Isotopes and Ions


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 types of 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, 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, which means it has 10 neutrons.

Carbon occurs naturally in three isotopes:

  • Carbon-12
  • Carbon-13
  • Carbon-14

A set of diagrams illustrating the atomic structure of three isotopes of carbon. The first, "CARBON-12", shows an atomic nucleus with 6 neutrons and 6 protons, surrounded by orbiting electrons, totalling 12 particles in the nucleus. The second, "CARBON-13", displays a nucleus with 7 neutrons and 6 protons, amounting to 13 particles in the nucleus. The last, "CARBON-14", represents a nucleus containing 8 neutrons and 6 protons, equalling 14 particles in the nucleus. Each diagram is accompanied by a corresponding carbon symbol.

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, atoms can lose or gain electrons, giving them a positive or negative charge. At this point, we call the atom an ion.

  • If an atom loses electrons, it becomes a positively charged ion (cation)
  • If an atom gains electrons, it becomes a negatively charged ion (anion)

A diagram depicting the formation of ions from a neutral atom. At the top, a "Neutral Atom" is shown in yellow. From this atom, two arrows emerge. The pink arrow, labelled "Loss of Electron(s) e", points to a red sphere named "CATION" with a '(+)' symbol, indicating a positively charged ion. The blue arrow, labelled "Gain of Electron(s) e", points to a purple sphere named "ANION" with a '(-)' symbol, denoting a negatively charged ion. Below are three atomic representations: the leftmost depicts a neutral atom with 8 protons and 8 electrons, carrying a "0 Neutral Charge". The centre one illustrates an atom gaining an electron, with 8 protons and 9 electrons, bearing a "-1 Negative Charge". The rightmost showcases an atom losing an electron, having 8 protons and 7 electrons, with a "+1 Positive Charge".

When an atom’s electron absorbs too much energy, it can be ejected from the atom. This results in the atom having more protons than electrons, and therefore becoming a positive ion.

Ionisation is a process by which neutral atoms or molecules are converted to electrically charged atoms or molecules. Therefore, ionising radiation is radiation that can cause ionisation in the media it passes through. It does this by knocking electrons off atoms, forming free electrons.

A graphic representation of an atom undergoing ionisation. The atom, labelled "Atom", consists of a central cluster of protons (depicted as 'p' in blue) and neutrons (depicted as 'n' in purple). Orbiting this nucleus are electrons (marked as 'e' in orange) on pink pathways. From the left, an arrow labelled "Energy" is shown pointing towards the atom. As a result of this energy, one electron is being ejected from its orbit, highlighted by a sunburst effect. This dislodged electron is moving towards the right, labelled "Free electron", following a green arrow indicating its trajectory away from the atom. At the bottom right corner, there are three labelled legends: a blue 'p' for proton, a purple 'n' for neutron, and an orange 'e' for electron.

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