Question

The mass of an atom may be determined by adding the masses of the protons and neutrons in the nucleus. Why is it unnecessary to include the electrons when determining the mass of an atom?

Answer 1

The mass of an electron is much lower than the mass of either a proton or a neutron.

Answer 2

Electrons have a very low mass (9.10938356 × 10-31 kilograms) compared to protons (1.6726 x 10-27 kg) or a neutron (1.674929 x 10-27 kg).