The mass of an atom is primarily determined by the combined masses of its subatomic particles: protons, neutrons, and electrons.
Protons and neutrons, collectively known as nucleons, reside in the atom's nucleus. Protons carry a positive charge, while neutrons are electrically neutral. Both protons and neutrons have a mass of approximately 1 atomic mass unit (AMU).
Electrons, on the other hand, orbit around the nucleus in specific energy levels or electron shells. Electrons are much lighter than protons and neutrons, with a mass of approximately 1/1836 AMU. Consequently, the mass contribution of electrons to the total mass of an atom is negligible compared to the nucleons.
The number of protons in the nucleus determines the atomic number of the atom, which defines its element. For example, an atom with six protons is carbon, while an atom with eight protons is oxygen.
The total number of protons and neutrons in the nucleus is known as the mass number of the atom. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different mass numbers. For example, carbon-12 has 6 protons and 6 neutrons, while carbon-14 has 6 protons and 8 neutrons.
The mass of an atom is determined by summing the masses of its protons and neutrons, taking into account the isotope abundances if applicable. The atomic mass listed on the periodic table represents the weighted average of the isotopic masses of an element, considering the relative abundances of each isotope found on Earth.
In summary, the mass of an atom is determined by the masses of its protons and neutrons in the nucleus, while the contribution of electrons to the overall mass is relatively insignificant.