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An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Isotopes are atoms that have the same number of protons, but different numbers of neutrons.
The nuclei of some isotopes are unstable. They emit radiation and break down to form smaller nuclei.
An early modelmodel: a geographical model demonstrates how things may happen in a particular place or situation. of the atom was the plum pudding model. It was disproved by Rutherford’s scattering experiment and replaced by the nuclear model.
Structure of the atom
You should be able to describe the nuclear model of the atom.
The nuclear model
The structure of the atom
Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The electrons are arranged in energy levels, or shells, around the nucleus.
The table shows the properties of these three sub-atomic particles.
Properties of sub-atomic particles
| particle | relative mass | relative charge |
|---|---|---|
| proton | 1 | +1 |
| neutron | 1 | 0 |
| electron | almost zero | –1 |
The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:
- if an atom loses one or more electrons, it becomes a positively charged ion
- if an atom gains one or more electrons, it becomes a negatively charged ion
Check that you remember and understand the things you need to know about the atom for your Chemistry examination questions. Go to this section in the BBC Bitesize AQA Additional Science.
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