Some carbon atoms have a greater mass than others - Leaving Cert Chemistry - Question (a) - 2021

Carbon atoms can have different masses due to the presence of isotopes. An isotope is a variant of a chemical element that has the same number of protons but a different number of neutrons in the nucleus.

For carbon, the most common isotope is Carbon-12, which has 6 protons and 6 neutrons, giving it a mass of approximately 12 atomic mass units (amu). However, there are also heavier isotopes, such as Carbon-13, which contains 6 protons and 7 neutrons, and has a mass of about 13 amu, and Carbon-14, which includes 6 protons and 8 neutrons, with a mass of approximately 14 amu.

The variation in neutron numbers among these isotopes leads to the differences in mass. Therefore, while all carbon atoms have the same number of protons (and thus the same chemical properties), the differing number of neutrons accounts for the variation in mass among carbon isotopes.