A bit of history

In 1805 the English Chemist John Dalton published his atomic theory that said:

• Everything is made up from tiny particles called atoms
• Atoms are tiny hard spheres (= balls) that cannot be broken down into smaller parts
• Atoms in one element are all identical

This helped to explain many properties of materials. However, later experiments showed that atoms contained even smaller particles. In 1897 another British scientist, JJ Thomson, discovered the electron. The nucleus which makes up the middle of an atom was discovered by Ernest Rutherford in 1913.

The Structure of the atom

Today we know that atoms are made from three subatomic particles: proton, neutron and electron.

Protons and neutrons are found in the centre of the atom which is called the nucleus. Both have a mass of 1. Protons have a positive (+) charge and neutrons are neutral (= no charge).

Electrons have a negative (-) charge and have almost no mass at all. They are found on the electron shells on the outside of the atom, circling the nucleus.

What you need to remember

• Atoms are made from protons, neutrons and electrons called subatomic particles
• Protons: found in nucleus, positive (+) charge and a mass of 1
• Neutrons: found in nucleus, neutral (no charge) and a mass of 1
• Electrons: found on electron shells, negative (-) charge and almost no mass

Things to do

1. Name the scientist who first introduced atomic theory.
2. Name the scientists who discovered the electron and the nucleus.
3. State the names of the three subatomic particles as well as their masses and charges.
4. State where in the atom protons and neutrons are found.
5. State where in the atom electrons are found.
6. Copy and label the image of the atom. Words: electron, proton, neutron, shell
7. Describe in your own words what an atom looks like. Include information about the charges and masses of the subatomic particles.