Chemical formulae

A chemical formula uses element symbols to show which elements are present in a substance. Small numbers called subscripts show the relative numbers of their atoms.

For example, the formula for water is $$\ce{H2O}$$. It contains hydrogen, represented by $$\ce{H}$$, and oxygen, represented by $$\ce{O}$$. The subscript 2 applies to hydrogen. There is no subscript after oxygen, which means one oxygen atom. The ratio of hydrogen atoms to oxygen atoms is therefore 2:1.

Element symbols must be written with the correct capital and lower-case letters. For example, $$\ce{Na}$$ is the symbol for sodium, whereas $$\ce{N}$$ is the symbol for nitrogen.

Molecular formulae

Some substances consist of molecules. A molecular formula shows the type and number of atoms in one molecule. One water molecule, $$\ce{H2O}$$, contains two hydrogen atoms and one oxygen atom. One carbon dioxide molecule, $$\ce{CO2}$$, contains one carbon atom and two oxygen atoms.

Not every compound consists of separate molecules. For example, the formula $$\ce{NaCl}$$ shows that sodium chloride contains sodium and chlorine atoms in a 1:1 ratio; it does not describe one sodium chloride molecule.

Deducing a formula from a model

To find the formula of a simple compound from a model or diagram, identify each type of atom using the labels or key. Count the atoms of each type, then write their element symbols with subscripts for the numbers present. Do not write a subscript when the number is one.

For example, if a model shows one carbon atom joined to two oxygen atoms, write the symbols $$\ce{C}$$ and $$\ce{O}$$. Carbon needs no subscript, while oxygen needs a subscript 2. The formula is $$\ce{CO2}$$.

Familiar formulae

Some elements also exist as molecules. Hydrogen, oxygen and nitrogen have the formulae $$\ce{H2}$$, $$\ce{O2}$$ and $$\ce{N2}$$ respectively. Each of these molecules contains two atoms of the same element.

Familiar compounds include water, $$\ce{H2O}$$; carbon dioxide, $$\ce{CO2}$$; methane, $$\ce{CH4}$$; and sodium chloride, $$\ce{NaCl}$$.

Word equations for chemical reactions

A word equation represents a chemical reaction using the names of the substances involved. It shows which substances react and which new substances are formed.

The starting substances are called reactants and are written on the left of an arrow. The substances formed are called products and are written on the right. The arrow means “forms” or “produces”.

A plus sign separates the names when there is more than one reactant or more than one product.

Constructing a word equation

First identify the substances that react and the substances formed. Write the reactant names on the left, followed by an arrow, then write the product names on the right. Separate multiple names on the same side with plus signs.

For example, magnesium reacts with oxygen to form magnesium oxide:

Magnesium and oxygen are the reactants, so they appear on the left. Magnesium oxide is the product, so it appears on the right.

Symbol equations for chemical reactions

A symbol equation uses chemical formulae to show the substances that react and the substances formed. The reactants are written on the left and the products on the right. An arrow points from reactants to products, and plus signs separate substances on the same side.

For example, hydrogen reacts with oxygen to form water:

Balancing symbol equations

Atoms are rearranged in a chemical reaction, so a balanced equation must have the same number of atoms of each element on both sides. In the equation above, there are two hydrogen atoms on each side, but two oxygen atoms on the left and only one on the right.

To balance an equation, place numbers called coefficients before formulae. A coefficient applies to the whole formula: $$\ce{2H2O}$$ represents two water molecules, containing four hydrogen atoms and two oxygen atoms. If no coefficient is written, it is understood to be one.

Placing a coefficient of two before water balances the oxygen atoms, but gives four hydrogen atoms on the right. Placing a coefficient of two before hydrogen then balances the hydrogen atoms:

Each side now has four hydrogen atoms and two oxygen atoms. The coefficients give the relative amounts: two molecules of hydrogen react with one molecule of oxygen to form two molecules of water.

Balance an equation by changing coefficients, not the small numbers called subscripts within a formula. Changing a subscript changes the formula of the substance itself; for example, $$\ce{H2O2}$$ is a different substance from $$\ce{H2O}$$.

State symbols in symbol equations

A state symbol after a formula shows the physical state of that substance:

• $$\ce{(s)}$$ means solid.
• $$\ce{(l)}$$ means liquid.
• $$\ce{(g)}$$ means gas.
• $$\ce{(aq)}$$ means aqueous: the substance is dissolved in water.

For example, the balanced equation can show hydrogen and oxygen as gases and the water formed as a liquid:

A substance marked $$\ce{(aq)}$$ is dissolved in water; it is not necessarily a liquid in its pure form. State symbols describe the substances but do not change the balance of the equation.