How are ionic compounds formed?

Lets take the ionic formula for Calcium Chloride is #CaCl^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 #Ca^(+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 #Ca^+2# cation.

This makes the formula for calcium chloride, #CaCl_2#.

For the example Aluminum Oxide #Al_2O_3#

Aluminum has an oxidation state of +3 or #Al^(+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 #Al_2O_3#.

I hope this is helpful.
SMARTERTEACHER