Carbon-12 nucleus: The carbon-12 nucleus is used as the standard reference for assigning atomic masses. It has a mass of exactly 12 atomic mass units.
Protons: Protons are positively charged subatomic particles found in the nucleus of an atom. They contribute to the overall mass and determine the element's identity.
Neutrons:Neutrons are neutral subatomic particles found in the nucleus of an atom. They also contribute to the overall mass but do not affect the element's identity.