Question

Compare the elements of group 15 with those of group 16 in relation to the following property:

Ionisation energy

Answer 1

1. Group 16 elements generally have higher ionisation energies than Group 15 due to smaller atomic size and higher effective nuclear charge.
2. However, the lighter elements (O, S, Se) of Group 16 have unexpectedly lower ionisation energies than the corresponding Group 15 elements (N, P, and As).
3. This is because Group 15 elements have a half-filled p-subshell (np³) which is more stable and symmetrical, making their electrons harder to remove.
4. The extra stability of half-filled p-orbitals in Group 15 elements requires more energy to ionise compared to Group 16 elements, which have one more electron (np⁴).
5. Ionisation energies decrease down the groups for both Group 15 and Group 16, due to increasing atomic size and the shielding effect.
6. Overall, except for the lighter elements, Group 16 shows higher ionisation energy but Group 15’s half-filled configuration makes their ionisation energies notably high too