Choose the element with the higher ionization energy from each pair.
(a) Al or In
(b) Cl or Sb
(c) K or Ge
(d) S or Se
Here, we are going to identify the element with higher ionization energy from each pair.
Ionization energy represents the energy required to remove an electron from an isolated gaseous atom in its ground state.
As we move from left to right across a period, the ionization energy increases. This is because, as we move across a period, successive electrons are added to orbitals in the same energy level and the shielding of the nuclear charge by the inner core of electrons does not increase very much to compensate for the increased attraction of the electron to the nucleus. Thus, across a period, increasing nuclear charge outweighs the shielding. Consequently, the outermost electrons are held more and more tightly and the ionization energy increases.
On the other hand, as we move down a group, the outermost electron being increasingly farther from the nucleus, there is an increased shielding of the nuclear charge by the electrons in the inner levels. In this case, increase in shielding outweighs the increasing nuclear charge and the removal of the outermost electron requires less energy down a group. Thus, the ionization energy decreases down a group.