Size of anion:
An anion is formed by adding one or more electrons to the neutral atom.
F = 1s22s22p5 , F– = 1s22s22p6
O = 1s22s22p4 and O2- = 1s22s22p6
An anion has more electrons than its parent atom. With the increase of the number of electrons, the magnitude of the screening constant also increases. Therefore, the magnitude of the effective nuclear charge also decreases. The decreased effective nuclear charge pulls the electron cloud of the anion outward away from the nucleus. Thus makes the anion larger in size than its parent atom.
Nitrogen, N has 7 electrons and 7 protons, if we add an electron then there are 8 electrons and 7 protons, the additional electron increases the repulsive forces between the electrons making the radius increase.
From the above discussion, we can explain the following:
- The size of Cl– ion is bigger than the Cl atom
- Anion is large in size than its parent atom
- Ionic radius of K+ is smaller than that of Cl–, although they possess iso-electronic configurations.