The constituent particles of molecules solids are molecules of the same compound. The molecular solids are classified into three types according to the type of molecules involved in crystal formation and the nature of intermolecular force of attraction between the neighbouring molecules.
a) Polar molecular solids :
The covalently bonded bonded molecules of the substances like HCl, SO2, etc. exist in gaseous state as discrete molecules under normal conditions of temperature and pressure. On cooling and subjecting to high pressures the gases first liquefy and then solidify to yield polar molecular solids. Polar molecules possess permanent dipole moment. The molecules in solid state are held together by strong dipole-dipole interaction.
As the molecules are polar, there is separation of positive and negative charges. Hence the polar molecules arrange themselves in such a way that opposite charges of the neighbouring molecules are brought closer. The solids are soft and do not conduct electricity. As the bonding is relatively weak the melting points of the solids are relatively low.
b) Non polar molecular solids :
None polar molecules like CO2, H2, Cl2, CH4 and weakly polar molecules like CO and other hydrocarbons form non polar molecular solids at relatively lower temperature. Iodine ( I2 ) exits in solid state are also soft and do not conduct electricity. The melting point of non polar molecular solids are very low, lower than polar molecular solids. The solids are formed by the interplay of weak dispersion forces or London forces.
c) Hydrogen bonded molecular solids :
If hydrogen atom in a molecule is bonded covalently to any of the strongly electronegative atom like oxygen, nitrogen or fluorine than the hydrogen atom acquires additional positive charge and shows ability to form additional bond with strongly electronegative atoms in the vicinity. The additional bond formed is called hydrogen bond. Solid ice is formed by the hydrogen bonding between hydrogen atoms of one water molecule with Oxygen atoms of neighbouring water molecules. Hydrogen bonded solids are non conductors electricity. At normal temperatures and pressures these solids exist as liquids or even gases. The liquids solidify only on cooling.
Hydrogen bonding in ice and water
From the knowledge of Atomic radii of hydrogen and oxygen atoms, the volume of a single water molecules is found to be equal to 15 X 10-24 cm3. Assuming that all water molecules in liquid state are closely packed, the total volume of 1 mole of water i.e. 18g water may be obtained by multiplying volume of 1 molecule of water by Avogadro's Number, 6.022 X 1023 = 9 cm3 . Hence, volume of 1 mole of water = 15 X10-24 cm3 X 6.022 X 1023cm3. However volume of 1 mole of water with density 1gmL-1 is found to be equal to 18cm3 . Hence it is evident that out of total 18 cm3, the molar volume of water, the actual volume occupied by water molecules is about 9cm3 and remaining 9cm3 i.e. about half of the total volume is empty, unoccupied.
X-Ray crystallography study of ice shows that it has hexagonal three dimensional crystal structure formed by intermolecular hydrogen bonding, which leave almost half the space vacant, unoccupied. X-ray studies of liquid water shows that structure of liquid water and solid ice are almost identical. However on melting of ice some of the hydrogen bonds are broken and some of the empty spaces are occupied by water molecules. And astonishingly density of liquid water molecules become more closely spaced. Generally density of solid state is always greater than the density of its liquid state. Substance water has abnormal behaviour, its liquid state has more density than its solid state. Hence ice floats on water. This is due to more empty space in hexagonal structure of ice. Hydrogen bonding between water molecules forming tetrahedral arrangement of ice.