Periodic Classification Of Elements - Class 10 Chemistry - Extra Questions

Newlands assumed that only 56 elements existed in nature and that no more elements would be discovered in the future. Later on, many new elements were discovered, whose properties did not fit into Newlands' Law of Octaves.

(i) Atomic size decreases from left to right, as the valence electrons are attracted more by the nucleus due to increase in the nuclear force.
(ii) The atomic size increases from top to bottom in a group because the number of shells keep on increasing. Therefore, distance between nucleus and valence electrons increases.

Law of octaves,  given by J.A.R. Newlands, states that "if the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements."

The atomic mass of Selenium (Se) can be calculated as shown below.

Five major differences between Mendeleev's periodic table and Modern periodic table are:-

S.No.	Mendeleev's periodic table	Modern Periodic table
(i)	Elements have been arranged in increasing order of atomic masses.	Elements have been arranged in increasing order of atomic number.
(ii)	It consists of 8 groups	It consists of 18 groups
(iii)	All the groups from I to VIII are divided into two sub groups	No sub-groups.
(iv)	He left some gaps in the table for undiscovered elements.	All elements are filled in increasing order of atomic number. No gaps left in the periodic table.
(v)	Isotopes were placed in different groups.	Isotopes were placed in same groups.

Dobereiner arranged the elements in increasing order of atomic masses. He found that the atomic mass of the middle element was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad when they are arranged in their increasing order of atomic mass.

The law used by Newland to arrange elements is known as Newlands law of octaves. According to the law of octaves elements, when arranged in increasing order of their atomic masses, the property of every eighth element is similar to the first element.

Newland proposed the Law of Octaves, which states that when elements are arranged in order of increasing atomic masses, then every eighth element has properties similar to that of the first element."

Dobereiner grouped elements in a set of $$3$$ called triads. Thus each group had $$3$$ elements. Dobereiner classified elements based on their properties. He illustrates the relationship between the atomic masses of elements and their properties with his law of triads.

• A German scientist called Johann Döbereiner put forward his law of triads in 1817. Each of Döbereiner's triads was a group of three elements. The appearance and reactions of the elements in a triad were similar to each other.
  
• Hence the arrangement of elements in a group of three is known as triads.

The increasing order of the number of electron shells is $$He <  Ne < Ar$$.
$$He,  \:  Ne,  \: Ar$$ have one, two, and three electron shells respectively.
$$He,  \:  Ne,  \: Ar$$ have atomic numbers 2, 10, and 18 respectively.

Only one triad of Dobereiner's triads exists in the columns of Newland's octaves. For example, the second column of Newlands' classification has the elements lithium (Li), sodium (Na) and potassium (K), which is a part of a Dobereiner’s triad. 

The element $$'E'$$ is having least metallic characters because As we go from left to right the metallic character decreases and the nonmetallic character increases. Furthermore, the tendency of an element to gain electrons increases with increases in effective nuclear charge, so non-metallic character increases on moving from left to right in a period.

The combining capacity of an atom is known as its valency. The number of bonds that an atom can form as part of a compound is expressed by the valency of the element.

The valence of hydrogen is $$1$$.

J.A.R. Newland in 1865 represented the "Law of Octave".

(a) Although the atomic mass of silicon $$\left( Si \right)$$ is almost average of atomic masses of sodium $$\left( Na \right)$$ and chlorine $$\left( Cl \right)$$, they don't have similar properties. Hence, even though they follow the law of the triad, they don't have similar properties. Therefore cannot be called Dobereiner's triad.

Both the elements with atomic numbers $$11$$ and $$14$$ lie in the same group so when we move from right to left in a period table atomic size decreases. So, an element with the atomic number $$14$$ will be bigger in size.

A belongs to Group 13, B belongs to Group 14 and C belongs to Group 2. Valency of A is 3, that of B is 4 and that of C is 2.

(a) No, because all these elements do not have similar properties although the atomic mass of silicon is average of atomic masses of sodium (Na) and chlorine (CI).
(b) Yes, because they have similar properties and the mass of magnesium (Mg) is roughly the average of the atomic mass of Be and Ca. 

Such an arrangement of elements is called as triad. Triad of elements was proposed by Dobereiner. He arranged elements in triads. In a triad, the atomic mass of the middle element was equal to the average of atomic massed of the other two elements. 

(a) Magnesium 
(b)  2, 8, 2 
(c) $$2Mg + O_2 \rightarrow 2MgO$$
(d) $$MgO + H_2O \rightarrow Mg(OH)_2$$
(e) Electron dot structure for the formation of magnesium oxide is shown in image: 

On moving across a period, nuclear pull increases because valence shell electrons increase across a period, and the tendency to lose these electrons will get decreases which means elements cannot lose their electrons easily. Hence, Group $$ 17 $$ elements are strong non-metals, while Group $$ 1 $$ elements are strong metals. For groups $$1$$ element there is less nuclear attraction and group elements can easily lose these electrons hence group $$1$$ are strong metals.

1. Hydrogen position : Hydrogen resembled the properties of both alkali metals like  lithium and also that of halogens like iodine. Hence the position of hydrogen whether hydrogen is to be placed with halogen or alkali metals was not specified

2. position for isotopes: Merdelew's periodic table was based on arranging elements in increasing order of the atomic masses but isotopes were not included in hid periodic table 

3. Certain elements were arranged in reverse order. Elements having higher atomic mass were placed in front or before the elements with less atomic mass. Ex: cobalt and Nickel.

4.Severals Gaps were left in the periodic table as he believed that several elements are yet to be discovered for instance  Gallium was not discovered at that time.

5.Position of VII the group is inappropriate.

Mendeleev left some gaps in his periodic table of elements for the undiscovered elements. For example Mendeleev discovered eka-silicon with atomic mass of $$72$$, which was later named Germanium with atomic mass $$72.6$$.

The atom having the smallest size = $$Hydrogen\ (H)$$

Moving left to right in the second period, the number of valence electrons $$increases$$ from $$1$$ to $$8.$$.

Since it has one valence electron, it is a metal. Potassium with atomic number $$19$$ has the electronic configuration $$(2,8,8,1)$$.

Metals are placed on the left-hand side of the Periodic Table while non-metals are found on the right-hand side. A zig-zag line separates the metals from the non-metals.

Dobernier could identify only three triads.

Atomic radius decreases from top to bottom in a group and, it decreases from left to right across a period. Therefore, helium is the smallest, and francium is the largest.

Law of triads was given by Dobereiner.

The Law of Octaves was introduced by Newlands.

The length covered by 1 carbon atom corresponds to its diameter.
The diameter of carbon atom is $$\displaystyle \frac {2.4}{2 \times 10^{8}}=1.2 \times 10^{-8  } $$ cm.
The radius of carbon atom is $$\displaystyle \frac {1.2 \times 10^{-8} }{2} = 0.6 \times 10^{-8} cm =0.06\ nm $$.

$$\displaystyle  K > Mg > Al > B  $$ represents the correct order of metallic character.
On going from left to right in a period, the metallic character decreases. Thus the order $$\displaystyle K > Mg > Al $$. On moving down the group, the metallic character decreases.Hence, $$B$$ is less metallic than $$Al$$.