The structure of an atom involves a central nucleus containing protons (positive charge) and neutrons (no charge). Electrons (negative charge) orbit the nucleus in specific energy levels or shells. The atomic number* (Z) of an element is equal to the number of protons in the nucleus, which defines the element. The mass number* (A) is the total number of protons and neutrons in the nucleus. Isotopes* are atoms of the same element that have different numbers of neutrons, and therefore different mass numbers. Electrons are arranged in electron shells and subshells* (s, p, d, f) according to their energy levels. The electron configuration describes this arrangement.