The view of atoms as electrons orbiting a nucleus as the planets orbit the sun
is not an accurate one. The temptation is to think of electrons, protons and
even photons as behaving like miniature billiard balls. But the at subatomic
scales this kind of understanding based on everyday experience simply does not
work. These particles have no definite position and it is more useful to think
in terms of probability distributions or wave functions. Their existence must be
deduced from subtle interactions with other particles and the detectors
physicists use to study them.
In this way, physicists have discovered whole families of fundamental particles,
most of which exist only fleetingly, and which are able to transform into each
other provided that energy, spin, charge and other properties are conserved. The
Standard Model is a theoretical framework used to organise and understand these
fundamental particles; the quarks, gauge bosons and leptons (which include the
electron). They can be categorised graphically in an analogous way to the
Periodic Table of elements, with each row having the same charge (photon and
gluon excepted) and each column representing the three generations of matter and
the force carriers. Other particles, called hadrons, are made up of quarks in
groups of two (mesons, containing a quark/antiquark pair) or three (baryons).
The neutron and proton are types of baryon.
You can see a representation of the Standard Model, together with a selection of
hadrons using the Particles option from the Tools menu.
Fermions are on the left of the vertical line (spin 1/2, 3/2 etc.). These are
matter constituents. To the right of the vertical line are the Bosons (particles
with integer spin) which are force carriers. On palm-size format devices you can
view either fermions or bosons at any one time. The fundamental particles
comprising the Standard Model are in the white area. The particles are arranged
vertically according to their charge. The charge of each row, excepting the
quarks and gluon, is indicated at the right hand side in units of elementary
charge. The quarks are also grouped in rows of equal charge of 2/3 and -1/3.
All particles have corresponding anti-particles where properties such as charge
are reversed. Particles and their antiparticles can also be thought of as being
the same entity travelling in opposite directions in time. The antineutrinos
have the same properties as the neutrinos but their spin direction with respect
to the direction of travel is reversed (i.e. opposite handedness). Some
particles, such as the photon, do not have distinct anti-particles, or rather
they are their own anti-particles. The right-hand horizontal line separates the
matter (from which the Universe is almost entirely made) and anti-matter. The
antimatter particles are shown with inverted colours.
The bosons are lined up with particles opposite their corresponding
anti-particles. A horizontal line divides them, but there is no definitive way
of saying which is the "particle" and which the "antiparticle". Particles on
either side of the line should simply be regarded as anti-particles of each
other, for example the pi+ and pi- (which are conventionally referred to as pion
and anti-pion). The pi0 is its own antiparticle because it consists of an
up/anti-up quark pair.
You can click on a particle to get more information about it. In the case of
mesons and baryons the constituent quarks are also shown. In the example the
proton can be seen to be one of the few stable particles. The properties of the
particle can be entered into the calculator by clicking on the buttons. It can
also be seen to be made of a down quark and two up quarks. The quarks are bound
together by the strong nuclear force, mediated by gluons. The strong nuclear
force is so great that it is not possible to separate the quarks so that they
can exist independently. The energy of pulling them apart is enough to cause
more quarks to snap into existence which then decay into other particles.
Excellent introductions to Particle Physics and the Standard Model can be found
in the Particle Physics
Big Bang Science.