Explain why magnesium oxide has a higher melting point than sodium chloride?

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The main property of ionic compounds which affects melting point is the strength of the ionic bonds.

Sodium is a Group I (or IA) representative element which means it will form a cation of +1 charge.  Chlorine is a Group XVII (or VIIA) representative element indicating it will form an anion of -1 charge.  This means the electrostatic attraction between the two ions is determined by Coulomb's law and is proportional a single positive and negative charge.

Magnesium is a Group II (IIA) element meaning it produces a +2 charge and oxygen is a Group XVI (VIA) element producing a -2 ion.  Coulombs law predicts this will have a force of electrostatic attraction that is based on a +2 and -2  charge which is 4 times greater than that of NaCl.

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