The Rules of Chemical Notation

The rules of chemical notation are used to write the chemical formula (also called molecular formula) of a compound.

An element that tends to gain |x| electron(s) forms ions with an |x-| charge (|1-,2-,3-,| etc.). A negatively charged ion is called an anion.
An element that tends to lose |x| electron(s) forms ions with an |x+| charge (|1+,2+,3+,| etc.). A positively charged ion is called a cation.

The charges indicated in the table represent the most likely charges carried by an ion in a binary ionic compound. Other charges are rare but possible.

In addition, the charges indicated in the table apply to binary ionic compounds. If a compound is ionic, but not binary, other ionic charges are possible. Ions are not involved in the formation of nonionic compounds.

In a binary ionic compound, what is the most likely charge of a magnesium ion?

In a binary ionic compound, what is the most likely charge of an aluminum ion?

In a binary ionic compound, what is the most likely charge of a copper ion?

1. Determine the ionic charges carried by the elements that make up the compound.

2. Determine the number of ions of each element required to create an electrically neutral compound. The sum of all ionic charges must be equal to |0.|

3. Write the chemical formula with the metal first and the nonmetal last.

A compound is made of calcium |(\text{Ca})| and chlorine |(\text{Cl}).| What is the chemical formula of the compound?

A compound is made of lithium |(\text{Li})| and hydrogen |(\text{H}).| What is the chemical formula of the compound?

A compound is made of aluminum |(\text{Al})| and oxygen |(\text{O}).| What is the chemical formula of the compound?