed through the gold foil
b) Some of the alpha particles were deflected by the foil by small angles.
c) 12000 particles appeared to rebound.

Conclusion:
a) Most of the atom is hollow.
b) The small angle deviation of the alpha particles confirms the presence of a positively charged center called the nucleus.
c) The nucleus of an atom is solid as the ray returns on its path and is very small in size.

Drawbacks of this model:
1. He failed to explain the stability of the atom.

(b) Given: 
Mass no. = 23
No. of electrons = 11
Then, no. of protons = 11
No. of neutrons = 23 - 11 = 12
Atomic no. = 11


Draw the structure of the following compound:
$$HOClO_{2}$$.


1638446_1778047_ans_2acfb06412464212bb6c1bc325fcda0e.png


Write one pair of isobars. Why the chemical properties of isobars are not similar? 

Isobars are the atoms of different elements having different atomic numbers but same mass number. One pair of isobars are $$ ^{40} Ar_{18} $$ and $$ ^{40} Ca_{20} $$ 
Isobars do not have the same number of protons or electrons so the chemical properties are not similar.


Define isobar. Give an example. 

Isobars are the atoms of different elements having a different atomic number but the same mass number. 
Example- $$ _{18}Ar^{40}\, ,\, _{20}Ca^{40} $$ 


Draw the sketch of Rutherford's $$\alpha-particle$$ scattering experiment.


1624812_1772677_ans_68dad4e4ed6a4e51a32a8c53e5cabc63.png


Draw the structures of the following molecules:
a. $$XeOF_{4}$$
b. $$H_{2}SO_{4}$$.


1638389_1777954_ans_ec092df49b404cbba4dd0e5431424c94.png


Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17).

Since the atomic number (Z) of chlorine is 17, the electronic configuration of chlorine will be 2, 8, 7. There are eight electrons in the L shell (second shell).


Draw the structures of the following:
a. $$H_{2}S_{2}O_{7}$$
b. $$HClO_{3}$$.


1638596_1778471_ans_532e7e3d8e23467c9acd19150f5e3b76.png


Rutherford’s $$\alpha$$ -particle scattering experiment led to the discovery of the _______.

Rutherford's $$\alpha$$-ray scattering experiment led to the discovery of the nucleus.


Why did Rutherford select a gold foil in his $$\alpha$$-ray scattering experiment?

For the scattering experiment, Rutherford wanted a metal sheet which could be as thin as possible. Gold is the most malleable of all known metals. It can easily be converted into very thin sheets. Hence, Rutherford selected a gold foil for his alpha-ray scattering experiment.


What is Atomic number?

Atomic number is defined as the number of protons present in the nucleus of an atom. It is denoted by $$Z$$.
For hydrogen, $$Z = 1$$, because in hydrogen atom, only one proton is present in the nucleus. Similarly, for carbon, $$Z = 6$$.


In what way is the Rutherfords atomic model different from that of Thomsons atomic model?

Rutherford had proposed a model in which the electrons revolved around the nucleus in well-defined orbits. There is a positively charged centre, called the nucleus, in an atom. He also proposed that the size of the nucleus was very small as compared to the size of the atom and nearly all the mass of an atom is centred at the nucleus. 

Thomson had proposed the model of an atom that was similar to a Christmas pudding. According to Thomson, electrons are studded like currants in a positively charged sphere of pudding. The mass and positive charge of the atom was supposed to be uniformly distributed according to this theory.


What is the atomic number of X?

Answer = 17

Atomic number = Number of protons

Atomic number of an atom is always equal to the number of protons. However, the number of electrons can vary based on whether the atom is neutral or charged species. Here, this information is not given. So, we will take the number of protons as the atomic number. So, answer is 17.


What do you think would be the observation if $$\alpha-$$ particle scattering experiment is carried out on a metal foil like platinum instead of gold foil?

If the $$\alpha-$$particle scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. Because the basic structure of atom remains the same irrespective of the material. In the original experiment by Rutherford, a gold foil was taken because gold is malleable and can be easily made as thin as possible when compared to other metals.


What do you think would be the observation of a particle scattering experiment is carried out on 
light nuclei like lithium.

If light metal like lithium is used for a particle scattering experiment, then massive alpha particles will push the nucleus and may not be deflected back.


Which sub-atomic particle was discovered by Goldstein?

E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle known as protons.


Draw diagram representing the atomic structures of the Sodium atom


1696611_1809458_ans_4b94b32385c74ab59c7dca6ac1537384.png


What is the smallest particle of matter? What is this known as? What is its structure?

The smallest particle of a substance is known as $$\text{atom}$$. We cannot see those atoms through naked eyes. Atom is made up of three particles - proton, electron, and neutron. Atoms are nearly spherical in shape where protons and neutrons are in the center of an atom (nucleus) and electrons revolving around them in different levels or orbitals.


______ and _______ are present in the nucleus of the atom.

The particles present in the nucleus of an atom are the proton and the neutron. Electrons are present outside the nucleus.
Hence, the correct answers are proton and neutron.


Draw a labeled diagram of the structure of the atom.


1718504_1819701_ans_80d18b0f684440a4afeb5e29d69c9ddf.png


Draw diagram representing the atomic structures of the Chlorine


1696617_1809459_ans_672383095b87475d892234ae8d7d1258.png


Find out the valency of the atoms represented by the figure (i) and (i).
1821436_9d6b8a29ddd247b9b42c532be6c5f089.png

(1) Its electronic configuration is $$2, 8, 8$$.  So, its valency is $$0$$. 

(2) Its electronic configuration is $$2, 7$$. So, its valency is $$1$$.


Explain the following:
Mass of an atom is concentrated inside the nucleus of the atom.

The mass of an atom is contributed by proton and neutron. An electron has a negligible mass which does not contribute much to the mass of an atom. As protons and neutrons lie inside the nucleus, so mass of an atom is majorly present in the nucleus. 


Draw a diagram representing the atomic structure of the Carbon atom

Carbon-12 (or $$^{12}C$$) contains 6 protons, 6 neutrons, and 6 electrons in one atom; therefore, it has a mass number of 12 amu and atomic number 6.

1696624_1809460_ans_705482f755224cf7977dcb0e13a9da04.png


Make the model of the structure of atom on cardboard with the help of balls or marbles.


1719930_1819825_ans_28b45c1845294f45aaa2f55f41e055ca.png


The atomic number and mass number of sodium are $$11$$ and $$23$$ respectively. What information is conveyed by this statement?

Atomic number $$(11)$$ of sodium conveys information that the number of protons and electrons is the same. 
Mass number $$(23)$$ of sodium indicates the sum of protons and neutrons.


Explain isobar with examples. Give any two uses of isotopes.

$$\text{Isobars:}$$ Atoms of different elements with different atomic numbers, which have the same mass number are known as isobars.
Eg: Atomic number of $$Calcium$$ is $$ 20 $$ and $$Argon$$ has atomic number $$ 18.$$ The number of electrons in these atoms is different but the mass number of both these elements is $$ 40 $$. That is the total number of nucleons is the same in the atoms of this pair of elements.
Use of isotopes are:
1. An isotope of $$Uranium$$ is used as a fuel in nuclear reactors,
2. An isotope of $$Cobalt$$ is used in the treatment of cancer.


Explain the gold foil experiment by Rutherford. What were the observations and conclusions made by Rutherford?

Rutherford's Gold Foil Experiment:
Rutherford took a very thin gold foil with 100 nm thickness. He bombarded the gold foil with alpha rays. It is important to note that alpha particles are positively charged.

Observations made by Rutherford:
$$\cdot$$ Most of the alpha rays passed through the gold foil without any deviation in the path.
$$\cdot$$ Some of the alpha rays showed slight deviation.
$$\cdot$$ A very few alpha radiations bounced back to their source.

Rutherford's Conclusions:
$$\cdot$$ A major portion of atom is hollow and bears no charge. Hence, most of the alpha rays passed through without deviation.
$$\cdot$$ Some of the alpha-rays showed slight deviations means that there is a positively charged portion in atom. The positive charge in atom offered repulsive force on alpha particles.
$$\cdot$$ Very few alpha rays bounced back means that the positively charged portion's size is very small compared to the overall size of the atom.


From the symbol $$_{19}K^{40}$$, state the mass number of potassium.

The mass number of potassium $$(A)= 40$$
the mass number is defined as the total number of protons and neutrons in an atom.


From the symbol, $$_{17}^{35}Cl$$, deduce the mass number of chlorine.

The symbol of an element is represented as: $$_{Z}^{A}X$$, where A is the mass number, Z is the atomic number and X is the element.
The mass number of chlorine $$(A) = 35$$ and $$17$$ indicates its atomic number($$Z$$).


What is Rutherford's Atomic Model? What are the reason of failure?

Rutherford's atomic model 
1. Atom is very small, spherical, and electrically neutral containing maximum space.
2. In an atom, most of the mass is concentrated in a very small region at the center known as the nucleus.
3. The nucleus of the atom is positive because all positive charges of the atom situated in the nucleus.
4. The electrons revolve around the nucleus in circular paths.

 Reason for failure of Rutherford's model.
1. It was not able to explain the stability of the atom.
2. This model could not explain the distribution of electrons.


What is the charge on neutrons ?

Neutrons are electrically neutral species. So, the charge on them is zero.


Explain the limitations of Rutherford's model of atom?

According to James Clerk Maxwells' electromagnetic theory, a charged particle (such as electron) in circular motion continuously emits energy as radiations. Electrons which revolve around the nucleus are negatively charged. Hence while electrons revolve around the nucleus, they should emit electromagnetic radiations lose energy and come closer to the nucleus. Finally, the negatively charged electrons should collapse into the nucleus. But this does not happen in an atom. Rutherford's model failed to explain that.


Explain the postulates of Rutherford model of atom?

The postulates of the Rutherford model of atom are:
$$\bullet$$ An atom has a center called the nucleus.
$$\bullet$$ Compared to the size of an atom the size of the nucleus is very small.
$$\bullet$$ All the positively charged particles and most of the mass is concentrated in the nucleus of atoms.
$$\bullet$$ Negatively charged electrons revolve around the nucleus in a circular path.
$$\bullet$$ This atom model is known as the planetary model of the atom because the sun at the center is compared to the nucleus planets revolving around the sun is compared to electrons.


Atoms are electrically neutral. Justify it?

Atoms are electrically neutral because they have equal numbers of protons (positively charged) and electrons (negatively charged). This also means the overall charge on the atom is zero. If an atom gains or loses one or more electrons, it becomes an ion.


Mention the mass of neutron.

Mass of neutron is $$1.675 \times 10^{27} \,kg \ (1.00866 \,amu)$$.


What are isobars ?

Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.


Which are the particles whose numbers you can find out if you know the atomic number of an atom?

Number of protons and electrons are the particles whose numbers we can find out if we know the atomic number of an atom.

Atomic number = Number of protons = Number of electrons

For example, atomic number of hydrogen is 1. So, we can say it has 1 proton and 1 electron.


Explain Rutherford's experiment in brief and give the result.

Rutherford conducted an experiment to explain the arrangement of electrons and protons in an atom.

Rutherford made alpha rays fall incident on a gold foil. He used radioactive element polonium as a source of alpha rays. If the distribution of electrons and protons in an atom is uniform, then all the alpha rays which strike the atoms should reflect back. However, he observed that most of the alpha rays passed through the gold foil in a straight line and few reflected back in different directions.

From his observations, he concluded that;
1. The center of an atom has a very small, heavy and positive electric charge possessing a particle called nucleus.
2. Electrons with negligible weight and negative charge must be arranged around this nucleus.
3. As very less amount of alpha rays reflected back, most part of the atom must be hollow.

1764624_1840380_ans_5ea4cbd6054549f1aa08ce0bdb643525.png


Write a short note on discovery of neutron.

In 1932, J. Chadwick discovered a subatomic particle that had no charge and a mass nearly equal to that of a proton. It was eventually named neutron. Neutrons are present in the nucleus of all atoms, except hydrogen.


On what basis Rutherford said that atom is empty?

In his experiment, Rutherford made $$\alpha$$-particles fall on the gold foil. He observed that most of the $$\alpha$$-particles passed straight through the gold foil without deviation. He thus concluded that most part of the atom is empty.


What information about atomic nucleus was given by Rutherford from his experiment?

From his experiment, Rutherford determined that in an atom, the middle part is very small, heavy, and possesses a positive electric charge. This middle part is called the atomic nucleus. Electrons having a negative charge and negligible weight are arranged around the center (nucleus). 


Write the atomic number of fluorine? 

The atomic number of fluorine is $$9.$$


Which element does not have any neutron in it?

Hydrogen does not have any neutron in it. It contains one proton inside the nucleus and one electron outside the nucleus.


How can you say that nucleus is small in size but heavy in mass ?

A proton's or neutron’s mass is thousands times greater than an electron’s mass. Protons and neutrons are also greater in mass than most subatomic particles. Since the protons and neutrons make up the nucleus, the nucleus will be relatively heavier in mass than most sub-atomic particles.
The Rutherford’s alpha scattering experiment and deflection of lighter particles like electrons on collision with the nucleus gives us the proof that nucleus is small and heavy. 


How was the nuclear model of atom discovered ?

Rutherford's atomic model is also known as the nuclear model. In the nuclear atom, the protons and neutrons, which comprise nearly all of the mass of the atom, are located in the nucleus at the center of the atom. The electrons are distributed around the nucleus and occupy most of the volume of the atom. 

Rutherford, in his experiment, directed high energy streams of α-particles from a radioactive source at a thin sheet (100 nm thickness) of gold. In order to study the deflection caused to the α-particles, he placed a fluorescent zinc sulphide screen around the thin gold foil.

He made the following observations:

  1. A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
  2. Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume.
  3. Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180⁰ angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.


Protons are positively charged particles. Which early discovery led to the discovery of these particles?

The discovery of canal rays, which is positively charged radiation, by Goldstein eventually led to the discovery of protons later on.


What was the drawback of Rutherford's model of the atom? Describe Bohr's model of atom and explain its usefulness over Rutherford's model.

Drawbacks of Rutherford's atomic model : 
As per Rutherford’s model, electrons revolve around the nucleus in a circular path. But particles that are in motion on a circular path would undergo acceleration, and acceleration causes radiation of energy by charged particles. Eventually, electrons should lose energy and fall into the nucleus. And this points to the instability of the atom. But this is not possible because atoms are stable. Hence, Rutherford failed to give an explanation on account of this.

Bohr modified the atomic structure model by explaining that electrons move in fixed orbitals (shells) and not anywhere in between and he also explained that each orbit (shell) has a fixed energy level. Bohr improved Rutherford’s atomic model by proposing that electrons travelled in circular orbits with specific energy levels. It also explained why the light emitted by atoms consists of lines of certain colours.
When a metal atom is heated, it absorbs energy and the electrons jump to higher energy levels. When the electrons return to lower levels, they emit this energy in packets of specific energies. Some of these energies correspond to the colours that we see in the spectra. Others are at energies that our eyes cannot detect.


Fill in the blanks:
The nucleus is nearly _________ in size as compared to the total size of the atom.

The nucleus is nearly 1/100,000th in size as compared to the total size of the atom.


Fill in the blanks:
Proton was discovered by _________.

E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of a sub-atomic
particle. This sub-atomic particle had a charge equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times that of the electron. It was given the name proton.


Protons and neutrons can be found in the ................ of the atom.


2077591_270583_ans_cdc8096267a64f788b173c59bbfb8aae.jpg


Describe Rutherford's scattering experiment about the discovery of the nucleus.

Rutherford, in his experiment, directed high-energy streams of α-particles from a radioactive source at a thin sheet ($$100$$ nm thickness) of gold. To study the deflection caused to the α-particles, he placed a fluorescent zinc sulfide screen around the thin gold foil.


He made the following observations:

  1. A major fraction of the α-particles bombarded towards the gold sheet passed through it without any deflection, and hence most of the space in an atom is empty.
  2. Some of the α-particles were deflected by the gold sheet by very small angles, and hence the positive charge in an atom is not uniformly distributed. The positive charge in an atom is concentrated in a very small volume.
  3. Very few of the α-particles were deflected back, that is only a few α-particles had nearly 180⁰ angle of deflection. So the volume occupied by the positively charged particles in an atom is very small as compared to the total volume of an atom.


Based on the above observations and conclusions, Rutherford proposed the atomic structure of elements. According to the Rutherford atomic model:

  1. The positively charged particles and most of the mass of an atom were concentrated in an extremely small volume. He called this region of the atom a nucleus.
  2. Rutherford's model proposed that the negatively charged electrons surround the nucleus of an atom. He also claimed that the electrons surrounding the nucleus revolve around it at very high speed in circular paths. He named these circular paths orbits.
  3. Electrons being negatively charged and nucleus being a densely concentrated mass of positively charged particles are held together by a strong electrostatic force of attraction.



The ratio of the radii of hydrogen atom and its nucleus is $$\sim 10^{5}$$. Assuming the atom and the nucleus to be spherical:
(i) What will be the ratio of their sizes?
(ii) If the atom is represented by planet earth $$' \mathrm{R}_{\mathrm{e}}^{\prime}=6.4 \times 10^{6} \mathrm{m},$$ estimate the size of the nucleus.

(i) Volume of a sphere is given by $$\mathrm{V}=\dfrac{4}{3} \pi r^{3}$$
Let "R" be the radius of the atom and "r" be that of the nucleus.
The ratio of the radii of hydrogen atom and its nucleus is $$\sim 10^{5}$$
$$\Rightarrow \mathrm{R}=10^{5} r$$
Volume of the atom
$$\begin{array}{l}=\dfrac{4}{3} \pi \mathrm{R}^{3}=\dfrac{4}{3} \pi\left(10^{5} r\right)^{3}\left(\because \mathrm{R}=10^{5} r\right) \\=\dfrac{4}{3} \pi r^{3} \times 10^{15}\end{array}$$
Volume of the nucleus $$=\dfrac{4}{3} \pi r^{3}$$
Ratio of the size of the atom to that of the nucleus
$$=\dfrac{\dfrac{4}{3} \times 10^{15} \times \pi r^{3}}{\dfrac{4}{3} \pi r^{3}}=10^{15}$$

(ii) If the atom is represented by the planet Earth, then the radius of the nucleus would be given by :-
$$\begin{array}{l}r_{n}=\dfrac{R_{e}}{10^{5}} \\r_{n}=\dfrac{6.4 \times 10^{6} m}{10^{5}}\left(R_{e}=6.4 \times 10^{6} \mathrm{m}\right) \\=6.4 \times 10 \mathrm{m} \\=64 \mathrm{m}\end{array}$$


Class 9 Chemistry Extra Questions