The variation observed in the metallic and nonmetallic character of elements can be explained in the terms of ionization enthalpy and electron gain enthalpy.
i. Metallic character:
- The ionization enthalpy decreases down the group. Thus, the tendency to lose valence electrons increases down the group and the metallic character increases down a group.
- However, the ionization enthalpy increases across the period and as a result metallic character decreases across a period.
ii. Nonmetallic character:
- Electron gain enthalpy becomes less negative as we move down the group and hence, nonmetallic character decreases down the group.
- However, electron gain enthalpy becomes more and more negative across the period and thus, nonmetallic character increases across the period.