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PARTICLE THEORY

We traditionally think of the atom as behaving like a tiny solar system with electron 'planets' orbiting a nuclear 'sun' of protons and neutrons. This is a lie. Protons, neutrons and electrons are not really lumps of stuff, they are sort of waves and particles at the same time. And we cannot really see them, all we can do is guess where they are. This means that a picture of an atom is a fuzzy cloud - a bunch of statistical predictions about to where the bits are likely to be:

Atoms are very complex structures. The traditional three components are in fact made up of more fundamental particles which are affected by all sorts of strange forces.

So from the bottom:

Quarks combine to form hadrons. The quarks are glued together by the strong nuclear force.

Hadrons (in the form of protons and neutrons) combine to form a nucleus. Hadrons are glued together by the weak nuclear force.

Electrons are attracted to the nucleus by electromagnetic forces. The position of the electron is determined by its energy.

FUNDAMENTAL PARTICLES

There are two types of fundamental particle: fermions and bosons. They are called fundamental in that they cannot be broken done into smaller components.

FERMIONS

There are two types of fermion: leptons and quarks.

Leptons are indestructible and highly unreactive: they may join in the atomic party but do not dance. You cannot combine leptons to make bigger particles. Examples of leptons include: electrons, positrons, neutrinos and muons.

Quarks are incapable of independent existence - they are the life and soul of the atomic party and are only found in groups of two or three making bigger and more stable particles. Three quark particles are called hadrons the most common of which are protons and neutrons. Two quark particles are called mesons and are usually the result of radioactive decay in atoms. There are six types or 'colours' of quark: up, down, top, bottom, quark and charm.

BOSONS

Bosons are the glue that holds the universe together. Their job is to transfer energy from one particle to another. There are four bosons: gravitons, photons, W and Z particles. When a boson collides with a particle its energy is absorbed and it ceases to exist. It follows that bosons only exist to transfer energy from one particle to another. Each is linked to one of the four forces of nature.

All of these particles and their interactions form the Standard Model. A poster displaying the Standard model is here: Medium (728kB) or here: Large (1.4MB).

THE FORCES OF NATURE

The four fundamental forces of nature are: gravity, electromagnetism, the weak and strong nuclear forces.

No matter where you are in the universe, gravity is pulling you down. Gravity affects everything from atoms all the way up to galaxies. Gravity works by pulling every thing together; it is the prime force of nature because it affects everything.

If an object has an electrical charge it will be attracted to an object of opposite electrical charge. This is why electrons stick close to protons. Electromagnetic attraction is infinite in range but only effective on charged particles.

Because neutrons have no electrical charge something else is needed to hold them together. This is the weak nuclear force, it holds protons and neutrons together and is therefore only found inside the atomic nucleus.

Protons and neutrons are made up of quarks, and need a very strong force to hold them together. This force is called the strong nuclear force, it is the strongest glue in the universe and only found within hadrons and mesons.

BACK TO THE ATOM

Using the standard model, we can see that atoms are built up as follows:

Quarks combine to form hadrons. The quarks are glued together by the strong nuclear force.

Hadrons (in the form of protons and neutrons) combine to form a nucleus. Hadrons are glued together by the weak nuclear force.

Electrons are attracted to the nucleus by electromagnetic forces. The position of the electron is determined by its energy.

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