Question
Explain the trend in oxidation states of group 15 elements (N, P, As, Sb, Bi) and predict why does not exist while does.
Solution — Step by Step
Group 15 elements have valence configuration . They can show oxidation states from to .
Common: (in NH, PH), (in PCl, BiCl), (in HNO, PCl).
Going down the group, the energy gap between and orbitals increases. The pair becomes increasingly reluctant to participate in bonding — this is the inert pair effect.
So the oxidation state (using only ) becomes more stable down the group, while (using both and ) becomes less stable.
For Bi (heaviest in the group), the pair is very tightly held due to poor shielding by the filled and orbitals (lanthanide and -block contraction). Promoting electrons from to bond is energetically unfavorable.
So Bi prefers . exists; does not (or is highly unstable).
P, much smaller, has accessible orbitals and easier promotion. So both PCl and PCl are stable.
state stability: N P > As > Sb > Bi.
state stability: N < P < As < Sb < Bi.
Final answer: doesn’t exist due to the inert pair effect — Bi’s electrons are too tightly bound to participate in bonding. PCl exists because P’s is not subject to this effect.
Why This Works
The inert pair effect is one of the most important down-group trends in p-block chemistry. It shows up in:
- Sn vs Pb: SnCl stable, PbCl less stable; PbCl more stable than SnCl.
- Tl in group 13: favored over .
- Bi in group 15: favored over .
The root cause: poor shielding by filled inner orbitals (especially in heavy elements) makes the pair feel a high effective nuclear charge, lowering its energy and making it a “nuclear-bound spectator”.
Alternative Method
Argue from bond energies: the average M-Cl bond energy in BiCl is much lower than in PCl due to weaker Bi-Cl overlap (size mismatch). Five Bi-Cl bonds release less energy than the cost of promoting Bi’s pair, so the state is thermodynamically unfavorable.
Common Mistake
Students confuse “inert pair effect” with “loss of -orbitals”. The inert pair effect is about the -orbital pair becoming reluctant to bond, not about -orbital availability. Both factors contribute to instability of higher oxidation states down a group, but they’re distinct phenomena.