A Brief History of Ideas About the Atom
Greek philosophers first wondered about how far it was possible to divide matter into ever smaller quantities. They wondered if there was a limit to how small a piece of matter could get. In 440 BC the Greek philosopher Democritus suggested there was a limit. The limit to divisibility came when matter was no bigger than an atom. He put forward no proof in favour of his arguments.
John Dalton, an English chemist, used the atom concept to explain differences in the elements and the Russian Mendeleev gave the concept a large push forward with the periodic table idea. Gaps in the table were to be filled later, with apparently, different elements, each with a different species of atom. About the end of the nineteenth century the atomic concept received a blow with the apparent discovery of particles smaller than the atom – notably the electron.
Later observations of radioactivity implied that atoms have structure, which implied smaller particles than atoms should exist and this was confirmed by the Rutherford alpha particle scattering experiment, which showed that most of the mass of an atom is concentrated at the centre.
In 1913 Neils Bohr developed the idea of electron orbits. Electrons orbited the nucleus of an atom much as planets orbited the Sun. This simple idea gave rise to the attractive picture of atoms as miniature solar systems.
Later it was discovered that even the nucleus inside the atom had a structure all its own. The discovery of the neutron in 1932, and nuclear fission, leading to the atom bomb, seemed to make man the master of the atom at last. This was only a step however, because even the particles inside the nucleus were shown to have structure after the second world war. Protons and neutrons were shown to be made of of smaller particles called quarks, and we cannot conclusively yet say that even the smallest particles are truly indivisible.