Three kinds of particles make up atoms: the electron, mentioned in the previous section of this article, the proton, and the neutron. They are commonly referred to as subatomic particles.

carries a single positive charge of electricity equal in magnitude to the negative charge of the electron.

carries no electric charge and is therefore electrically neutral. Its mass is approximately equal to that of the proton. A single, isolated neutron is an unstable particle—that is, it rapidly decays into other particles.

has a mass much less than that of the proton or neutron (about 1/1836 as great). The electron carries a single negative charge of electricity.

Protons and neutrons make up the nucleus of an atom. For this reason, they are sometimes referred to as nucleons. The nucleus of the simplest atom, ordinary hydrogen, consists of a single proton. In all other atoms, the nucleus is made up of both protons and neutrons.

As the result of many kinds of experiments with devices called particle accelerators, physicists believe that protons and neutrons are themselves composed of smaller particles. These particles are called quarks. Electrons, unlike protons and neutrons, are elementary particles—that is, they are not composed of smaller particles.

Each type of particle has corresponding to it an antiparticle of the same mass but of opposite magnetic and electrical properties. Physicists believe antiprotons, antineutrons, and positrons (antielectrons) can form atoms of antimatter.

By using devices such as particle accelerators, bubble chambers, and mass spectrographs, physicists have obtained precise measurements of particle masses.

The mass of a proton, neutron, or electron is often expressed in terms of atomic weight, for which the carbon 12 atom (a carbon atom that has 6 protons and 6 neutrons) serves as the standard. The carbon 12 atom is assigned a mass of 12. A free proton (one that is not bound with one or more other particles in an atomic nucleus) has a mass of 1.0072766; a free neutron, a mass of 1.0086654; and an electron, a mass of 0.000548597. The actual mass of the electron is about 9.1 x 10-31 kilogram, which means that it would take 1,100,000,000,000,000,000,000,000,000,000 electrons to weigh one kilogram.

When protons and neutrons combine to form a nucleus, their combined mass is slightly less than the total sum of their individual masses.