The mass of the atom is determined by the neutrons and protons.
Explanation:
The mass of an atom is primarily determined by the number of neutrons and protons it contains. Here is a detailed explanation of why this is the case:
1. Nucleons: Neutrons and protons are collectively known as nucleons. They are located in the nucleus of an atom, which is the dense core at the center of the atom.
2. Neutrons: Neutrons are subatomic particles that have no electric charge. They contribute to the overall mass of the atom but do not participate in chemical reactions. The number of neutrons in an atom can vary, resulting in different isotopes of an element.
3. Protons: Protons are positively charged subatomic particles. They also contribute to the overall mass of the atom. The number of protons in an atom determines its atomic number and, therefore, its identity as a specific element.
4. Electrons: Electrons are negatively charged particles that orbit the nucleus in energy levels called shells. While electrons do have mass, it is extremely small compared to the mass of neutrons and protons. Therefore, electrons do not significantly contribute to the overall mass of the atom.
5. Atomic Mass: The atomic mass of an atom is determined by adding up the masses of its protons and neutrons. It is usually expressed in atomic mass units (amu) or grams per mole (g/mol). The mass of an electron is so small that it is generally not considered when calculating the atomic mass.
In conclusion, the mass of an atom is primarily determined by the number of neutrons and protons it contains. Electrons, although they have mass, do not significantly contribute to the overall mass of the atom.

Because neutron and proton are inside the atom from which atom mass can determine. and one thing that there is electron also but it moves outside the atom on special orbit called cell or energy cell