Which of the following elements does not form hydride by direct heating with dihydrogen?

1. Be

2. Mg

3. Sr

4. Ba

Hint: If the element has a low oxidation potential then it will not react with H2 directly.

Beryllium does not react directly with hydrogen unlike other alkaline earth metals to form ionic hydride. This is because the oxidation potential of Beryllium is very low and thus it does not donate its electrons easily to hydrogen.

Thus, except for Be, all alkaline earth metals form hydrides (MH2) on directly heating with H2. BeH2 can't be prepared by the direct action of H2 on Be.

It is prepared by the action of LiAlH4 on BeCl2.

$2{\mathrm{BeCl}}_{2}+{\mathrm{LiAlH}}_{4}\to 2{\mathrm{BeH}}_{2}+\mathrm{LiCl}+{\mathrm{AlCl}}_{3}$