A theory of the structure and behavior of atoms has taken more than two millenia to evolve, from the abstract musings of ancient Greek philosophers to the high-tech experiments of modern scientists. However, prior to the scientific revolution and the development of the scientific method starting in the 16th century, ideas about the atom were mainly speculative. It wasn't until the very end of the 19th century that technology became advanced enough to allow scientists a glimpse of the atom's constituent parts: the electron, nucleus, proton, and neutron.
The idea that all matter is made up of tiny, indivisible particles, or atoms, is believed to have originated with the Greek philosopher Leucippus of Miletus and his student Democritus of Abdera in the 5th century B.C. (The word atom comes from the Greek word atomos, which means “indivisible.”) These thinkers held that, in addition to being too small to be seen, unchangeable, and indestructible, atoms were also completely solid, with no internal structure, and came in an infinite variety of shapes and sizes, which accounted for the different kinds of matter. Color, taste, and other intangible qualities were also thought to be composed of atoms.
While the idea of the atom was supported by some later Greek philosophers, it was fiercely attacked by others, including Aristotle, who argued against the existence of such particles. During the Middle Ages in Europe, Roman Catholic theologians were heavily influenced by Aristotle's ideas, and so atomic philosophy was largely dismissed for centuries. However, the Greeks' conception of the atom survived, both in Aristotle's works (his arguments against) and in another classical work by the Roman author Lucretius, De rerum natura (“On the Nature of Things”), which was rediscovered in Europe at the start of the Renaissance.
Modern atomic theory is generally said to begin with John Dalton, an English chemist and meteorologist who in 1808 published a book on the atmosphere and the behavior of gases that was entitled A New System of Chemical Philosophy. Dalton's theory of atoms rested on four basic ideas: chemical elements were composed of atoms; the atoms of an element were identical in weight; the atoms of different elements had different weights; and atoms combined only in small whole-number ratios, such as 1:1, 1:2, 2:1, 2:3, to form compounds.
Not all of these ideas were new; the Greeks had already introduced the idea that elements were composed of atoms and that atoms of different elements had different physical properties. Dalton's particular contribution, which distinguished his work from what had been done before, was his method for actually determining atomic weight. In an essay published in 1805, Dalton had included a list of atomic weights for 21 elements. Dalton was also the first to propose standard symbols for the elements.
Dalton's work was mainly about the chemistry of atoms—how they combined to form new compounds—rather than the physical, internal structure of atoms, although he never denied the possibility of atoms' having a substructure. Modern theories about the physical structure of atoms did not begin until 1897, with J. J. Thomson's discovery of the electron.
Actually, what Thomson discovered was that cathode rays were streams of negatively charged particles with a mass about 1,000 times smaller than a hydrogen atom. He claimed that these particles, which he called “corpuscles,” were the things that atoms were made from. The term “electron” predated Thomson's discovery—a few years earlier Irish physicist G. J. Stoney had proposed that electricity was made of negative particles called “electrons,” and scientists had adopted the word to refer to anything with an electric charge. However, Thomson, who was a physicist at Cambridge University, was the first to suggest that these particles were a building block of the atom.
Thomson also tried to show how the electrons were situated in the atom. Since atoms were known to be electrically neutral, Thomson proposed (1904) a model in which the atom was a positively charged sphere studded with negatively charged electrons. It was called the “plum-pudding” model, since the electrons in the atom resembled the raisins in a plum pudding. This model did not survive unchallenged for long. In 1911, Ernest Rutherford's experiments with alpha rays led him to describe the atom as a small, heavy nucleus with electrons in orbit around it. This nuclear model of the atom became the basis for the one that is still accepted today.
Bohr and Beyond
In 1913, Danish physicist Niels Bohr, who had studied under both Thomson and Rutherford, further refined the nuclear model by proposing that electrons moved only in restricted, successive orbital shells and that the outer, higher-energy orbits determined the chemical properties of the different elements. Furthermore, Bohr was able to explain the spectral lines of the different elements by suggesting that as electrons jumped from higher to lower orbits, they emitted energy in the form of light. In the 1920s, Bohr's theory became the basis for quantum mechanics, which explained in greater detail the complex structure and behavior of atoms.
Protons and Neutrons
Since Thomson's discovery of the electron in 1897, scientists had realized that an atom must contain a positive charge to counterbalance the electrons' negative charge. In 1919, as a byproduct of his experiments on the splitting of atomic nuclei, Rutherford discovered the proton, which constitutes the nucleus of a hydrogen atom. A proton carries a single positive electrical charge, and every atomic nucleus contains one or more protons. Although Rutherford proposed the existence of a neutral subatomic particle, the neutron, in 1920, the actual discovery was made by English physicist James Chadwick, a former student of Rutherford, in 1932.