Nuclear charge number is the number of protons in the nucleus of an atom.

See also Mass Number, Ordinal Number.

The nucleus is the positively charged center of an atom and contains most of the mass. Except for 1H, the atom core consists of a combination of the subatomic particles protons and neutrons.

Characterization of atoms by their nuclear properties, as the number of protons and the number of neutrons. The different nuclides of an element are its isotopes (equal proton number, but different neutron numbers). Isomers of this particular nuclide are equal in the proton and mass numbers, but differ in their energy content. Unstable nuclides which are radioactive are called radionuclides.

See also Isotope, Isomer and Decay.

In the periodic systems of elements the ordinal number sorts elements by their nuclear charge (their protons).

See also Nucleus, Proton and Mass Number.

The process of pair production is the creation of a particle and its antiparticle. If a photon with a minimum energy of the rest mass of an electron and a positron hits a nucleus an electron and a positron is emitted. The surplus energy appears as the kinetic energy of the two created particles. Conservation of energy and momentum makes pair production possible or not.