Thermal Stability of Hydrides ($$MH_3, MH_2, HX$$)

Trend Orders:

$$\begin{aligned} \text{Group 15:} &\quad \text{NH}_3 > \text{PH}_3 > \text{AsH}_3 \\ \text{Group 16:} &\quad \text{H}_2\text{O} > \text{H}_2\text{S} \\ \text{Group 17:} &\quad \text{HF} > \text{HCl} > \text{HBr} > \text{HI} \end{aligned}$$

Concept: Thermal stability decreases systematically down each group because the central atom's atomic size increases, resulting in longer

$$M\text{-H}$$ bonds and a steep drop in bond dissociation enthalpy.

Notation:

• $$M$$ = Group 15 or 16 central atom;
• $$X$$ = Group 17 halogen atom;
• $$\text{NH}_3, \text{H}_2\text{O}, \text{HF}$$ = Most thermally stable hydrides in their respective groups.