The number of protons in the nucleus, which determines the chemical properties of an element, is called the atomic number. Hence the nominal mass, based on the mass number, approximates the actual atomic mass. The nominal mass of an atom is not affected by the number of electrons, which are very light. The nominal mass of an atom of an element is measured by the sum of the protons and neutrons in it. The energy balance in the decay of a neutron is achieved by theĪnti-neutrino, a neutral particle that carries off surplus energy as the Protons are about 1,836 times heavier than electrons, and neutrons are about 1,838 times heavier than electrons. However, it is remarkable that neutrons, when they exist together with protons in the nucleus of atoms, are stable. Neutron => proton + electron + antineutrino The mass of an atom lies almost entirely in its nucleus since protons and neutrons are far heavier than electrons.įree neutrons are unstable particles which decay naturally into a proton and electron, with a half-life of about 12 minutes.

As a result, atoms of elements are normally electrically neutral. The number of electrons in an atom is normally equal to the number of protons in the nucleus. Electrons are electrically negative and have a charge equal in magnitude to that of a proton.

The nuclei of atoms are composed of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. The atoms of which every element of matter is composed have a nucleus at the center and electrons whirling about this nucleus that can be visualized as planets circling around a sun, though it is impossible to locate them precisely within the atom. Some of the terms used in this factsheet can be found in IEER’s on-line glossary. A basic background in nuclear physics for those who want to start at the beginning.

Nuclear fission uranium 235