Name the particles which determine the mass of an atom.

Mass of an atom:

1. The atom consists of two parts: Nucleus and the shells.
2. The nucleus contains of protons and neutrons and the shells contain electrons.
3. The mass of electron is very less $9.1\times {10}^{-28}\mathrm{g}$.
4. Thus, the mass of electron is considered to be negligible as compared to the mass of protons and neutrons together.
5. Thus, the mass of an atom is the mass of protons and neutrons in the nucleus.

Thus, the particles that determine the mass of an atom are protons and neutrons.