Question

sodium is greater than magnesium

Answer 1

Magnesium comes after sodium in the same period. Atoms of both elements have three electron shells. But the nuclear charge of sodium is +11 and that of magnesium is +12. Hence, the electron shells are pulled inward more strongly Mg atom than in Na atom. Hence, sodium atom is greater than magnesium atom.

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Answer 2

Because sodium has only 1 valence electrons as compared to 2 of magnesium