An isotope of an element that contains the same number of protons but a different number of protons.
For example:
C-12 and C-14 are the two isotopes of the Carbon Element. The 12 and 14 denote the mass numbers and are different because of the difference in ONLY the number of neutrons and NOT the number of protons i.e. C-12 and C-14 both have 6 protons. They have 6 and 8 neutrons respectively.
