Atoms of the same element that have different numbers of neutrons

If you learned about protons and neutrons the answer would be isotopes. isotopes are atoms that are the same element but have different numbers of neutrons. 

There is more of the lighter isotope, because the average is close to the lighter isotope. When doing the calculation for the average atomic mass, it considers the fraction of the isotopes available in a sample. Therefore samples of the element that have a values that do not deviate far from the sample are more abundant.