The mass of atoms is primarily determined by the number of protons and neutrons in their nucleus. Protons and neutrons are collectively known as nucleons, and each has a mass of approximately one atomic mass unit (amu). Electrons, which orbit the nucleus, have a much smaller mass and do not significantly contribute to the overall mass of the atom. The atomic mass of an element is often listed on the periodic table, represented in amu. For example, the atomic mass of Carbon is about 12 amu, indicating that a single atom of Carbon weighs roughly 12 times that of a hydrogen atom, the lightest element.