Question

How do you write ionic formulas for binary compounds?

Answer 1

The term binary means two pieces. Binary compounds contain two elements.

• 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 `17^(th)` 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^(− )` anion.

Ionic bonds form when the charges between the metal cation and non-metal anion are equal and opposite. This means that two `Cl^(−)` 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`

• Aluminium 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