Mass number 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 mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number ("Z") which denotes the
Atomic spacing Atomic spacing Atomic spacing refers to the distance between the nuclei of atoms in a material. This space is extremely large compared to the size of the atomic nucleus, and is related to the chemical bonds which bind atoms together. The spacing between atoms in most ordered solids is on the order of a few ångströms (a few tenths of a nanometer). In very low density gasses (for example, in outer space) the average distance between atoms can be as large as a meter. The atomic spacing of crystalline structures is usually determined by passing an electromagnetic wave of known