Write a note on covalent or molecular hydrides.

(N/A) Dihydrogen forms molecular compounds with most of the $p$-block elements. Most familiar examples are $CH_{4}$,$NH_{3}$,$H_{2}O$,and $HF$.
For convenience,hydrogen compounds of non-metals are considered as hydrides.
Being covalent,they are volatile compounds.
Molecular hydrides are further classified according to the relative numbers of electrons and bonds in their Lewis structure into: $(i)$ electron-deficient,$(ii)$ electron-precise,and $(iii)$ electron-rich hydrides.
An electron-deficient hydride has too few electrons for writing its conventional Lewis structure. Example: Diborane $(B_{2}H_{6})$. All elements of group $13$ form electron-deficient compounds and act as Lewis acids (electron acceptors).
Electron-precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group $14$ form such compounds (e.g.,$CH_{4}$),which are tetrahedral in geometry.
Electron-rich hydrides have excess electrons present as lone pairs. Elements of group $15-17$ form such compounds. For example,$NH_{3}$ has $1$ lone pair,$H_{2}O$ has $2$ lone pairs,and $HF$ has $3$ lone pairs. They behave as Lewis bases (electron donors). The presence of lone pairs on highly electronegative atoms like $N$,$O$,and $F$ results in hydrogen bond formation,leading to the association of molecules.