Here's why:

* Electrons have a very small mass, approximately 9.11 x 10^-31 kilograms.

* Protons and neutrons, the other two main subatomic particles, are significantly heavier. Protons have a mass of approximately 1.6726 x 10^-27 kilograms, and neutrons are only slightly heavier than protons.

Therefore, the electron is the lightest subatomic particle found in an atom.