The atom is the smallest unit of a given chemical element that maintains an identity of its own and which cannot be split by means of chemical processes. However, the development of nuclear physics in the course of the twentieth century showed that that the atom can be split into smaller particles: electrons, protons and neutrons.
Electrons are negatively-charged and small-mass particles that orbit the atomic nucleus clustered into different energy levels. The number of protons in an atom is equal to the number of electrons.
The atomic nucleus is very dense and is composed of protons and neutrons. Protons are positively-charged particles with a mass 1837 times that of an electron.
Neutrons are electrically neutral particles with a slightly greater mass than a proton. The nuclear forces that keep these combinations of protons and neutrons together are sufficiently strong to overcome the electrical repulsion force among the positively-charged protons. The nucleus accounts for 99.9% of the atom mass, although it only occupies 0.0001% of its entire volume.
The mass of the atomic nucleus is less than the sum of the masses of the individual particles that conform the nucleus. This reduction in mass represents the nuclear binding energy, through the conversion of mass into energy, as expressed by Einstein’s famous equation: E=mc2
Isotopes and radioisotopes
An unstable isotope has an excess of protons or neutrons in the nucleus, or an excess of energy in the nucleus. In order to reach stability, the parent nuclide disintegrates into one or more offspring nuclides, emitting particles and energy. Atoms with this nuclear imbalance are called radioisotopes, and the atomic transmutation process is referred to as radioactive decay or disintegration.
When the offspring isotope is also unstable, it transforms into another one, and so on successively, generating what is called a radioactive family, until it finally produces a stable isotope.
Radionuclides are unstable atomic nuclei that decay spontaneously into another more stable nucleus through:
- The emission of particles: electrons (β- particles), positrons (β+ particles), and α particles or neutrons.
- The emission of electromagnetic radiation (X-rays and gamma-rays [γ]).
- Isomeric transition: a nucleus in an excited energy state releases energy and returns to its stable form.
- Electron capture: the nucleus captures an electron from an inner atomic orbital, converting a proton into a neutron. Its place is then occupied by another electron from an outer orbital, emitting characteristic X-rays.