why do both calcium oxide and sodium chloride have a simple cubic structure and similar interionic?

Question

distances, yet calcium oxide melts at 2973K whereas sodium chloride melts at 1074K

Answer 1

Atoms arrange themselves so as to balance the charge around them, ie, the electron clouds. These are negative and have repulsive forces, so the atoms are arranged in the best way that balances the forces. The simple cubic is the best balanced arrangement for these two ionic compounds.

Oxides have very high melting points because oxygen is very electronegative. It has a strong negative pole which creates a strong polarity within its compounds. The strong negative and positive poles create strong attractive forces between the two atoms and within the crystal lattice. Thus it takes a lot of heat to loosen up the bonds in the crystal and force the individual molecules apart during the melting phase. Hence the high melting point.

Answer 2

1. Look at the radius ratio between Na+/Cl- and Ca2+/O2-. You should find them similar, hence the crystal structures.
2. The attraction between a 2+ and a 2- is theoretically 4 times as strong as between a + and a -. This explains the higher melting point.