atom is the smallest component of an elementary substance, i.e. of a substance which is not a compound of several elementary substances. An atom consists of an atomic nucleus and an atomic shell. The nucleus comprises several protons and neutrons (in case of hydrogen, of one proton only), and the atomic shell several electrons (in case of hydrogen, one electron only). The number of protons is always identical with the number of electrons in neutral state. If the numbers differ, the particle is not an atom but an electrically charged ion .