Here's why:

* Protons and Neutrons: The nucleus of an atom contains protons and neutrons. These particles are much heavier than electrons.

* Electrons: Electrons are much lighter and orbit the nucleus.

* Atomic Mass: The atomic mass of an element is primarily determined by the number of protons and neutrons in its nucleus.

So, while the electrons contribute to the overall structure and chemical properties of an atom, the vast majority of its mass resides within the tiny nucleus.