Atomic theory has evolved since ancient times. Scientists have taken the hypothesis of Greek scholars and have built on it with their different discoveries and theories regarding the atom, which derives from the Greek word "atomos," meaning indivisible. Since then, the scientific community has discovered that these particles further divide into subparticles called protons, neutrons and electrons. Nevertheless, the name "atom" has stuck.
Leucippus and Democritus
Leucippus and Democritus were the first to propose, in the fifth century B.C., that all matter is made of tiny units called atoms. According to Infoplease.com, the two philosophers held that these were solid particles without internal structure, and came in a variety of shapes and sizes. Intangible qualities such as taste and color, according to this theory, were made of atoms. However, Aristotle strongly opposed this idea, and the scientific community failed to pay serious attention to it for centuries.
In 1808, English chemist John Dalton further built on the Greek notion of atoms. He postulated that matter is made of atoms, which are small indivisible particles. He also proposed that while all atoms of one element are identical, they are totally different from those that make up other elements.
English physicist Joseph J. Thomson proposed the "plum pudding" theory of the divisible atom in 1904, after discovering electrons in 1897. According to Ausetute.com, his model postulated that atoms consist of a big positively charged sphere studded with negatively charged electrons (he called them "corpuscles") like fruit in a plum pudding. He further hypothesized that the charge of the positive sphere's charge is equal to the negative charges of the electrons. Today we call the positive charges protons, and the negative ones electrons.
British physicist Ernest Rutherford proposed a nuclear model of the atom, in which a nucleus exists, in 1911. He also discovered activity in this part, namely the movement of protons and electrons within the central part of the atom. He further postulated that the number of protons in an atom equals that of the electrons. He also hypothesized that more neutral particles exist. These have come to be known as neutrons.
Danish physicist Niels Bohr proposed in 1913 what Ausetute.com refers to as the planetary model, in which electrons revolve about the nucleus just as the planets orbit the sun. While the electrons are in orbit, they have what Bohr termed "constant energy." When these particles absorb energy and transition into a higher orbit, Bohr's theory refers to them as "excited" electrons. When the electrons return to their original orbit, they give off this energy as electromagnetic radiation.
Modern Atomic Theory
Modern theory has centered on a principle that governs electrons. As the CB Physical Sciences textbook points out, a person knows that electrons move, but he cannot tell a given one's speed, direction or location at any given time. The textbook likens it to a situation with the blades on an airplane propeller. When watching those blades move, the observer cannot tell where a single blade is at any given point in time, because while they are moving, they do so in a blurry cloud. It is in like manner that electrons move around a nucleus.