The stability of hydrides is related to the bond strength between the element and hydrogen. Among the given hydrides, \( \text{BeH}_2 \) is less stable due to its electron deficiency or hypovalency.
Final Answer: \( \text{BeH}_2 \) is less stable among the given hydrides.
A group 15 element forms \( d\pi - d\pi \) bond with transition metals. It also forms a hydride, which is the strongest base among the hydrides of other group members that form \( d\pi - d\pi \) bonds. The atomic number of the element is …….