Q.Which among the following is a molecular hydride?

Hydrides are classified as ionic (saline), molecular (covalent), or metallic (interstitial), based on how hydrogen bonds with the other element; NH3 is the molecular one here.

Hydrogen forms three broad classes of binary hydrides depending on the electronegativity/nature of the partner element:

• Ionic (saline) hydrides — formed by highly electropositive s-block metals (Li, Na, K, Ca, etc.), where H exists as H⁻. LiH is an example.
• Molecular (covalent) hydrides — formed by p-block non-metals/metalloids that share electrons with H in discrete covalent molecules. NH3 (nitrogen trihydride) is a classic molecular hydride, along with CH4, H2O, HF, etc. …

Among AsH3, NH3, BiH3 and PH3, NH3 is the most thermally stable hydride because bond dissociation enthalpy decreases down the group.

AsH3, NH3, BiH3 and PH3 are hydrides of Group 15 elements (N, P, As, Bi). Going down the group, atomic size increases, so the bond formed between the central atom and hydrogen (M-H bond) becomes longer and weaker, i.e., the bond dissociation enthalpy decreases: N-H > P-H > As-H > Bi-H. A hydride with stronger, higher-energy bonds is more thermally stable (harder to decompose on heating). Since nitrogen is the smallest …