Question

Kindly explain both the points. These two points are for group-15 elements. [Explain both of these here only as they are similar and short.]
• Boiling points increase down the group
• Melting point increases up to arsenic, and then decreases up to bismuth

Answer 1

1.

Boiling point increases down the group due to the increase in the vander waals forces of attraction. As we go down the group size of the atom increases, this increase in the size means that the strength of vander waals forces of attraction also increases. And as the strength increases we require more energy to break the bonds to change it into gas phase. Thus the boiling point increases.
Factors on which the boiling point depends -
1) Strength of bonds- If the bond is stronger, then boiling point is high.
2) It depends on the size of the atoms
3) Branching decreases the boiling point as it decrease the surface area.

2.

• The melting point increases down the group due to the increase in atomic size.
• However, the unexpected decrease in the melting point of Sb and Bi is because of their tendency to form 3 covalent bonds instead of 5 covalent bonds due to inert pair effect.
• As a result, the attraction among their atoms is weak and hence their melting point are low.
• Because of large size of atoms, Bi has weaker interatomic forces than Sb and therefore, has still lower melting point.