An atom is the smallest possible particle of a chemical element. Smaller particles exist, but they do not have the chemical characteristics that identify an element.
The nature of matter was one of the key concerns of the early Greek philosophers. Empedocles proposed that all matter was composed of just four elements: earth, air, fire, and water. The Greek philosopher Leucippus and his disciple Democritus were the first to suggest that matter was composed of tiny particles that could not be divided. In fact, the word atom derives from the Greek word atomos, meaning "indivisible." Only a few years after Democritus put forward his theory, the Greek philosopher Aristotle put forward an alternative theory of matter that added a fifth element, "quintessence," of which the stars and planets were made, to Empedocles’ list. Aristotle also believed that matter could be divided indefinitely and that an object was the result of an abstract "form" being imposed on matter. Because Aristotle’s theory was more consistent with the teachings of the Christian religion, his theories were endorsed, while those of Democritus were dismissed for nearly two thousand years.
Dalton’s atomic theory
In the late 18th century, chemists discovered a number of substances that could not be broken down into other substances by the chemical methods of the time. They realized that there were more elements than the four of Empedocles’ theory. A French chemist, Joseph Proust, made a great breakthrough when he observed that elements always combine in fixed proportions by weight when they form compounds, so a gram of hydrogen will always react with eight grams of oxygen to form water.
In 1803, the British scientist John Dalton proposed that the fixed proportions of combination observed by Proust could be explained if the elements consisted of atoms—just as Democritus had suggested—and if atoms of different elements formed compounds by joining together in simple, fixed ratios. Dalton also drew up the first table of atomic weights, but it contained some errors: thinking that the formula of water was HO, Dalton gave oxygen an atomic weight of 8 units relative to hydrogen, which he gave an atomic weight of 1 unit. In fact, the formula of water is H2O and the atomic weight of oxygen is 16.
Although Dalton had proposed the existence of atoms, he was unable to describe their composition. The first clue to the structure of atoms came in 1897, when the British physicist Joseph Thomson discovered the electron while studying cathode rays—a form of radiation emitted by a hot metal when it is connected to a negative voltage. By studying how these rays were deflected by electric fields, Thomson calculated that they consisted of negatively charged particles with a mass nearly 1/2000 the mass of a hydrogen atom.
Realizing that he had managed to split negatively charged fragments from atoms, Thomson proposed the "plum pudding" model of atomic structure, which envisaged tiny, negatively charged electrons embedded in a mass of positive charges. The number of positive charges in a neutral atom had to be equal to the number of electrons, and Thomson gave the name atomic number to the number of electrons and positive charges in an atom of a given element.
