Hydrogen combines almost with all the elements and forms a class of compounds called hydrides. Let us briefly explain about different types;
Mostly s- block elements forms this type of hydride which conduct electricity only in the molten state, but BeH2 is exceptionally covalent and polymeric in structure. They react with water violently to give H2 and the corresponding hydroxide. (eg. NaH(s) + H2O NaOH(aq) + H2(g)
Covalent or molecular hydrides
Mostly these kind of hydrides are formed by p-block elements situated in the periodic table. According to the relative no. of electrons and bonds present in the Lewis structure covalent hydrides can be briefly categorised as follows;
Electron - deficient hydride
This type of hydride has very few electrons for writing the conventional Lewis structure. Eg. B2H6, diborane, these are also called Lewis acids.
Electron precise hydrides
They have exactly the required no of electrons for writing their conventional Lewis structures. Eg. All elements of group 14.
Electron rich hydrides
They possess excess of electrons than the required, which are present as lone pairs. Elements of group 15-17 particularly N, O, F which forms hydrides in the associated forms called hydrogen bonding. These are also called as Lewis bases.
Metallic (or) non stoichiometric (or) interstitial hydrides
Unlike the formerly discussed ones these are non stochiometric in nature. Eg. NiH, PdH, TiH etc. These are always conducting in nature. Except for metals like Pd, Ni etc. where hydrogen occupies the intersticies, rest of the hydrides provide a large volume for the accommodation of hydrogen. Hence this property leads a way for hydrogen storage .