Elementary Particles

Aristotle believed that all the matter in the universe was composed of four basic elements:- air, earth, fire and water. These elements were acted on by two forces: gravity, the tendency for earth and water to sink, and levity, the tendency for air and fire to rise. The division of the universe into matter and forces is used today.

Aristotle believed that matter could be divided into smaller parts without any limit: one never came up against a grain of matter that could be further divided. A Greek named Democritus, held that matter was inherently grainy and everything was made up of large numbers of different atoms. A British chemist, Dalton pointed out that chemical compounds always combined in certain proportions could be explained by the grouping together of atoms to form molecules.

Picture of Albert Einstein

The argument between the two theories was solved by Einstein. Physical evidence was provided by Brownian motion - the random motion of small particles of dust suspended in a liquid colliding with dust particles.

Picture of Sir J.J. Thomson

J.J Thomson demonstarted the existence of an electron that had the mass of 1/1000th atom. He used a cathode ray tube which gave off electrons and due to their negative charges could be accelerated towards a phosphor-coated screen by a electric field producing flashes of light. Rutherford discovered the internal structure of an atom consisted of a tiny, positively charged nucleus orbitted by electrons. This was deduced by the Chadwick discovered the nucleus contained another particle called the neutron which almost the same mass as a proton but was chargeless.

Picture of Earnest Rutherford

It was thought that protons & neutrons were "elementary particles", but experiments in which protons were collided with other protons or protons or electrons at high speeds indicated that they were in fact made up of smaller particles named quarks by Murray Gell-Man.

There are 6 of different quarks('flavours'):- up, down, strange, charmed, bottom and top. Each flavour can come in 3 'colours':- red, green and blue. For eg. a proton or neutron is made up of 3 quarks, 2 up quarks and 1 down quark; a neutron contains 1 up and 2 down quarks. Other particles can be created using other quarks, but all these have much greater mass and decay rapidly into protons and neutrons.

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