_​Atoms, elements and compounds  

In a nutshell

Atoms are the small building blocks of life. Atoms of two or more elements chemically bond together to form compounds.

Atoms

Definition

Atoms are the small particles which make up everything in the universe. 

Elements 

Definition

An element is a substance made up of only one type of atom.  

Example

Sodium is an element. The element sodium is made up of sodium atoms only.

1. Atoms 2. Element 3. Compound 4. Mixture

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Compounds

Definition

Substances made up of more than one type of atom are called compounds. 

Compounds form when atoms of different elements react. The atoms of the different elements are held together by chemical bonds.

There is a fixed ratio of different atoms throughout a compound. 

Example

Water is a compound. All water molecules are made up of two hydrogen atoms chemically bound to one oxygen atom. 

Mixtures

Definition

If a substance contains atoms of different elements, but the atoms are not bonded together then it is not a compound, it is referred to as a mixture.

Chemical symbols 

To avoid writing out the full name of elements, atoms of each element can be referred to using a one or two letter symbols known as a chemical symbol. 

Example

$$C$$​ = carbon, $$Ag$$​ = silver and $$N$$​ = nitrogen.

Chemical formulas 

Compounds can be represented by formulas. Formulas are made up of chemical symbols and they display the proportions of each element in a given compound.

Formulas can contain brackets. Everything inside the brackets is multiplied by the small number outside of the bracket.

Example 1

Water is made up of two hydrogen atoms and one oxygen atom. The formula for water is $$H_2O$$.

Example 2

The formula for acetic acid is $$C_2H_4O_2$$. Each molecule contains two carbon atoms, four hydrogen atoms and two oxygen atoms.

Example 3

The formula for magnesium nitrate is $$Mg(NO_3)_2$$​ . Each molecule contains one magnesium atom, two nitrogen atoms and six oxygen atoms.