Question

Question #62ac8

Answer 1

Electron mass is very small, but it's finite and known. It's only considered negligible in comparison with the nuclear mass (Born-Oppenheimer approximation).

Explanation:

The electron mass is `9.11*10^(-31) Kg`. It's very small but finite and not negligible, unlike the electron radius, which has no known lower bound.

In theoretical calculations, it's a commonly used approximation to assume that the mass of the atomic nuclei is much larger than the mass of the electrons, so the kinetic energies of those two groups can be calculated separately (furthermore, we usually approximate the nuclei to be static, so their kinetic energy is zero). This is called the Born-Oppenheimer approximation.

Additionally, massless particles can exist. Photons, for example, are massless, and they are particles (although they are also waves, but that's just quantum nature). Neutrinos were believed to be massless until recently, and it didn't cause any problem with the theory. Mass isn't an intrinsic property of particles in order to exist, although most particles have mass. It's like charge: most elementary particles are charged, but neutral particles exist.