Question

Discuss the trends in chemical reactivity of group 15 elements.

Answer 1

1. Reactivity towards hydrogen: The elements of group 15 react with hydrogen to form hydrides of type EH₃, where E = N, P, As, Sb, or Bi. The stability of hydrides decreases on moving down from NH₃ to BiH₃.
2. Reactivity towards oxygen: The elements of group 15 form two types of oxides, E₂O₃ and E₂O₅, where E = N, P, As, Sb, or Bi. The oxide with the element in the higher oxidation state is more acidic than the other. However, the acidic character decreases on moving down a group.
3. Reactivity towards halogens: The group 15 elements react with halogens to form two series of salts: EX₃ and EX₅. However, nitrogen does not form NX₅ as it lacks the d-orbital. All trihalides (except NX₃) are stable.
4. Reactivity towards metals: The group 15 elements react with metals to form binary compounds in which metals exhibit −3 oxidation states.

Answer 2

1. Nitrogen is less reactive than other group 15 elements due to the very high bond dissociation enthalpy (941.4 kJ/mol) of the N≡N triple bond.
2. Phosphorus and other heavier elements have weaker single bonds, so they are more reactive.
3. Reactivity increases down the group as atomic size increases and ionisation enthalpy decreases.
4. Nitrogen tends to form N³⁻ ions due to its small size and high electronegativity, while heavier elements are less likely to form such negative ions.
5. The stability of +3 oxidation state increases down the group due to the inert pair effect, while +5 oxidation state stability decreases.
6. Group 15 elements form covalent compounds by sharing their three unpaired p-electrons.
7. Overall, reactivity trends depend on atomic size, bond strength, and oxidation state preferences of the elements.