ABSTRACT
The number of protons determines the identity of an element. For example, all atoms with six protons are carbon atoms. The specific version of carbon, the isotope, is determined by the number of neutrons, i.e. carbon-13 has six protons and seven neutrons. While the number of protons and neutrons is important to determine the element under consideration, the number of electrons dictates the stability and reactivity of an atom. Electrons occupy very specific energy levels, which chemists term “shells” and “subshells.” In this chapter, the pattern in electron configurations is investigated to understand what makes an atom stable, filled shells, and unstable, unfilled shells. The arrangement of elements in the periodic table reflects the valence, highest energy, electron configuration. More specifically, elements in the same group have the same number of valence electrons, which means they show similar chemical properties. The chapter concludes by considering the common charges that ions, charged atoms, take on in order to achieve electronic stability.
