F.4 homework question (聽日交)

Question

Na2O, MgO, SiO2 and SO2 are oxides of period 3 elements in the Periodic Table. Discuss how the melting points of these oxides are related to their bonding and structure.

Answer 1

SO2 has molecular structure. The molecules of SO2 are held together by weak van der Waals' forces.
Hence, SO2 has the lowest melting point.

SiO2 has giant covalent structure. Si atoms and O atoms are held together by a network of strong covalent bonds. Since covalent bonds are directional, almost all covalent bonds are broken in melting.
Hence, SiO2 has the highest melting point.

Both Na2O and MgO have giant ionic structure. In each of them, the opposite ions are held together by rather strong electrostatic attractions. In melting, only part of the electrostatic attractions is broken because ionic bonds (electrostatic attractions) are non-directional.
Hence, the melting points of Na2O and MgO are higher than that of SO2, but lower than that of SiO2.

Mg^2+ ion carries a greater charge than Na^+ ion, and Mg^2+ has a smaller size. Therefore, the electrostatic forces between Mg^2+ and O^2- are stronger than those between Na^+ and O^2-, because electrostatic forces are directly proportional to the charges of the opposite ions and inversely proportional to the distance between the opposite ions.

Hence, MgO has a higher melting point than Na2O.

Melting points : SiO2 > MgO > Na2O > SO2

Answer 2

Because these atoms in the ionic bond all got a full outer shell, which all atoms are trying to achieve, so they "are happy", and don't want to move around anymore, meaning that there will be much more heat energy to be applied to break the bonds between each atoms, also they are in a form of lattice (structure of ionic bonds, cube), the forces are far greater than a convalent bond (non-metal with non-metal, share electrons), because the forces between atoms in a covalent bond are weaker, so the heat energy to melt them should be far greater than the heat energy needed to melt a covalent bond