Nitride: Classification, Preparation and Uses of Nitrides

Nitride is a rare natural substance. It is formed when nitrogen combines with an element of similar or lower electronegativity. The nitride ion (N^3-) is found in nitrides. Nitrides are further classified into three categories i.e iconic, covalent and interstitial. In this article, we will discuss the classification of nitrides, preparation of nitrides and its uses. 

What is Nitride?

• Nitride is a compound of nitrogen in which nitrogen is present in the oxidation state of -3. 
• Nitride has an ionic radius of approximately 140 pm. 
• Most of the metal nitrides are unstable. 
• In most cases, when nitrides react with water, they form ammonia. But there are some exceptions also. 
• Nitrides of some elements, such as boron, vanadium, silicon, titanium are very hard and resistant to chemicals.

Classification of Nitrides

Nitrides are classified into four different groups on the basis of their nature of bonding character. The four groups are:- Ionic or saline nitrides, covalent nitrides, Transition metal nitrides, and Molecule forming nitrides. 

(i) Ionic or Saline nitrides:- 

These nitrides form when nitrogen is combined with alkali metals, alkaline earth metals, and the metals of Group 3. They form ammonium ions after reacting with hydrogen.

• Among all alkali metals, only Lithium is able to form nitride. But in the case of alkaline earth metal, all the elements form nitride in the formula of M₃N₂. When these compounds (which consist of metal cations and N^3- ions) undergo the hydrolysis process, Ammonia and metal hydroxide are the final results obtained. 
• Ionic nitrides show a wide range of variability in their stability.

(ii) Covalent nitrides:-

When nitrogen combines with the elements of groups 13-15, it forms covalent nitrides. Some examples of covalent nitrides are BN, Si₃N4, etc. 

• These nitrides are very hard. 
• Most of them are nonconductors.
• These nitrides show a wide range of properties on the basis of that element to which nitrogen is bonded. Some of the important covalent nitrides are following:-

Boron nitride (BN)

Boron nitride is a colorless crystal with a molar mass of 24.82 g/mol. It is insoluble in water. Boron nitride is said to be isoelectronic. This is because both boron and nitrogen share the same number of valence electrons with two bonded carbon atoms. 

Cyanogen (CN)₂

Cyanogen is colorless, with an almond-like odor and a toxic gas. It is prepared by the oxidation of hydrogen cyanide(HCN). For the preparation of cyanogen, a variety of oxidizing agents can be used. Some of them are oxygen gas(O₂), chlorine gas(Cl₂), and nitrogen dioxide(NO₂).

2HCN + NO₂ → (CN)₂ + NO + H₂O

• The structure of the cyanogen molecules is linear. 
• It is highly inflammable.

(iii) Transition metal nitrides:- 

These are the interstitial compounds that possess the properties of covalent compounds and ionic crystals. 

• These are the largest groups of nitrides. 
• In these nitrides, the nitrogen atom occupies the interstices, holes in the closely packed metal ions.
• MN, MN₂, and MN₄ are the general formulas for these nitrides.
• Their melting point is very high and they are very hard in nature.

(iv) Molecules forming nitrides:- 

Mostly the elements of groups 15-17 form these nitrides. They are highly volatile and have a low melting point. 

Preparation of Nitrides

There are two most frequently used methods for the preparation of the Nitrides. 

• First, one is the direct reaction of elements at an elevated temperature. For example :-

3Ca + N₂ → Ca₃N₂

• The second method is the loss of ammonia by the thermal decomposition of a metal amide. An example of one such process is:-

3Ba(NH₂)₂ → Ba₃N₂ + 4NH₃

• Nitrides can also be formed by the reduction of an oxide and metal halide in the presence of nitrogen gas. 

Al₂O₃+3C+N₂→2AlN+3CO

Uses of Nitrides

• Compounds of nitrides like the silicon nitride and titanium nitride are used for the hard coatings and cutting of materials.
• Some compounds of nitrides are used as lubricants. For Example: hexagonal boron nitride. 
• Nitrides are mainly used as insulators.
• Some nitrides like lithium nitride are also used for hydrogen storage purposes as they can absorb hydrogen-like metal oxides.

Things to Remember Based on Nitrides

• In nitrides, nitrogen is present in the oxidation state of -3. 
• Nitride has an ionic radius of approximately 140 pm. 
• Nitrides form ammonia when they are reacted with water. 
• Nitrides of some elements, such as boron, vanadium, silicon, and titanium are very hard and resistant to chemicals.
• Nitrides are classified into four different groups such as ionic or saline nitrides, covalent nitrides, transition metal nitrides, and molecule forming nitrides. 
• They are mostly used as insulators. 
• Hexagonal boron nitride is lubricant in nature. 
• Silicon nitride and titanium nitride are used for the hard coatings and cutting of materials.