Question

How do you write the formula for ionic compounds?

Answer 1

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