Mass number number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: . The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number ("Z") as a subscript to the left of the element symbol directly below the mass number:
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