How do you write ionic formulas for binary compounds?

May 2, 2014

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

• 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 ${17}^{t h}$ 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 $C {a}^{2 +}$ cation.

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

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

• Aluminium 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 $A {l}_{2} {O}_{3}$.