Here's why:

* Electrons: Electrons are the negatively charged particles that orbit the nucleus of an atom. They have a very small mass compared to protons and neutrons.

* Protons and Neutrons: Protons and neutrons reside in the nucleus, which is the dense, positively charged center of an atom. They have significantly more mass than electrons.

Therefore, while the nucleus contains most of the atom's mass, the electron cloud, which is the region where the electrons are located, has the least mass.