Atomic number, mass & isotopes

An element is defined by its atomic number Z: the number of protons in its nucleus. Hydrogen has Z = 1; carbon has Z = 6. Changing the proton count changes the element.

The mass number A belongs to an isotope. It is the number of protons plus neutrons. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons.

Standard atomic weight describes the relative atomic masses and isotopic composition of normal terrestrial material. It is usually not an integer. It is dimensionless and is not the mass number of one isotope.

Practice: read an isotope

Open magnesium in the table: its atomic number is 12. For a neutral magnesium-26 atom, the proton count is 12, the neutron count is 26 − 12 = 14, and the electron count is 12. If it becomes Mg²⁺, it has 10 electrons; the nucleus has not changed

Check your understanding

Does rounding the atomic weight on an element card give the neutron count?

Try it in the periodic table

Scientific reference: OpenStax Chemistry 2e

Return to lessons