A group 15 element forms d π-d π bond with transition metals. It also forms hydride, which is a strongest base among the hydrides of other group members that form d π-d π bond.
The atomic number of the element is _______ .

Solution

Chemistry ma’am · 4 steps

Strike out nitrogen first because it has no d orbitals, then pick the most basic hydride from the rest.

1. Who can dπ-dπ bond. A dπ-dπ bond needs vacant d orbitals on the group 15 atom. Nitrogen is in period 2 and has no d orbitals, so it cannot form one. P, As, Sb and Bi can. For example, ligands such as P(C₂H₅)₃ and As(C₆H₅)₃ bond to transition metals this way.
2. Basicity of hydrides. Basic strength of the hydrides falls down the group: NH₃ > PH₃ > AsH₃ > SbH₃ > BiH₃. As the atom gets bigger, its lone pair spreads over a larger volume and is less available to donate.
3. Combine both clues. NH₃ is the strongest base overall, but nitrogen is already out. Among P, As, Sb and Bi, the most basic hydride is PH₃.
4. So. The element is phosphorus, and its atomic number is 15.

The common slip: 7. Choosing nitrogen because NH₃ is the most basic hydride gives 7. Nitrogen has no d orbitals, so it cannot form dπ-dπ bonds.

The idea to keep. In group 15, any clue that needs d orbitals rules nitrogen out at once. Hydride basicity, like most hydride trends here, decreases steadily from NH₃ to BiH₃.

Answer from the NTA final answer key. Solution by PrepWiser.