Na2O, MgO and Al2O3 - all have high melting points because they form giant ionic lattices.
MgO has a higher melting point than Na2O because Mg forms 2+ ions which attract O^2- ions more strongly than 1+ Sodium ions in Na2O.
Al2O3 has a melting between Na2O and MgO because it's 3+ ions distort the oxygen's electron cloud making the bonds partially covalent.
SiO2 has a higher melting point than other non-metal oxides because it has a giant macromolecular structure. Strong covalent bonds hold it together, so lots of energy is needed to break the bonds and the melting temperature is high.
P4O10 and SO2 are covalent molecules. They have relatively low melting points because they form simple molecular structures. The molecules are held together by weak intermolecular forces (dipole-dipole and van der Waals), which take little energy to overcome.
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