The majority of an atom's mass is made up of protons and neutrons, which reside in the atom's nucleus.

Here's why:

* Protons and Neutrons: These particles are much heavier than electrons, contributing almost the entire mass of an atom.

* Electrons: These particles orbit the nucleus and are significantly lighter than protons and neutrons. Their mass contribution is negligible compared to the nucleus.

While electrons play a crucial role in chemical bonding and determining an atom's properties, their mass is so small that the nucleus accounts for nearly all the atom's weight.