Although matter is composed of atoms, for the purposes of describing it in a chemical sense it is more conveniently represented in terms of elements. Elements are pure substances that cannot be decomposed or resolved into simpler substances by chemical means. The simplest element is hydrogen, which consists primarily of atoms that have one proton in their nuclei and one extra-nuclear electron, although some of its atoms also have a neutron in the nucleus. The particles within the nucleus, the protons and neutrons, are collectively called nucleons and the total number of nucleons constitutes the atom’s mass number (A). The next element, helium, has atoms with two protons, two extra-nuclear electrons, and usually two neutrons, although some atoms have only one neutron. Such atoms of an element having the same number of protons (Z) in their nuclei but different numbers of neutrons (N) are called isotopes of that element, and all have essentially the same chemical properties.