Current Practice GROWING POINTS Radioactive Isotopes in Clinical Medicine

There are four fundamental atomic particles-the proton, the neutron, the electron, and the positron. The proton has mass and a unit positive electrical charge. The neutron has mass and no charge. The electron has very small mass compared with the proton and neutron, and has unit negative electrical charge. The positron is the positively charged counterpart of the electron, having equal mass and unit positive charge. The atom is composed of a positively charged central nucleus surrounded by a number of electrons. The nucleus is made up of protons and neutrons. The number of protons determines the positive charge and is known as the atomic number. The number of protons plus neutrons is the atomic mass number and determines the atomic weight. The number of orbital electrons surrounding the nucleus of an atom is equal to the number of protons in the nucleus. Hence the atom is electrically neutral. Since chemical reactions concern only interactions between the outer electrons, the chemical properties of an atom are determined by the number of orbital electrons-that is, they are characterized by the atomic number.