Let's take a look at a different group of particles, the leptons. Like quarks, leptons are fundamental particles, meaning they aren't built up of smaller particles. Also like quarks, there are six different flavors of leptons (remember, flavor just means different kinds): the electron, the muon and the tau, and their three corresponding neutrinos: the electron neutrino, the muon neutrino, and the tau neutrino.

Leptons do not carry a color charge. This means that they are not affected by the color force and therefore don't get glued together into groups: leptons are particles which are a whole on their own, in contrary to quarks which are always a part of a hadron.

However, the various neutrinos don't carry an electric charge either. That means that they don't get affected by the color force, the nuclear force, nor the electric force. They can fly through huge collections of particles without interacting with any of them. In fact, zillions of neutrinos fly right through our body all the time and then they go straight on right through our earth without feeling the slightest thing.

The most important lepton for us is the electron. The electron has a negative electric charge and interacts with other electrically charged particles like the proton. In fact, the electron and the proton are the only free standing electrically charged particles that exist for a longer period of time. Knowing that the electron and proton have opposite charges, and that opposite charges attract, it won't be surprising that electrons and protons tend to be close together. You can see such a bond, called atom or specifically in this case hydrogen atom, on the right.

Actually, "atom" isn't such a good word for such bond. Atom is a Greek word and means as much as "undivisible". It got this name because until a little more than 100 years ago, atoms were thought to be undividable. Then, however, it was found that the hydrogen atom is built up of an electon dancing around a proton, and later, even worse, that the protons are built up of even smaller particles, the quarks! Today, atoms are split all the time. Electric current, for one example, is nothing more than a bunch of electrons stolen from their atoms moving through matter. The word atom stuck however, and we still use it even though it's slightly outdated.

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