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.
Ancient Greek Beliefs
Leucippus and Democritus were the first to propose, in the fifth century B.C., that all matter is made of tiny units called atoms. 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.
J.J. Thomson's Theory
English physicist Joseph J. Thomson proposed the "plum pudding" theory of the divisible atom in 1904, after discovering electrons in 1897. 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 charged particles 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 a 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.
Einstein, Heisenberg and Quantum Mechanics
From decades of painstaking research from thousands of scientists, the current atomic theory builds on work done in the 1930s by Albert Einstein, Werner Heisenberg and others. As with the earlier theories, the atom consists of a central, heavy nucleus surrounded by a number of electrons. Unlike earlier theories that treated electrons, protons and other tiny particles as definite solid "lumps," modern quantum theory treats them as statistical "clouds;" oddly, you can measure their speed exactly, or their locations, but not both at the same time. Instead of electrons behaving as planets orbiting in well-behaved elliptical paths, they whirl around in fuzzy clouds of various shapes. Atoms, then become less like hard, precise billiard balls and more like springy, round sponges. And despite being "solid" matter, they can exhibit wavelike properties such as wave length and interference patterns.
As scientists looked at atoms with increasingly more powerful instruments, they discovered that the protons and neutrons that made up the nucleus were in turn made of even smaller particles. In the 1960s, physicists Murray Gell-Mann and George Zweig called these particles "quarks," borrowing a word used in a James Joyce novel. Quarks come in varieties such as "up," "down," "top" and "bottom." Protons and neutrons are formed from bundles of three quarks each: "up," "down" and "up" and "down," "up" and "down," respectively.