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Atomic mass unit
1 atomic mass unit (1 u) is defined as one-twelfth of the mass of a carbon-12 atom.
Atomic number ($Z$)
It defines which element the atom is by indicating the number of protons in the nucleus.
Isotopes
Atoms of the same element (same protons/atomic number) that have different numbers of neutrons, resulting in different mass numbers
Mass number ($A$)
The total number of protons and neutrons in the nucleus.
Relative atomic mass $A_r$
The weighted average mass of all the atoms (isotopes) of that element in a sample, relative to 1/12 of the mass of a carbon-12 atom.