Hos do you calculate a mass of 1 atom for a isotope and a element? examples would be appreciated.?

Answer 1

You divide the atomic mass of the isotope in grams
by avogadro's number 6.023 10^23
Example
The hydrogen atom has the most frequent isotope, with atomic mass of 1 (for the common isotope)

So, the mass of the atom is 1/6.02310^23 = 1.66 10^-24 g

For the less common isotope heavy hydrogen atomic mass =2

you found the mass of the atom 2/6.02310^23 = 3.32 10^-24 g

Answer 2

The molecular mass (Mr) of an element is determined by its nucleons. For example, Carbon-12 has 6 Protons and 6 Neutrons. When a sample contains two isotopes the equation below is applied:
Mr = (Mr(1) x %abundance + Mr(2) x %abundance) / 100

Where Mr(1) and Mr(2) are the molecular masses of each individual isotope, and %abundance is the percentage abundance of that isotope in the sample.

Read more: http://en.wikipedia.org/wiki/Isotope#Molecular_mass_of_isotopes