The radius of a calcium ion is 94 pm and of an oxide ion is 146 pm. Predict the crystal structure of

The radius of a calcium ion is 94 pm and of an oxide ion is 146 pm. Predict the crystal structure of calcium oxide.

Answer - The ratio  r+/r- = 94/146 = 0.644 . The prediction is an octahedral arrangement of the oxide ions around the calcium. Because the ions have equal but opposite charges, there must also been an octahedral arrangement of calcium ions around oxide ions. Thus we would expect a rock salt (NaCl) structure.