Question

why do we say that electrons have zero mass?
they move so fast that einstein theory of relativity (e = mc²) makes their mass zero
their charge is negative, so their mass is always cancelled out by the atoms protons
they are outside the nucleus, so their mass is never considered in the mass of the atom
their mass is so small compared to protons and neutrons that we can ignore them

Explanation:

Electrons have a very small mass compared to protons and neutrons. The mass of an electron is approximately 1/1836 times the mass of a proton. In many calculations and considerations of atomic mass, the mass of electrons is so negligible that it can be ignored, giving the impression of zero - mass in some contexts.

Answer:

Their mass is so small compared to protons and neutrons that we can ignore them.