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Most matter is composed of molecules (groups of atoms) e.g. water, salt, proteins or atoms themselves e.g. hydrogen, iron.

Atoms are made up of protons & neutrons in a nucleus and electrons which surround the nucleus. This is the Bohr model of the atom based on the discovery of the neutron in 1921.
The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The 'energy levels' or 'shells' or 'orbits' are represented in two ways: either by the numbers 1 - 6 or by letters K- P. The energy levels are counted from centre outwards.
See Electron Configuration at answers.com

The current, 2005, structure, called the Standard Model of Fundamental Particles and Interactions, describes neutrons and protons made up of quarks. Based on decades of experiments showing the existance of quarks first proposed in 1964.
Electrons are a type of lepton.

There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.)

Each member of a higher generation has greater mass than the corresponding particle of the previous generation, with the possible exception of the neutrinos (whose small but non-zero masses have not been accurately determined).

This mass hierarchy causes particles of higher generations to decay to the first generation, which explains why everyday matter (atoms) is made of particles from the first generation.

The second and third generations of charged particles do not occur in normal matter and are only seen in extremely high-energy environments such as cosmic rays or particle accelerators.

Quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called Hadrons.
There are two classes of hadrons:
baryons are any hadron which is made of three quarks (qqq).
mesons contain one quark (q) and one antiquark ().

Protrons and neutrons are baryons:
protons are made of two up quarks and one down quark (uud),
neutrons are made of one up quark and two down quarks (udd).

One example of a meson is a pion+), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so an antipion (π-) is made up a down quark and an up antiquark.

Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K-) meson lives much longer than most mesons, which is why it was called "strange" and gave this name to the strange quark, one of its components.

There are six leptons, three of which have electrical charge and three of which do not. They appear to be point-like particles without internal structure. The best known lepton is the electron (e-). The other two charged leptons are the muon() and the tau(), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos which are created when a neutron decays. They have no electrical charge, very little mass, and they are very hard to find.

BOSONS - Forces
In particle processes the forces are described as due to the exchange of particles; for each type of force there is an associated carrier particle.
Particle Context Range Effects
Gravitation Gravity infinite No effect at nuclear level
Photon Electro-
infinite Holds molecules together and electrons to the nucleus in an atom.
Gamma ray is the name given a photon from a nuclear transition.
Gluon Strong force 10-13 m Holds quarks together to form hadrons; Keeps positive charged protons from flying apart from electrical force.
The alpha particle is a helium nucleus - one of the products of a nuclear fission, a residual strong interaction effect . Fission is the breakup of a massive nucleus into smaller nuclei; this occurs when the sum of the masses of the smaller nuclei is less than the mass of the parent nucleus.
Weak Gauge Bosons weak force 10-16 m Nuclear interactions associated with particle decay.
Weak interactions are the only processes in which a quark can change to another type of quark, or a lepton to another lepton. in a nucleus where there is sufficient energy a neutron becomes a proton and gives off an electron and an antielectron neutrino. This decay changes the atomic number of the nucleus. Beta ray is the name given to the emerging electron.
Higgs Boson weak force ? Responsible for mass.
In 1964 Scottish physicist Peter W. Higgs of Edinburgh Univ. proposed a way to explain how the fundamental particles could have mass. He theorized that the whole of space is permeated by a field, now called the Higgs field, and as particles tragvel through this field, and if they interact with it they acquire what appears to be mass.

Fermions - Matter See The Particle Adventure | What holds it together? | Fermions and bosons.
Leptons -
  Electron - Atoms
  Electron Neutrino - Radioactive Decay
Quarks - Atomic Nuclei

Particles have attributes of spin, mass and charge.
Spin - Intrinsic angular momentum
Charge - Relative to a proton
Mass - E/c2 where E is electronvolts (eV)

Where do these particles, which are not part of atoms, exist?
They are created when other particles decay or are broken up by high speed collisions, created in the laboratory or from cosmic rays hitting the atmosphere. There may be other sources; I couldn't find a good website that explains it.

See Also:

Elementary Particles
List of subatomic particles at answers.com
Particle Physics at reade.com Interactive adventure
Particle Adventure Glossary
Elementary Particles at NOVA

last updated 26 Dec 2005