Question

Which of the following ions is the smallest in size?
A) $C{l^ - }$
B) $N{a^ + }$
C) $M{g^{2 + }}$
D) $S_2^ -$

Answer 1

The size of atoms increase due to the effect of extra shells of electrons being added. In a given period, the alkali metal is the largest size and halogen is the smallest.

Complete answer:
All the options above are ions. Metals usually form positive ions which are formed by removing one or more electrons from the metal atom. Metal ions are smaller than the atoms from which they are formed as the whole of the outer shell of electrons is ionized, i.e.removed. This is one reason why cations are much smaller than the original metal atom.
In a positive ion, the more electrons which are removed greater will be the charge on the ion and smaller the ion becomes. Hence greater the positive charge smaller is the ion.
Now let us observe how a negatively charged ion or anion is formed. when a negative ion is formed, one or more electrons are added to an atom, the effective nuclear charge is reduced and hence the electron cloud expands. Therefore negative ions are bigger than their corresponding atoms.
Applying the above concept to the question we understand that the cations among the options are $M{g^{2 + }}$ and $N{a^ + }$ and the one with the higher charge is magnesium ion hence it will have the smaller size. The other two ions are negatively charged and as discussed above anions are greater in size from both the atoms and cations.

Therefore we can conclude that the smallest ion is $M{g^{2 + }}$ and the correct option is C.

Note: Effective nuclear charge is the ratio of the number of charges on the nucleus to the number of electrons is increased. It increases when the positive ion is formed as the number of positive charges on the nucleus exceeds the number of orbital electrons.