Mass Number
Definition
The total count of protons plus neutrons in an atom's nucleus. It tells you which isotope of an element you have.
Symbol: ·
Technical definition
The mass number (A) is the total number of nucleons, protons plus neutrons, in an atomic nucleus. Different mass numbers of the same element are isotopes, and the mass number is approximately (but not exactly) the atom's mass in atomic mass units.
Example
Carbon-14 has a mass number of 14 (6 protons + 8 neutrons), while the common carbon-12 has a mass number of 12.
Common confusion
Mass number is a simple count, not a measured mass. The atomic weight on the periodic table is a weighted average of isotope masses and is usually not a whole number.