Atoms and molecules

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Key points

  • are the building blocks of everything.
  • Atoms can form strong bonds with each other, making .
  • A pure substance made from only one type of atom is called an element. Elements are listed on the periodic table.
  • Elements can combine to make compounds.

Why is it difficult to see an individual atom?

It is difficult to see an individual atom because they are so small. We use microscopes to see tiny things like bacteria but these are about 10,000 times bigger than an atom.

Atoms and molecules

Watch this video to find out more about atoms and molecules.

What is the difference between an atom and a molecule?

An atom is the smallest particle of an element. A molecule is made of two or more atoms chemically bonded together.


Atoms are the building blocks of all matter. Everything is made of atoms - even yourself.

Atoms are the smallest particle of an , which are far too small to see. Even the most powerful microscopes cannot visualise a single atom. If the earth’s population of 7 billion people were the size of an atom, they would take up less than 1mm of space.

In science, the word can be used in many ways. It is a single piece of matter from an element or a compound, which is too small to be seen. Particles can be atoms, molecules or ions.


An element is a pure substance which is made from only one type of atom. Everything in the universe contains the atoms of one or more elements. The atoms in one element are all the same as each other, but they are different from the atoms of any other elements. There are 118 different elements. They are listed on the . Examples of elements include oxygen, hydrogen and carbon.

Gold is an element. Which atoms are pure gold bars made of?

In pure gold bars, all of the atoms are gold.


Molecules are made when two or more atoms chemically bond together. Atoms from different elements can combine. When the atoms are from different elements, the molecule can also be called a compound.

The atoms of some elements like helium do not form bonds. Helium atoms are single atoms.

The atoms of elements such as hydrogen, nitrogen and oxygen join in pairs to make molecules.

Water is made of molecules. Each water molecule is made from two hydrogen atoms chemically bonded to one oxygen atom. This means that the chemical formula of water is H₂O.

Carbon dioxide is made of molecules of a carbon atom bonded to two oxygen atoms (CO₂).

A water molecule

Drawing atoms and molecules

Atoms can be drawn as circles. But all the atoms of an element need to look exactly the same.

When drawing a diagram of molecules made from the atoms of more than one element, different sizes and colours can show the different elements.

There are no rules for the colours of the atoms when drawing them.

They can be any colour.


One red oxygen atom and two white hydrogen atoms.
Water molecules

Water (H₂O) molecules contain one oxygen atom and two hydrogen atoms.

Here, the oxygen atom is red and the hydrogen atoms are white.

Ethanol molecule made of one red atom, two black atoms and white atoms. Red is oxygen, black is carbon and white is hydrogen.
Ethanol molecules

Ethanol is a much larger molecule made of three types of atoms. Its formula is CH₃CH₂OH.

Here, the oxygen atom is red, carbon is black and hydrogen is white.

A ball-and-stick model of methane

Molecules can be drawn in different ways. In this diagram of methane (CH₄) the chemical bond between atoms is shown as a stick.

One molecule of the compound methane is made from one carbon atom and four hydrogen atoms.

These are carbon dioxide molecules. Can you work out what elements the different coloured circles are?

The black circles are carbon atoms and the red ones are oxygen. Carbon dioxide molecules contain one carbon atom and two oxygen atoms. The formula for carbon dioxide is written as CO₂.

Single element molecules

  • Some molecules are made from one type of atom. This means the molecule is an element.
  • Oxygen is a non-metal element and is found naturally as a molecule. Each one is made up of two oxygen atoms that are strongly bonded together.
  • At room temperature oxygen is a gas.
Three pairs of red circles attached together.
An oxygen molecule

An oxygen molecule always exists as a pair of atoms.

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