Atomic number: it is the number of protons in an atomic nucleus.
Mass number: it is the number of protons+neutrons (or number of nucleons) in an atomic nucleus.
Since they represent a number of subatomic particles, atomic number and mass number are always whole numbers.
Relative atomic mass: the average mass of an atom of an element in atomic mass unit.
The relative atomic mass is most often not a whole number since it represents the mean value of the masses of the various stable isotopes of that element, weighed by their relative abundance.
The following example helps illustrate these differences:
The relative atomic mass of element magnesium is 24.305 u. This number is the average mass of a magnesium atom, although not a single magnesium atom has a mass of exactly 24.305 u. All magnesium atoms have an atomic number of 12 but varying number of neutrons, therefore varying mass number. The element magnesium exist naturally as a mixture of 3 isotopes, corresponding the mass numbers 24, 25 and 26.
This can be compared to the birth rate per woman in Australia: although a woman has in average 1.92 children, each woman can only have a whole number of children (0, 1, 2, 3, ...).
Mass number: it is the number of protons+neutrons (or number of nucleons) in an atomic nucleus.
Since they represent a number of subatomic particles, atomic number and mass number are always whole numbers.
Relative atomic mass: the average mass of an atom of an element in atomic mass unit.
The relative atomic mass is most often not a whole number since it represents the mean value of the masses of the various stable isotopes of that element, weighed by their relative abundance.
The following example helps illustrate these differences:
The relative atomic mass of element magnesium is 24.305 u. This number is the average mass of a magnesium atom, although not a single magnesium atom has a mass of exactly 24.305 u. All magnesium atoms have an atomic number of 12 but varying number of neutrons, therefore varying mass number. The element magnesium exist naturally as a mixture of 3 isotopes, corresponding the mass numbers 24, 25 and 26.
This can be compared to the birth rate per woman in Australia: although a woman has in average 1.92 children, each woman can only have a whole number of children (0, 1, 2, 3, ...).