One of the leptons, the electron, is part of the atom. Inside the atom are the electrons spread out around the positive nucleus, forming a "cloud" of negative charge with a diameter of about 10-10 meters. Theoretically electrons are considered to be point-like. Experiments show that the diameter is at least smaller than 10-18 m.
The electron is stable; meaning it does not decay. If it meets its antiparticle, the positron, it can be annihilated. In this process, the masses of both particles will be transformed into energy in the form of gamma photons (photons of very high energy).