A much debated subject is that of radioactivity. The danger that radioactivity can cause to the health of humans is a very large cause of concern. Hermann Muller was awarded the Nobel prize in 1946 for identifying the ability of radiation to mutate human genes and potentially cause cancer. Put more scientifically, the radiation emitted has the ability to cause ionisation of molecules in living cells which means the electrons leave their parent nucleus. However, the environment has always produced this radiation, known as background radiation. This does not necessarily mean that we are all at risk from the radiation. Our cells rapidly repair the damage caused by the low dose of radiation from the environment. Now, in addition to this background radiation, there are also man-made sources of radiation. The theory of radioactivity and radiation was then put to use in industry, medicine and as power sources.
An atom can be classified by the number of protons, neutrons and electrons. The term radioactivity, refers to the particles which are emitted from a particle as a result of nuclear instability. The three most common types of radiation are called alpha, beta and gamma rays. An alpha particle has two neutrons and two protons and no electrons. Therefore they are positively charged. Beta particles can be thought of as pseudo-electrons. In other words, a neutron emitting a beta particle would then turn into a proton. Therefore, they are negatively charged. Lastly, gamma rays are similar to rays of light at much higher energies. Excited electrons decaying to lower energy states are one of the main causes of gamma rays. This radiation was first discovered by Henri Becquerel in 1896 and then later a theory of this radiation was put forward by Marie Curie in 1899.
Put simply, this radiation occurs when an atom is not in its 'neutral' state. An atom will decay through alpha particles if there is too much mass. If there are too many neutrons, then a beta particle will be emitted. Likewise, if there is too much energy then a gamma ray will be emitted. However, these decays are described more accurately by looking to the forces of nature. The atom is held together by the strong force. However, this force only covers a very short distance (~atomic dimensions). Therefore, if the atom is too large, then it is ironically this strong force that causes a decay (and often gamma rays as a bi-product) . However, the beta decays are not caused by this strong force. It was Enrico Fermi that found that the weak force is responsible for this decay.
One can measure how radioactive these particles are by studying the number of disintegrations per second of the nuclide. This is measured in Becquerels (Bq), where 1Bq is equal to one disintegration per second. This activity decays exponentially with time. The half life of a substance is the time taken for the activity to drop by half its initial activity. The rate at which each nuclide decays is important to know for a number of reasons including the measure of how old a substance is or for health reasons.