Mass number can be near-integer, but at the same time not corresponding to the mass of any natural isotope. For example, bromine has only two stable isotopes, Br and Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol), even though the isotope Br with such mass is unstable. Mass number The mass number (symbol "A", from the German word "Atomgewicht" (atomic weight), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It determines the atomic
Neutron number Neutron number The neutron number, symbol N, is the number of neutrons in a nuclide. Atomic number (proton number) plus neutron number equals mass number: Z+N=A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N - Z = A - 2Z. Neutron number is rarely written explicitly in nuclide symbol notation, but appears as a subscript to the right of the element symbol. In order of increasing explicitness and decreasing frequency of usage: Nuclides that have the same neutron number but a different proton number are called isotones. This word