Hydrides

The high boiling point of water is due to H-bonding.

Phosphorus cannot expand its octet with hydrogen to form PH₅.

Step 1: Phosphorus can expand its octet only with highly electronegative atoms like fluorine. Step 2: \beginitemize • PH₃ exists (stable hydride) • PF₅ exists (expanded octet with F) • LuH₃ exists (lanthanide hydride) • PH₅ does not exist (P cannot form stable pentahydride) \enditemize

Step 1: Understanding the Concept:
The stability of hydrides generally depends on the electronegativity difference, the nature of the bond (ionic vs covalent), and the bond dissociation energy.

Step 2: Detailed Explanation:
1. HF and NH : These are covalent hydrides of highly electronegative p-block elements. They are quite stable due to strong covalent bonding and, in the case of HF, significant ionic character. 2. LiH: This is a saline (ionic) hydride. While reactive, it is thermally stable compared to Group 2 hydrides. 3. BeH : Beryllium hydride is an electron-deficient polymer. Because Beryllium is very small and has a high polarization power, its hydride is not a simple molecule but forms a complex bridged structure to gain stability. Among the choices, it has the lowest thermal stability and is the least stable in terms of standard formation.

Step 3: Final Answer
BeH is the least stable among the given hydrides.