Explanation
Sodium ion has a larger size than lithium because it has one extra shell than lithium and consequently its size will be greater than lithium.
Now, lithium has a larger size than magnesium because more the charge on an ion lesser will be its size. Also, Mg has a larger size than beryllium because of the presence of extra shell compared to beryllium. Or we can say in other words, more the charge on an ion lesser will be its size due to the increase in effective nuclear charge after the loss of electrons.
The effective nuclear charge will be greater in the ion which has lose more electrons.
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