Question

The trinegative ions (M³⁻) are formed only by N and P among group 15 elements. Explain.

Answer 1

1. Group 15 elements have valence electronic configuration ns² np³, allowing them to gain 3 electrons to form M³⁻ ions (oxidation state −3) or lose electrons for positive states.
2. Formation of M³⁻ ions requires atoms to have a small size and high electronegativity to hold the extra electrons tightly.
3. Nitrogen is the smallest and most electronegative element in group 15, so it readily forms the nitride ion (N³⁻).
4. Phosphorus is next in size and electronegativity, and it can also form phosphide ion (P³⁻), though less easily than nitrogen.
5. Larger atoms like arsenic (As), antimony (Sb), and bismuth (Bi) have bigger atomic sizes and lower electronegativities, so they cannot stabilise the high charge density of M³⁻ ions.
6. Due to the larger size and less pull on added electrons, As, Sb, and Bi prefer to lose electrons and form positive ions like M³⁺ rather than gain three electrons.
7. Thus, the tendency to form trinegative ions decreases down the group and is observed prominently only for nitrogen and phosphorus.

This explains the order N³⁻ > P³⁻, while As, Sb, and Bi do not form stable trinegative ions due to size and electronegativity factors.