What evidence suggests that the electron is a fundamental particle?
The standard model of particle physics suggests that there are fundamental particles called leptons and quarks which are the basic building blocks of matter, i.e., they are seen as the "elementary particles". The concept is that these fundamental particles are not made out of anything else - they cannot be further subdivided into smaller constituents. To suggest that a particle is a fundamental particle in this sense is indeed a bold claim and requires defense. This document chooses the electron and seeks to make that defense.
1. Physical properties that do not vary
The electron was found to have a definite charge to mass ratio, e/me, in early experiments and later was found to have an intrinsic spin of 1/2.
2. Stable against decay
No decay of electrons has been observed - decay into other constituents would of course prove that it is not a fundamental particle.
3. No internal structure
The highest energy scattering experiments involving electrons reveal no internal structure for electrons.