Question

Which of the following is network crystalline solid?
A. AIN
B. ${{I}_{2}}$
C. Ice
D. NaCl

Answer 1

Before answering this question, we should first know It is a chemical compound or element where the atoms are bonded by covalent bonds in a continuous network extending throughout the material is known as a network solid or covalent network solid (also called as atomic crystalline solids). There are no individual molecules and the entire crystal or amorphous solid in a network solid may be considered a macromolecule.

Complete answer:
Formulas for network solids are simple ratios of the component atoms represented by a formula unit like those for ionic compounds.
Some examples of network solids are diamond with a continuous network of carbon atoms and silicon dioxide or quartz with a continuous three-dimensional network of $Si{{O}_{2}}$units. Graphite and mica group of silicate minerals structurally contains two dimensional sheets covalently bonded within the layer with other bond types holding the layers together whereas glasses are the disordered network solids. They are prepared by rapid cooling of melts so that little time is left for atomic ordering to happen.

So, Option (A) AIN is correct.

Note:
Properties-
They are very hard because of the strong covalent bonds throughout the lattice (deformation is easier, however in direction that do not need the breaking of any covalent bonds, as with flexing or sliding of sheets in graphite or mica)
They have a high melting point as melting means breaking of covalent bonds not merely overcoming weaker intermolecular forces.
The solid-phase electrical conductivity is variable, it depends on the nature of the bonding. The network solids with delocalized pi bonds like graphite or dopants can exhibit metal-like conductivity whereas network solids in which all electrons are used in sigma bonds like diamond etc are poor conductors.
The liquid-phase electrical conductivity is low because the macromolecule has neutral atoms that means the melting does not free up any new charge carries like for an ionic compound.
The solubility is less, it is generally insoluble in any solvent because of the difficulty of solving such large molecules.