Question

Which of these has the greatest ionization energy? Ar, Na+, Na, Mg, Kr? the answer says that it is Na+, because it is the smallest, but why isnt argon the smallest atom? The periodic trend for atom size is that the atom size decreases going up and across the periodic table.

Answer 1

Ne is smaller than Ar because we know, down the group the atomic size increases and Ar is placed down than Ne in the same group.

Na⁺ has same no. of electrons and so electronic configuration with Ne but Na⁺ has one positive charge , so Na⁺ has smaller size than Ne because size decreases with more positive charge with same no. of atom.

So, ultimately the smallest atom here is Na⁺ , that's why it has greatest ionization energy among them.