Which of the following nitrides exist as giant molecules?

Options

A. `BN`
B. `Si_(3)N_(4)`
C. `AIN`
D. Both (2) and (3)

Answer

4

Explanation

Like the carbides, nitrides can be classified as ionic, covalent and interstital.Ionic nitrides containing the nitride ion `(N^(3-))` are formed by `Li` and the Group `2` elements, either by direct synthesis or, more often, by heating the metal in ammonia to form the amide which is then decomposed:
`6Li(s)+N_(2)(g)rarr2(Li^(+))_(3)N^(3-)(s)`
`Ba(s)+2NH_(3)(g)rarrBa^(2+)(NH_(2)^(-))_(2)(s)+H_(2)(g)`
`3Ba^(2+)(NH_(2)^(-))_(2)(s)rarr(Ba^(2+))_(3)(N^(3-))_(2)(s)+4NH_(3)(g)`
They are high melting-point solids and readily hydrolysed by water to the hydroxide and ammonia,
`N^(3-)+3H_(2)O(l)rarrNH_(3)(g)+3OH^(-)(aq.)`
Covalent nitrides which consist of molecules, are formed by a multitude of elementswhich are nonmetallic. Examples include sillicon nitride `(Si_(3)N_(4))` and boron nitride `(BN)` which are giant molecules. Boron nitride exists in the 'graphite' and diamond forms.
Nitrigen atoms are able to fit into the spaces between transition metal atoms, without causing too much distortion of the basic metal lattice, forming nonstoichiometric interstitial nitrides. Like the corresponding carbides, these nitrides are extremely hard, very inter and possess electrical conductivity. They are generally prepared by heating the finely divided metal in a stream of nitrogen or ammonia at a temperature in excess of `1000^(@)C`.