Isotopes - atomic structure

Question 1

What difference does isotopes have to normal atoms?

Answer 1

Same number of protons but different number of neutrons.
So same atomic number but different mass numbers.

Question 2

If an element has a number of isotopes, what formula works out the relative atomic mass?

Answer 2

Relative atomic mass = (sum of (isotope abundance x isotope mass number) ) divided by sum of abundances of all the isotopes.