Glossary
atomic number
An element's atomic number is the number of protons in its nucleus, and it is what decides which element the atom is.
The atomic number of an element is the count of protons in one atom of it. That number is the element's identity: change the protons and the element changes, so every atom with six protons is carbon and every atom with eight is oxygen. In a neutral atom the number of electrons equals the number of protons, which is why the atomic number also fixes the electron count and therefore the arrangement in the shells. The number sits above the element's symbol on the periodic table, and the elements are ordered by it — hydrogen 1, helium 2, lithium 3 — which is why the table's shape is a pattern rather than a list. It is not the mass. The mass number is protons plus neutrons and differs between isotopes of one element, so a question that asks why two atoms of carbon weigh differently is asking about neutrons, not about the atomic number.
Taught in
Related terms
- isotope — Isotopes are atoms of the same element with different numbers of neutrons, so they share chemistry but differ in mass.
- ion — An ion is an atom or group of atoms with a charge, made by losing electrons (positive cation) or gaining them (negative anion).
- mole — A mole is 6.022 × 10²³ particles of a substance — the chemist's counting unit, chosen so that one mole of a compound weighs its formula mass in grams: 18 g of water is one mole of it.
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)