The mass (nucleon) number for atoms of a particular element may differ if the element exists as
isotopes.
Atoms of isotopes have the same number of protons but different numbers of neutrons.
Atoms of a particular
isotope are shown by the
element symbol preceded by the
mass number as a superscript.
Hydrogen exists as two naturally occurring isotopes: 1H and 2H.
Because many elements exist in nature as a mixture of different isotopes, the
relative mass (Ar) of an atom in a naturally-occurring sample of an element is a
weighted average of the isotopes.
Because the natural abundance of 1H is much much higher (99.98%) than 2H (0.02%), the relative atomic mass of an average hydrogen atom is only slightly higher than 1 (1.0079).
The existence of isotopes make a much more significant difference to the molar mass of some other elements. Consider copper that exists as two isotopes copper-63 (69%) and copper-65 (31%). The average mass of copper atoms in a sample from nature is 63.5.
It is important to remember that
isotopes of the same atom have the
same chemical properties; however they may react at a slightly different rate.