3.1 Chemical elements and their properties

The chemical elements are uniquely distinguished by their atomic number, Z, which is the number of protons in an atom. According to the IUPAC terminology, the atom is the smallest part of an element with no net electric charge, which can enter into chemical combinations. Each neutral atom contains as many electrons as protons in its nucleus. Atoms of the same element with the same atomic number, but different number of neutrons are called isotopes. The isotopic abundance (isotope abundance or atom abundance) is the relative number of atoms of a particular isotope in a mixture of the isotopes of a chemical element, expressed as a fraction (%) of all the atoms of the element. The isotopic abundances of the elements vary depending on the source of materials. The natural isotopic abundance is the isotopic abundance in the element as found on Earth. Because of natural radioactivity, this quantity may not be well defined for some isotopes.

The mass number, A, of an isotope of a given element is defined by the sum of the number of protons, Z, and neutrons, N, in its nucleus. Elements have more than one isotope with varying numbers of neutrons. For example, there are two common isotopes of carbon, ¹²C and ¹³C, which have 6 and 7 neutrons, respectively. The atomic mass is primarily determined by the number of protons and neutrons...