The mass of the atom is determined by

Answer 1

the total number of protons and neutrons in the nucleus.
The mass of an atom is primarily determined by the sum of the masses of its protons and neutrons. Protons and neutrons both have a mass of approximately 1 atomic mass unit (amu). Electrons, which orbit the nucleus, have a much smaller mass and therefore do not significantly contribute to the total mass of an atom. Therefore, the mass number of an element is determined by the sum of the number of protons and neutrons in its nucleus.