Here's why:
* Protons and Neutrons: These particles reside in the nucleus of an atom and have nearly identical mass. The mass of an electron is significantly smaller (about 1/1836th the mass of a proton).
* Electrons: Electrons orbit the nucleus and contribute very little to the overall mass of the atom.
Important Note: While protons and neutrons are the primary contributors to mass, there is a small amount of mass-energy equivalence that comes from the binding energy holding the nucleus together. This effect is described by Einstein's famous equation, E=mc², where energy (E) and mass (m) are equivalent.
