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Chemistry Paper 1 / Atomic structure and the periodic table
Lesson 1.1 · AQA Combined Science

Atoms, elements
and compounds

01 · Start small

What is an atom?

An atom is the smallest part of an element that can exist.

All substances are made of atoms. In our particle diagrams, each circle represents one atom.

Atoms are much too small to see with your eyes.

One helium atom. He is its chemical symbol.
02 · One type of atom

What is an element?

An element contains only one type of atom.

Every element has its own chemical symbol. The first letter is a capital. A second letter is lowercase.

HHydrogenOOxygenHeHeliumNaSodium
Helium is an element: all these atoms are the same type.
03 · Different atoms joined

What is a compound?

A compound contains two or more different elements chemically joined together.

Water is a compound made from hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom.

Its formula is H₂O.

Three water molecules. Each has the same combination of atoms.
H = hydrogenO = oxygen
04 · Look at what is joined

What is a molecule?

A molecule is a group of two or more atoms chemically bonded together.

The atoms can be from the same element or from different elements.

Oxygen: O₂

Two oxygen atoms joined together.

A molecule of an element.
There is only one type of atom.

Water: H₂O

Hydrogen and oxygen atoms joined together.

A molecule of a compound.
There are two different types of atom.
Joined atoms do not always mean a compound. O₂ is still an element because every atom is oxygen.

Water and carbon dioxide exist as molecules. Some other compounds have giant structures instead of separate molecules. You will meet these in the bonding topic.

These are models: the colours help distinguish elements, and lines show bonds. They are not photographs or diagrams to scale.

05 · Different substances together

What is a mixture?

A mixture contains two or more substances that are not chemically joined to each other.

A mixture can contain elements, compounds or both. The substances keep their own properties and can be separated by physical methods.

Example: air is a mixture of gases, including nitrogen and oxygen. These gases are not chemically joined to each other.

Look for different types of separate particle or different types of molecule in the same sample.

Helium atoms mixed with water molecules. The helium is not bonded to the water.

Remember these four ideas

Atom: one particle of an element.

Element: only one type of atom.

Compound: different elements chemically joined.

Mixture: different substances together, not chemically joined to each other.