# How do you write the formula for ionic compounds?

Mar 6, 2014

Lets take the ionic formula for Calcium Chloride is $C a C {l}^{2}$

Calcium is an Alkaline Earth Metal in the second column of the periodic table. This means that calcium has 2 valence electrons it readily gives away in order to seek the stability of the octet. This makes calcium a $C {a}^{+ 2}$ cation.

Chlorine is a Halogen in the 17th column or ${p}^{5}$ group.
Chlorine has 7 valence electrons. It needs one electron to make it stable at 8 electrons in its valence shells. This makes chlorine a Cl^(−1 ) anion.

Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two Cl^(−1) anions will balance with one $C {a}^{+} 2$ cation.

This makes the formula for calcium chloride, $C a C {l}_{2}$.

For the example Aluminum Oxide $A {l}_{2} {O}_{3}$

Aluminum has an oxidation state of +3 or $A {l}^{+ 3}$
Oxygen has an oxidation state of -2 or ${O}^{- 2}$

The common multiple of 2 and 3 is 6.

We will need 2 aluminum atoms to get a +6 charge and 3 oxygen atoms to get a -6 charge. When the charges are equal and opposite the atoms will bond as $A {l}_{2} {O}_{3}$.