Here's why:
* Protons: These particles carry a positive charge and have a mass of approximately 1 atomic mass unit (amu).
* Neutrons: These particles have no charge and also have a mass of approximately 1 amu.
Electrons, while contributing to the atom's overall structure and charge, have a mass that is significantly smaller than protons and neutrons (about 1/1836 amu). Therefore, their contribution to the atom's overall mass is negligible.
In summary: The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus, which is represented by the atom's atomic mass number.
