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Atoms and radioactivity


Gases, liquids and solids are composed of atoms. It was once thought that atoms could not be split: the word atom comes from átomos, an ancient Greek word meaning "indivisible". In fact, atoms consist of even smaller particles: protons and neutrons (nucleons) in the nucleus and electrons in the shell.

The protons in the nucleus have a positive electrical charge, whereas the neutrons are neutral. This means that the nucleus is positively charged. Negatively charged electrons are arranged around the positively charged nucleus in the electron shell. If the number of electrons in the shell is equal to the number of protons in the nucleus, the atom is electrically neutral; if an atom gains or loses one or more electrons, it becomes an ion.

The electron shell may be more than 100,000 times larger in diameter than the nucleus. However, as the masses of the proton and the neutron are almost 2,000 times greater than the mass of the electron, the nucleus makes up almost all the atomic mass.

  • Diameter of the nucleus: 10-15 m
  • Diameter of the atom: 10-10 m
  • Mass of the electron: 10-30 kg
  • Mass of the proton/neutron: 10-27 kg

Chemical elements, nuclides and isotopes

All atoms of a chemical element have the same number of (positively charged) protons and therefore the same charge of the nucleus. The number of protons in the nucleus is used to arrange the elements systematically in the periodic table. However, the number of neutrons in the atomic nuclei, and hence the atomic mass, may vary.

A type of atom whose nuclei have specific numbers of protons and neutrons is called a nuclide. Isotopes are variants of a particular chemical element which have the same number of protons in each atomic nucleus but differ in neutron number. The word isotope is formed from the Greek, meaning "the same place", indicating that different isotopes of a single element occupy the same position on the periodic table. A chemical element may thus have several isotopes, which differ in neutron number.

For example, hydrogen (H) has three naturally occurring isotopes. The most common hydrogen isotope, protium, consists of just one proton in the nucleus. The nucleus of deuterium (also known as heavy hydrogen) contains one proton and one neutron, while the nucleus of tritium (super-heavy hydrogen) contains one proton and two neutrons.


Some nuclides have an atomic nucleus that decays spontaneously without any external influence. Known as radionuclides, they transform themselves into other nuclei.

Ionising radiation is emitted during this process. This is known as radioactivity.

Some radionuclides – such as tritium, a radioactive isotope of hydrogen – occur naturally. Radionuclides are also artificially produced in nuclear reactors.


Each radionuclide has a half-life. This is the length of time it takes for half of the radioactive atoms of a specific radionuclide to decay. It is impossible to predict when an individual atom will decay. Half-life is based on large numbers of nuclei and is therefore merely a statistically anticipated value.