1.1: Structure of Matter


What is the fundamental particle of matter?

This question has been asked over the millennia. In Ancient Greece, the elements of fire, water, earth and air were thought to combine in different quantities to produce matter. In 450 BC, the philosopher Democritus first described an 'atom' as an indivisible part of matter that could not be seen.
Modern atomic theory begins with Dalton in 1803. He proposed that all matter was made up of elements; these elements were made up of indivisible units known as atoms. The size of atoms was determined by Loschmidt in 1865, and the first periodic table was published by Mendeleev in 1869.

Until 1897, atoms were thought to be fundamental and indivisible. Experiments with cathode ray tubes by Thomson discovered electrons. In 1909 Rutherford developed the concept that the mass of an atom was concentrated in a tiny, central nucleus. In 1913 Bohr developed the concept of electrons orbiting the atom in discrete shells, a view that still permeates today.

In 1932 the neutron was discovered, confirming that the nucleus of the atom consisted even smaller particles, neutrons and protons. Further expirements have shown that these particles are not fundamental; they are made up of various quarks. The quarks appear to be point sources of mass and if they are made up of even smaller particles it has not been discovered yet. The electron, discovered in 1897, is a member of a different type of elementary particles - the leptons.

For a more entertaining discussion of the structure of matter, try The Particle Adventure, which is fun for some light reading before you get serious about all of this physics stuff!