Answer:
Since he has not added the mass of electrons, one of the subatomic particles.
Step-by-step explanation:
An atom is made of electrons, protons, and neutrons. Electrons have negligible mass as compared to the masses of protons and neutrons. Being the major contributor to the mass of the atom, the total mass of protons and neutrons gives the approximate mass of atoms. The exact mass of atom would be sum total of the mass of electrons, protons, and neutrons.