How many of the following hydrides are electron deficient?
i)${\mathrm{SiH}}_4$             ii) ${\mathrm{AlH}}_3$                   iii)  ${\mathrm{B}}_2{\mathrm{H}}_6$           iv)  ${\mathrm{GaH}}_3$

Boron has 3 electrons in the valence shell and it requires 5 electrons to complete its octet.
From the structure we can see that diborane has a 3c-2e bond, hence it is electron deficient.

Aluminium has 3 electrons in the valence shell and it requires 5 electrons to complete its octet , if it forms a bond with 3 hydrogen it still requires more electrons to complete its octet. Hence it is electron deficient.

Silicon has 4 electrons in its valence shells and requires 4 electrons to complete its octet and here it forms a bond with 4 hydrogen atoms hence SiH₄ is an electron precise molecule.

Gallium has 3 valence electrons and it requires 5 electrons to complete its octet, if it forms a bond with 3 hydrogen it still requires more electrons to complete its octet. Hence GaH₃ is electron deficient.