Formulas of Ionic Compounds

For an ionic compound to be stable, the positive charges have to equal the negative charges. In NaCl, sodium (Na) is +1 and the chloride is –1. In MgO the magnesium is +2 and the oxygen (O) is –2 and so again the charges cancel each other out. Remember the charges come from the chart above.

Can we combine sodium (Na⁺) with oxygen (O^2-)? Yes! To make a stable compound we need 2 sodiums for every one oxygen.

There is rule for finding the correct formula. In every ionic formula the cation is written first and the anion written second. In the formula, the charge on one becomes the subscripts of the other.

The only corollary to this is that if you can divide both subscripts by an integer greater than one, you must do so. For example, Mg²⁺ and O^2- do not form Mg₂O₂ but MgO.

This works for many compounds:

Ba⁺² Cl^-	becomes	BaCl₂ _{barium chloride}
Al⁺³ O^-2	becomes	Al₂O₃ _{aluminum oxide}
Ca⁺² N^-3	becomes	Ca₃N₂ _{calsium nitride}
K⁺¹ P^-3	becomes	K₃P potassium phosphide

Please go to this site for more information on writing formulas. http://web.fccj.org/~smilczan/psc/bond.html