Isotopes are atoms of the same element, with the same number of protons but a different number of neutrons. All elements have isotopes, but only some are stable.
The unstable isotopes decay into other elements to become more stable. We call this process radioactive decay. When this happens, radiation is given out. The ionising radiation that you need to know about are alpha, beta and gamma.
The atomic number identifies an element. For example, if an element has 8 protons, it will always be oxygen. However, an element can have different mass numbers. Or in other words, it can have different numbers of neutrons. For example, oxygen has a mass number of 16, meaning it has 8 neutrons. But it can also have a mass number of 18, having 10 neutrons.
Carbon occurs naturally in three isotopes:
So these three atoms would all be classed as isotopes because they all have the same number of protons but a different number of neutrons.
Stable atoms are neutral, having the same number of protons in the nucleus and electrons orbiting the nucleus. However, electrons can lose or gain electrons, giving the atom a positive or negative charge. At this point, we call the atom an ion.
When an electron absorbs too much energy, it can leave the atom. This means that the atom will have more protons than electrons, becoming a positive ion.
Ionisation is a process by which neutral atoms or molecules are converted to electrically charged atoms or molecules. So ionising radiation is radiation that can cause ionisation in the mediums it passes through. It does this by knocking electrons off atoms, forming free electrons.