Atomic number:
It is the number of a chemical element in the periodic table, whereby the elements are arranged in order of increasing the number of protons in the nucleus. Accordingly, the number of protons, which is always equal to the number of electrons in a neutral atom, is also the atomic number.
Mass number:
It is the sum of the numbers of protons and neutrons present in the nucleus of an atom.
Isotope:
Each of 2 or more forms of the same element contains equal numbers of protons but different numbers of neutrons in their nucleus.
Relative atomic mass:
The weigh in grams of the number of atoms of the element in 12.00 g of carbon-12.
Or, we can say
The carbon-12 atom,
is the standard atom against which the masses of other atoms are compared. The relative atomic mass of an element is the average mass of its atoms, compared to 1/12th the mass of a carbon-12 atom. The relative atomic mass, Ar, of an element is calculated from:
The mass numbers of its isotopes
The abundance of the isotopes