Learn: Formulae of elements, compounds, and ions

Welcome!Today we'll learn about how to write chemical formulae for elements, compounds, and ions. This is important because it helps us understand how substances are made and how they react.

What are chemical formulae?Chemical formulae show the types and numbers of atoms in a substance. For example, the formula for carbon dioxide is CO2, meaning it contains one carbon atom and two oxygen atoms.

Formulae for elementsElements in their natural state often exist as single atoms or molecules. For example, oxygen exists as O2 (a molecule with two oxygen atoms), while helium exists as He (a single atom).

Writing formulae for compoundsCompounds are made of two or more elements chemically bonded together. For example, sodium chloride (table salt) has the formula NaCl because it contains one sodium atom and one chlorine atom.

How do we balance charges?In ionic compounds, the charges of the ions must balance. For example, magnesium chloride is MgCl2 because magnesium forms Mg2+ and each chlorine ion is Cl-. Two Cl- ions are needed to balance the charge of one Mg2+.

Polyatomic ionsSome ions are made of multiple atoms bonded together. For example, the sulphate ion (SO42-) contains one sulphur atom and four oxygen atoms. Its charge is 2-.

Review Time!Great work! You've learned about formulae for elements, compounds, and ions. Now let's test your understanding with a few questions.