mass number (A)
The number of protons and neutrons in one atom of a particular isotope of an element. The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of oxygen is oxygen-16, or 16O, which has 8 protons and 8 neutrons. Mass number is also known as the nucleon number since protons and neutrons are collectively known as nucleons. Although all atoms of a given element have the same atomic number, their mass number can differ depending on the number of neutrons they contain.
Related entries atomic mass
atomic mass unit
Related categories• ATOMIC AND NUCLEAR PHYSICS
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