By Kevin Beck
Updated Aug 30, 2022
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The atom, derived from the Greek word meaning "that which cannot be divided," is the fundamental unit of all matter. It is composed of subatomic particles—protons, neutrons, and electrons. The protons and neutrons reside in the nucleus, accounting for almost all of the atom’s mass, while electrons occupy orbitals at the periphery.
Protons are the defining particles of an element. The number of protons (atomic number Z) uniquely identifies each element. Hydrogen has one proton (Z = 1), the lightest element, whereas uranium has 92 protons (Z = 92), the heaviest naturally occurring element. Each proton has a mass of 1.00728 atomic mass units (amu) and carries a charge of +1. A nucleus containing only a single proton—such as hydrogen’s—still qualifies as an atom.
Neutrons, nearly the same size as protons, have a mass of 1.00867 amu and carry no electric charge. The number of neutrons in a nucleus varies, typically exceeding the proton count, especially for heavier elements. For example, a hydrogen atom has one proton and no neutrons; helium has two protons and two neutrons; tin has 50 protons and 69 neutrons; uranium has 92 protons and 146 neutrons. The sum of protons and neutrons gives the mass number M. Consequently, the neutron count equals M − Z. Changing the neutron number produces isotopes—atoms of the same element with different mass numbers, denoted as, e.g., ¹⁴C for carbon with eight neutrons.
Electrons are minuscule, negatively charged particles with a mass of 0.000549 amu. They orbit the nucleus in discrete energy levels—s, p, d, and f orbitals—rather than following simple planetary paths. In a neutral atom, the electron count equals the proton count (Z), ensuring overall charge neutrality. When the numbers differ, the atom becomes an ion, carrying a net positive or negative charge.
Atoms are exceedingly small and indivisible by ordinary means, analogous to the puzzle pieces that make up a jigsaw puzzle. Their structure—protons (positive), neutrons (neutral), and electrons (negative)—determines the element’s properties. Because electrons are practically massless compared to protons and neutrons, the atomic mass is effectively the sum of protons and neutrons.
