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
Mirror nuclei Mirror nuclei Mirror nuclei are nuclei where the number of protons of element one (Z) equals the number of neutrons of element two (N) and the number of protons of element two (Z) equals the number of neutrons in element one (N), such that the mass number is the same (A = N + Z = N + Z). As that Z = N and Z = N, A = N + N = Z + Z. By making the substitution Z = Z and Z = Z − 1, the mass number can be rewritten in the form 2Z