91Ó°ÊÓ

Melting point

Melting point is a temperature at which a solid form of a pure substance and liquid form of a substance coexist in equilibrium with each other. It is also the amount of energy required to break a bond that will convert a substance from solid phase to the liquid phase.

In general, on moving down the group the melting point decreases. This can be explained as follows: As the bonds between atoms of elements are stronger, high bond dissociation energy will be required and hence high temperature will be required and vice-versa. As we move down the group the size of the atoms increases and thus the bond length between atoms of elements also increases, which results in decrease of the bond dissociation energy required, and thus temperature required to break the bond is will also be lower. Therefore, the melting point also goes on decreasing on moving down the group. Melting points generally do not make a well-defined variation across the periodic table.

The reason for the decrease of melting point from C to Si is that, following a general trend, the size of Si is larger than the size of C, so the bond strength of Si-Si is lower as compared to that of C-C bond strength.

The reason for the sharp decrease in melting point of Sn is its distorted structure, which is a 12 co-ordinated structure and not a pure one.