A neutron has no electric charge. So you can easily find which combinations of quarks can form a proton or a neutron. The quarks have never been observed in isolation, but always in the close company of one or two other quarks. The electron e is the most familiar example of the other class of particle, the leptons. It is much lighter than its quark counterparts, and has an electric charge of -1.
Neutrinos are so elusive that they can pass through the entire earth without interacting.
These four particles, the up and down quarks, the electron and the neutrino, account for essentially all matter we see in the universe. It turns out, however, that these particles are not alone. These new particles behave in many ways like the familiar u, d, e and n, but are heavier and short-lived versions of them. For this reason, we call them the second and third generation particles. The second and third generation counterparts of the electron are known as the MUON and TAU .