Which hydride among the following is less stable?

We need to identify the least stable hydride among BeH$$_2$$, HF, NH$$_3$$, and LiH.

We start by noting that

The stability of a hydride depends on the strength of the bond between the element and hydrogen. Key factors include:

- Higher electronegativity of the bonded atom leads to stronger, more polar bonds with H, increasing stability.

- Higher bond energy means greater stability.

- Electron-deficient hydrides (like those of Group 2 and 13) tend to be less stable.

Next,

HF: Fluorine has the highest electronegativity (3.98). The H-F bond is very strong (bond energy ~568 kJ/mol). HF is very stable.

Nitrogen has high electronegativity (3.04). The N-H bond is strong (bond energy ~391 kJ/mol). NH$$_3$$ is stable.

LiH: This is an ionic hydride (Li$$^+$$ H$$^-$$). Despite the low electronegativity of Li, the ionic bond provides reasonable stability. LiH is moderately stable.

Beryllium has low electronegativity (1.57) and forms an electron-deficient covalent hydride. Be has only 2 valence electrons and forms only 2 bonds with H, leaving empty orbitals. BeH$$_2$$ is a polymeric hydride in solid state (with 3-centre 2-electron bonds) that readily decomposes. It is the least stable.

The least stable hydride is BeH$$_2$$.

The correct answer is Option 1: BeH$$_2$$.