Chemistry different mass numbers are known as isotopes. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, but atoms of carbon may have mass numbers of 12 or 13. The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number. The periodic table is arranged in groups, or columns, and periods, or rows. The periodic table is useful in identifying periodic trends. A "compound" is a pure chemical substance composed of more than one element. The properties of a compound bear little similarity to those of
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