Atoms are made up of three tiny particles called protons, neutrons and electrons.

Atoms have no overall electrical charge because the number of positive protons and negative electrons is equal, so they cancel each other out.

The number of protons in the nucleus of an atom determines what element it is. Atoms of the same element have the same number of protons.

At the centre of an atom is the nucleus, which is very small.

It has a positive charge because it contains protons.

It is very dense because it contains protons and neutrons which make up most of the mass of an atom.

This model of the atom was developed by Rutherford and Marsden. They directed a beam of alpha particles at a very thin piece of metal. Most particles went straight through the metal atoms, suggesting that most of the atom must be empty space.

A few particles were scattered at different angles.

Alpha particles carry a positive charge, so Rutherford concluded that they must be repelled by a small area of great positive charge. He called this the nucleus.

This model replaced an earlier ‘plum pudding’ model. The earlier model described an atom as having negative charges embedded in a positive dough. Rutherford and Marsden’s scattering experiment disproved this theory.

'Plum pudding' model of an atom

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Isotopes

Atoms of the same element which have different numbers of neutrons are called isotopes.

Carbon exists in three forms:

Some isotopes are radioactive. These are radioisotopes or radionuclides. Their nuclei are unstable and can spontaneously split up, emitting radiation and producing a new atom, with a different number of protons. This is called radioactive decay and is a random process – you don’t know which atoms may suddenly undergo a nuclear change.

Examples of isotopes

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Radioactive materials gradually decay and form new atoms. The time it takes for half the atoms in a sample to decay is called atom's half-life, so older samples emit less radiation. This idea is used to work out how old plant, animal and rock specimens are.

Carbon-14 is used to date things that were once living.

Uranium isotopes have very long half-lives and decay via a series of short-lived radioisotopes to produce stable isotopes of lead. The relative amounts of uranium and lead in a sample of igneous rack can be used to date the rock.

Potassium-40 decays to form argon. The proportions of radioactive potassium and stable argon can be used to date igneous rocks from which the gaseous argon has been unable to escape.

For example, the half-life of uranium-238 is 4 500 million years.

If a rock sample contains three times as many lead atoms as uranium atoms, and we assume there was no lead in the rock when it was formed, we can calculate its age.

After 4 500 million years, half the atoms would be uranium-238 and half would be lead.

After another 4 500 million years, half of these uranium atoms would have decayed to lead, producing a rock containing three times as much lead as uranium.

The age of the rock would therefore be 9 000 million years.

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Inside a nuclear reactor, unstable atoms with large nuclei are bombarded with neutrons. This causes the nuclei of the atoms to split into two smaller nuclei. This is called nuclear fission.

Further neutrons are released which can hit other atoms causing further nuclear fission. This is called a chain reaction.

The new atoms formed are also radioactive. Many of these have very long half-lives, so they will be dangerous for many years.

Large amounts of energy are released during radioactive decay or nuclear fission. This energy can be transferred into electrical energy in a nuclear power station, or used destructively as with a nuclear bomb.

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There are several types of nuclear radiation.

Alpha particles are helium nuclei, so they have two protons and two neutrons, but no electrons. They carry a 2+ charge.

Beta particles are high energy electrons emitted from the nucleus of an atom. They carry a negative charge.

Gamma rays are very short wavelength electromagnetic waves which travel at the speed of light. They do not have a charge.

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