The mass number of an atom is defined as the sum of which two subatomic particles?

The mass number of an atom is defined as the sum of protons and neutrons. This is because protons and neutrons, found in the nucleus of the atom, are the primary contributors to the atom's mass. Each proton has a mass of approximately 1 atomic mass unit (amu), and each neutron also has about 1 amu. Electrons, although they are subatomic particles associated with an atom, have a much smaller mass (approximately 1/1836 of a proton) and do not significantly contribute to the overall mass number.

Neutrons and quarks, while they are indeed subatomic particles, do not represent the conventional way of calculating mass in this context. Quarks are constituents of protons and neutrons; however, they are not counted in the mass number. Therefore, the summation of protons and neutrons is crucial in determining the mass number of an atom, which is central to understanding isotopes and atomic stability.