Question

Why is the size of sodium is greater than magnesium

Answer 1

The no. Of shells are same. But Na has less effective nuclear charge than Mg so electrons are more efficiently held in Mg resulting in lesser size.

Answer 2

size of sodium is greater than magnesium
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atomic number of Na is 11 and Atomic no of Mg is 12 and there are in horizontal manner with each other ie along the periods

along the periods the atomic size decreases as the atoms are added to the same shell and the nucleus tries to pull the outermost electron more effectively which decreases the size of the element