What is the mass of an electron?

The mass of an electron is often described as being "almost 0" when compared to the mass of protons and neutrons. Specifically, the mass of an electron is approximately (9.11 \times 10^{-31}) kg, which is significantly smaller than the mass of protons (around (1.67 \times 10^{-27}) kg) and neutrons (about (1.68 \times 10^{-27}) kg). This small mass is why, in many calculations or discussions, it is convenient to consider the electron's contribution to the overall mass of an atom as negligible.

In contexts such as atomic structure and chemical bonding, the electron's mass does not significantly impact calculations or the properties of atoms when compared to the more massive nucleons. Thus, stating that the mass of an electron is "almost 0" effectively captures the comparison between the masses of subatomic particles.