The two particles that account for most of the mass in an atom are the protons and neutrons. Protons and neutrons are collectively known as nucleons and are found in the nucleus of an atom.
Protons have a positive charge and a relative mass of approximately 1 atomic mass unit (u). Neutrons have no charge (they are electrically neutral) and also have a relative mass of approximately 1 u. Since electrons are much lighter in comparison, their mass is typically considered negligible when calculating the total mass of an atom.
The total mass of an atom is determined by the combined mass of its protons and neutrons in the nucleus. The number of protons in an atom defines its atomic number and determines the element to which it belongs. The sum of the protons and neutrons gives the mass number of the atom.
For example, in a carbon atom (with atomic number 6), the nucleus contains 6 protons and usually 6 neutrons, resulting in a total of 12 nucleons and a mass number of 12. The electrons, which orbit the nucleus, contribute very little to the total mass of the atom.