Q.Write the equation for the formation of calcium hydride from calcium metal and dihydrogen, and state why beryllium does not form an analogous ionic hydride.

On heating in dihydrogen gas, calcium (like strontium and barium) combines directly to form calcium hydride:

Here the strongly electropositive calcium atom transfers both its valence electrons essentially completely to two hydrogen atoms, giving a crystalline ionic lattice built from $\text{Ca}^{2+}$ cations and hydride ions, $\text{H}^-$ -- an ionic (saline) hydride, closely analogous to the alkali-metal hydrides.

Beryllium, however, does not behave the same way. Because $\text{Be}^{2+}$ is an extremely small, highly charge-dense ion, it strongly polarizes the electron cloud of any anion it is paired with, pulling electron density back towards itself and giving the bond substantial covalent character rather than allowing a purely ionic lattice to form. Consequently, $\text{BeH}_2$ is not an ionic solid at all: it exists as a covalent, polymeric solid (with hydrogen atoms bridging between beryllium centres), quite unlike the simple ionic lattice of $\text{CaH}_2$. (Magnesium hydride, $\text{MgH}_2$, shows an intermediate, lar …

Write and balance the direct combination equation for Ca + H2, then apply the same small-size/high-polarizing-power reasoning already used for Be's other ano …

Assuming all Group 2 metals form ionic hydrides without exception; describing BeH2 as simply 'unstable' rather tha …