Lewis dot and cross diagram for a group 2 hydride:     * 

H-Be-H  and H-Mg-H *  symmetrical linear shape, bond angle 180^o

From the periodic table and Z, the electron configurations are:

Be 1s²2s² , Mg 1s²2s²2p⁶3s² and H 1s¹

Be and Mg are group 2 elements with two outer valence electrons and so can form two single covalent bonds - 2 bond pairs with hydrogen atoms - but no 'octet' of valence electrons around the central atom.

There are no lone pairs of non-bonding valence electrons.

Beryllium hydride is an electron deficient molecule with the two single covalent bonds of linear geometry from sp hybridisation.

VSEPR theory argument predicts a linear shaped H-Be-H (or H-Mg-H) molecule because there are only two groups of bonding electrons around the central atom mutually repelling other to the maximum angle of 180^o (this assumes we are dealing an isolated molecule in the gaseous state).

There is also unstable gaseous MgH₂ at high temperatures, magnesium is also in group 2 with an electron configuration of 1s²2s²2p⁶3s², so the predicted shape and bond angle are the same i.e. linear shape and bond angle 180^o.)

When beryllium hydride or magnesium hydride are condensed they form solid stable complex polymeric structures.