Here's why:

* Electrons are negatively charged particles that orbit the nucleus of an atom. They have a mass of approximately 9.109 x 10^-31 kg.

* Protons are positively charged particles located in the nucleus. They have a mass of approximately 1.672 x 10^-27 kg, which is about 1836 times greater than the mass of an electron.

* Neutrons are neutral particles also located in the nucleus. They have a mass slightly greater than protons, at approximately 1.675 x 10^-27 kg.

Therefore, electrons have the least mass compared to protons and neutrons.