24.1    Physical Properties of Group IV Elements

Introduction to Group IV elements

• Group IV elements are p-block elements with a characteristic outer shell configuration of ns²np².

• Group IV elements exist in two different oxidation states, +2 and +4 states. The +2 state compounds become more stable going down the Group while the +4 state compounds become less stable going down the Group. The reasons will be explained later.

Variation in structure

• Going down the Group, there is a trend of non-metal to semi-metal to metal

• Carbon exists in two different allotropes, diamond and graphite(for the structures, refer to Chapter 5), they are both giant covalent compounds

• Silicon and germanium have the same type of structure as diamond. They are both giant covalent compounds

• There are two types of tin:
  1. Grey tin or alpha-tin has a giant covalent structure, same as the one diamond has.
  2. White tin or beta-tin(the more common one) has a distorted 12-co-ordinated metallic structure with metallic bonds

• Lead is a metal and it has a simple 12-co-ordinated metallic structure

Variation in melting and boiling point

• From carbon to germanium, the melting point decreases. This is because going down the Group, the covalent bond length increases. Longer bonds have lower bond energies. Less energy is required to overcome the covalent bonds, therefore melting point is lower
• 1. Tin has a lower melting point than lead. This is because in the solid state, metallic tin has a distorted structure, resulting the metallic bonds being less effective. This factor outweighs the larger atomic size of lead. Therefore, less energy is required to overcome the metallic bond in tin.
  2. Tin has a higher boiling point than lead, this is because in the liquid state, there is no ordered arrangement between the atoms. The larger atomic size of lead resulting the metallic bond being longer and weaker. Therefore, less energy is required to overcome the metallic bond in lead.

Variation in electrical conductivity

• The electrical conductivity increases down the Group as the nature of the elements changes from non-metal to semi-metal to metal
• 1. Carbon(diamond form) does not conduct electricity. This is because all the outer electrons are used in covalent bonding. Therefore there are no free and moving electrons to conduct the electricity.
  2. However, carbon(graphite form) conducts electricity. This is because in the graphite structure each atom donates one electron to a delocalised system of electrons(for more details, refer to Chapter 5). Therefore there are free electrons present to conduct the electricity.

• Silicon and germanium are semi-conductors, whose conductivity is between that of metals and non-metals.
• Tin and lead are metals, hence they are good conductors of electricity. This is because each atom involved in metallic bonding donates electrons into the sea of delocalised Therefore free electrons are present to conduct electricity.