The goal when forming compounds is to achieve a net charge of 0.
We are given aluminum with a charge of 3+ and nitrogen with a charge of 3-.
These charges are equal and opposite, so one of each will equal 0.3+ (Al) + 3- (N) = 0
Therefore, we do not need any subscripts. List the metal's symbol (aluminum) then the nonmetal.AlN
Th formula for the compound, which is aluminum nitride, is AlN.
Cations (positive ions) and anions (negative ions) combine to form ionic compounds. The charge of each ion is stated as the superscript in its formula.
The final compounds should be neutral (not carrying charge.) For that to happen, the ions should combine at a ratio that allows their charges to balance.
By convention, in chemical formula of ionic compounds, cations are written before anions.Example: Al³⁺ and SO₃²⁻
The charge on an Al³⁺ cation is +3. The charge on a SO₃²⁻ anion is -2.
Start by finding the least common multiple of the absolute value of the charges (drop the negative sign in front of the charge.)
The number of Al³⁺ ions in a formula unit of this compound will be equal to
Similarly, the number of SO₃²⁻ ions in a formula unit of this compound will be equal to
If a polyatomic ion (an ion with more than one atoms in it) appears more than once in the formula, enclose it in brackets and write the number of its occurrence outside the bracket.
Apply this rule to find the formula of other compounds. Additionally, keep in mind that if the charge on the cation and the anion is the same, each should occur only once in the formula.