Formulae as shorthand

A chemical formula tells you which elements or ions are present. It uses chemical symbols from the periodic table, capital letters carefully, and small numbers to show how many atoms are in one particle or formula unit.

For example, $\ce{H2O}$ is the formula of water. The H means hydrogen, the O means oxygen, and the small 2 means two hydrogen atoms. If there is no small number after a symbol, read it as one atom, so O in $\ce{H2O}$ means one oxygen atom.

Capital letters matter. $\ce{CO}$ means carbon monoxide, made from carbon and oxygen. $\ce{Co}$ means cobalt, an element. Changing a capital letter changes the meaning, so case is part of a correct formula.

Use this reading pattern first: split the formula into symbols, then read any numbers after each symbol. Do not change the formula to make it look balanced; balancing equations is a separate skill.

Formulae of elements

An element contains only one type of atom. Many element formulae are just the chemical symbol, such as $\ce{Mg}$ for magnesium, $\ce{Fe}$ for iron and C for carbon.

Some common non-metal elements are usually written as two-atom molecules when they are the element itself. The most useful ones to recall early are:

This does not mean every formula containing oxygen has $\ce{O2}$ in it. Water is $\ce{H2O}$, not $\ce{H2O2}$. Carbon dioxide is $\ce{CO2}$, not $\ce{CO}$. The 2 belongs only where the formula says it belongs.

The first check is whether the question asks for the element or for a compound containing that element. $\ce{Cl2}$ is chlorine as an element. $\ce{Cl-}$ is a chloride ion. $\ce{NaCl}$ is sodium chloride, a compound.

Formulae of simple compounds

A compound contains atoms of two or more different elements chemically combined. For this lesson, learn common formula cards rather than trying to generate every possible compound from rules.

The (aq) after $\ce{HCl}$ means dissolved in water: hydrochloric acid is hydrogen chloride in aqueous solution. Anhydrous copper(II) sulfate contains no water of crystallisation.

Some formulae use brackets. In $\ce{Ca(OH)2}$, the 2 after the bracket applies to the whole $\ce{OH}$ group. That is why calcium hydroxide contains one calcium ion for every two hydroxide ions. You do not need to deduce new ionic formulae here; just recognise the notation in formulae you are expected to recall.

Formulae of ions

An ion is a charged particle. Its formula includes the element symbol or group of symbols and its charge. The charge is part of the formula, so $\ce{Na}$ and $\ce{Na+}$ are not the same answer.

Positive ions are called cations. Negative ions are called anions. At this stage, recall the formulae and charges; the detailed formation of ions is taught later.

In plain text, ${}^{2+}$, ${}^{3+}$ or ${}^{2-}$ shows the ion charge that would normally be written as a superscript in printed chemistry.

Notice two common patterns. Metal ions are usually positive, such as $\ce{Na+}$ and $\ce{Mg^2+}$. Many non-metal or group ions are negative, such as $\ce{Cl-}$, $\ce{OH-}$ and $\ce{SO4^2-}$. The charge may be one plus, two plus, one minus or two minus, so do not leave it out.

Before writing a formula, decide whether it represents an element, a compound or an ion. Then check its capital letters, numbers and any charge.