Mass number

Mass number is the total number of protons and neutrons in an atomic nucleus. It distinguishes isotopes of the same chemical element.

Proton: A positively charged subatomic particle found in atomic nuclei. Each proton contributes one unit to an atom’s mass number.
Neutron number: The number of neutrons in an atomic nucleus, commonly denoted by N. Subtracting atomic number from mass number gives neutron number.
Carbon-14: A radioactive isotope of carbon with six protons and eight neutrons. Its mass number gives the total nucleons used in radiocarbon dating.
Relative atomic mass: The weighted mean mass of an element’s atoms relative to one twelfth the mass of carbon-12. It averages isotopic masses, unlike the integer count called mass number.
Neutron: An electrically neutral subatomic particle found in most atomic nuclei. Each neutron contributes one unit to an atom’s mass number.
Nuclide symbol: A notation placing mass number above and atomic number below an element symbol. It encodes the proton and neutron totals in a compact form.
Radiocarbon dating: A dating method that estimates the age of once-living material from its carbon-14 content. The method tracks an isotope identified by its mass number.
Isotopic mass: The mass of a particular isotope, usually expressed in atomic mass units. An isotope’s measured mass is not generally equal to its mass number.
Atomic number: The number of protons in an atomic nucleus, which determines the chemical element. Together with mass number, it specifies a nuclide’s neutron count.
Isotopic notation: A way of writing an isotope’s element symbol together with its mass number. It shows how mass number appears in names such as carbon-14.

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