2.1.4 Trends in Period 3 Elements : Melting Point

Question 1

How does melting point change from Na to Al?

Answer 1

• Na to Al increases because they are metallically bonded
• With increasing delocalised electrons within the structure
• So greater electrostatic forces of attraction between protons and electrons and so more energy required to overcome them, so a higher melting point

Question 2

How does melting point change from Si to Ar?

Answer 2

• Si has highest melting point because it forms covalent bonds, and is a giant covalent structure (4 covalent bonds to neighboring Si atoms)
• Lots of energy required to overcome these strong covalent bonds hence high melting point
• P to Ar
• Simple covalent molecules (P4, S8, Cl2 and Ar)
• Covalent bonds are strong but the forces of attraction between the molecules are weak dipole induced dipole forces
• Hence not much energy required to overcome them, so MP decreases

(S has higher melting point than P because S exists as S8 and P exists as P4 so more van der waals forces)

Question 3

Electrical Conductivity across period 3

Answer 3

• Na to Al increases
• More valence electrons donated as delocalised electrons
• So Al has more electrons available to move around and carry a charge
• So Al is a better conductor than Na
• Giant covalent structure of Si has no delocalised electrons so no electrons can freely move around and carry a charge so it is a bad conductor
• The rest do not have delocalised electrons so cannot carry charge so are bad conductors of electricity