Isotopes are the atoms of same element; they have same atomic number but different mass numbers. Isotopes have different number of neutrons in their nucleus. If an element exists in two isotopes having atomic masses ‘a’ and ‘b’ in the ratio m : n, then average atomic mass will be
mxa+nxb/m+n
Different isotopes of same element have same position in the periodic table. The elements which have single isotope are called monoisotopic elements. Greater is the percentage composition of an isotope, more will be its abundance in nature.