Atomic mass number in the context of "Baryon number"

The atomic number or nuclear charge number (symbol Z) of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (n_p) or the number of protons found in the nucleus of every atom of that element. The atomic number can be used to uniquely identify ordinary chemical elements. In an ordinary uncharged atom, the atomic number is also equal to the number of electrons.

For an ordinary atom which contains protons, neutrons and electrons, the sum of the atomic number Z and the neutron number N gives the atom's atomic mass number A. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of the nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in daltons (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.

Natural gallium (₃₁Ga) consists of a mixture of two stable isotopes: gallium-69 and gallium-71. Synthetic radioisotopes are known with atomic masses ranging from 60 to 89, along with seven nuclear isomers. Most of the isotopes with atomic mass numbers below 69 decay by electron capture and positron emission to isotopes of zinc, while most of the isotopes with masses above 71 beta decay to isotopes of germanium.

The medically important radioisotopes are gallium-67 and gallium-68, used for imaging, and further described below.