According to present-day theory, an atom consists of a nucleus[4] that is made up of protons and neutrons[5] and is surrounded by electrons. In each atom there is an equal number of protons (positively charged) in the nucleus and electrons (negatively charged) moving concentrically around the nucleus; since neutrons have no electrical charge and since protons and electrons cancel each other’s charges, the whole atom is electrically neutral, or uncharged. Each atom is identified by an atomic number and an atomic weight. The atomic number of an element (for example, carbon, nitrogen, oxygen) is determined by the number of protons, or positive charges, carried by the nucleus (or by the number of electrons surrounding the nucleus, which is the same). The atomic weight is the weight of an atom as compared with that of the atom of carbon, which is taken as a standard. The weight, or mass, of an atom is due chiefly to its protons and neutrons because the mass of its electrons is negligible. Isotopes Atoms of the same element, that is, atoms with the same number of protons and electrons, may vary slightly in mass because of having different numbers of neutrons. Since the chemical behavior of an element depends upon its electrons’ electrical charges, extra neutrons (which do not have an electrical charge) may affect the mass of an atom without disturbing its chemical properties. Atoms having the same atomic number but different atomic weights are called isotopes. For example, as shown in Figure 8, the isotope ¹H, or ordinary hydrogen, consists of a nucleus containing a proton (charge: +1; mass: 1) around which revolves an electron (charge: -1; mass: negligible); ²H, known as deuterium, contains an additional nuclear particle, a neutron (charge: 0; mass: 1); ³H, or tritium, contains two neutrons. Since the chemical behavior of an element depends upon the number of its electrons, these three atoms, although differing in weight, behave identically in chemical reactions. For convenience, the atomic weight is written as a superscript to the left of the element’s symbol. For instance ¹⁴C is the isotope of carbon with an atomic weight of 14 (ordinary carbon is the isotope with an atomic weight of 12, and it is written ¹²C). [Illustration: Figure 8 _Isotopes of hydrogen._] Hydrogen (¹H) Deuterium (²H) Tritium (³H)