A) Boiling Point, Melting Point and Enthalpy of Vaporisation
Enthalpy of vaporisation – the heat energy required to convert 1 mol of a liquid to its vapour at the boiling point of the liquid.
Example: Period 2 (Li, Be, B, C, N, O, F, Ne) & Period 3 (Na, Mg, Al, Si, P, S, Cl, Ar)
Across the period: the element become less metallic.
Li, Be, Na, Mg and Al are metals (metal lattice):
- B.P, M.P and enthalpy of vaporisation increase and the atoms are held together bystrong metallic bond.
- Increasing the number of valence electrons cause the strength of the metallic bondincrease.
B, C (graphite) and Si are metalloids & C (diamond) is non-metal (giant covalent molecule):
- B.P, M.P and enthalpy of vaporisation are very high.
- The atoms are held together by strong covalent bonds which form giant covalent structure (crystal lattice structure) in a 3-D structure.
- All the covalent bonds are needed to be broken before the solid melts.
N, O, F, P, S, Cl are non-metallic elements (simple molecular structure):
- B.P, M.P and enthalpy of vaporisation are relatively low that involves only the breaking of weak Van der Waals forces.
- N2, O2, F2, P4, S8, Cl2 consist of small and discrete molecules.
- The covalent bonds within the molecules are very strong, but the Van der Waals forcesof attraction between the molecules are very weak.
Ne and Ar are non-metallic (monoatomic structure):
- B.P, M.P and enthalpy of vaporisation are very low.
- Noble gases are uncombined atoms and have very weak Van der Waals forces of attraction between the atoms.
B) Electrical Conductivity
- All metals (Li, Be, Na, Mg and Al) are good conductors either in the solid or molten state.
- Metals have delocalised electrons which will move freely across the metal in the solid lattice structure when an electrical potential or voltage is applied.
- Non-metals (C diamond, N, O, F, Ne, P, S, Cl, Ar) are non-conductors.
- All the valence electrons in non-metals are used to form covalent bonds between atoms and there are no mobile electrons in the structure. Ne and Ar are noble gases and have the stable octet electronic configuration and do not have any mobile electrons.
- Metalloids (C graphite, Si) are semi-conductor.
- Conductivity of metalloid increases with the increasing of temperature.
