When two atoms of the same element have different numbers of neutrons and therefore different atomic masses, what are they?
a) isotopes
b) isomers
c) molecules

All atomic elements consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons. When two atoms of the same element have different numbers of neutrons and therefore a different atomic mass they are known as isotopes of the given element. An example of an isotope is deuterium, which is an isotope of hydrogen. 

The third choice .. (:

This would be canberra.