Melting points of period three oxides

What is the trend in the melting points of the metal oxides (Na2O, MgO and Al2O3)?

They have high melting points because they form giant ionic lattices. The strong forces of attraction between oppositely charged ions requires a lot of heat to be broken.

Why does MgO have a higher melting point than Na2O?

Mg forms Mg2+ ions which attract the O2- ions more strongly than Na+ ions.

Why does Al2O3 have a lower melting point than expected?

The difference in electronegativity is not as large between Al and O as between Mg and O. This means that the O2- ions in Al2O3 can’t attract the electrons in the metal-oxygen bond as strongly as in MgO. This makes the bonds in Al2O3 partially covalent.

What is the melting point of SiO2 like in comparison to the other non-metal oxides and why?

It is higher than the other non-metal oxides as it has a giant macromolecular structure with strong covalent bonds holding the structure together. These require a lot of energy to break.

What are the melting points of P4O10 and SO3 like and why?

The lowest melting points. They are covalent molecules with a simple molecular structure. Molecules are held together by weak intermolecular forces, such as dipole-dipole and vdws which require little energy to break.