What is radiation? 

Radioactivity is all around us

Everything around us is radioactive to a greater or lesser degree. Plants, water, rocks, even the air we breathe - everything contains radioactive substances. This has been so since the creation of the universe. Radiation even reaches us from outer space.

It's all about atoms and their nuclei

Radioactivity is a natural phenomenon. To understand it, we have to make a journey into the heart of matter. All matter is made up of atoms. We cannot see atoms: they are extremely small but complex building blocks.
Each atom consists of a nucleus around which particles called electrons circle. The nucleus is made up of particles called protons and neutrons. You could compare an atom to a mini-solar system: the sun is the nucleus and the planets are the electrons that orbit around it.
Figure 1: There are many different atoms, but they all consist of protons, neutrons and electrons (apart from hydrogen, which has no neutrons). Atoms differ according to the number of protons, neutrons and electrons they have.

Stable and unstable atomic nuclei

Some atomic nuclei are stable - they stay as they are. To be stable, the number of protons and neutrons in the nucleus must be balanced, though this does not mean the numbers must be equal. As nuclei are heavier this balance requires there to be more neutrons than protons.
In some atoms the balance is disturbed. There are too many protons compared to the number of neutrons, or too few of them, or even too many of both in some very heavy nuclei. The atomic nucleus is then unstable - it is said to be radioactive.

Unstable atoms release radiation energy

Sooner or later, each unstable atomic nucleus will spontaneously change to achieve a new and better balance. It will therefore release radiation energy and matter in the form of waves or particles.

Materials containing these kinds of atoms are called radioactive.

Figure 2: Radioactivity is a natural phenomenon through which unstable atomic nuclei achieve a better balance by emitting radiation.

Ionising radiation

The rays of the sun transfer energy in the form of heat. Radioactive rays also transfer energy. When these energetic rays pass through matter, they collide with atoms or molecules and transfer part of their energy to them: an electron is knocked off an atom, or an atom or a molecule absorbs an electron. In this way an electrically charged atom or molecule, an ion, comes into being. This phenomenon is called ionisation.
Ionising radiation is radiation that causes ionisation when it comes into contact with matter. This type of radiation is emitted by unstable atomic nuclei.
What is the difference between the rays of the sun and ionising radiation? The difference lies in the amount of energy that is transferred by the radiation. The rays of the sun transfer relatively little energy. Ionising radiation, on the other hand, transfers so much energy that it can cause changes in the matter into which it penetrates.