1. Evolution of the Atomic Theory By Micheal Raines

2. Democritus, 400 B.C. In 400 B.C., The Greek philosopher Democritus came up with a revolutionary theory. He theorized that if you kept cutting something into smaller and smaller pieces, eventually you would get an indivisible particle. He called this an atom, after the Greek word atomos, for indivisible. However, since he had not the technology to prove this theory, and since this contradicted Archimedes’ widely accepted theory, this was ignored for millenia.

3. John Dalton, 1809 In 1809, a British scientist known as John Dalton created experiments based on Democritus’ theory, despite the limited technology he had at the time. His theory followed thus: 1. All elements are made up of atoms, which are impossible to divide. 2. Atoms of the same element are identical. 3. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. 4. In chemical reactions, atoms are combined, separated, or rearranged – but never changed into another element. This theory is largely correct even to this day, however, he was wrong about atoms being indivisible.

4. J. J. Thomson, 1897 Another British scientist known as J. J. Thomson discovered that atoms can be divided in 1897. Using a cathode ray and plates of different polarity, he discovered that the ray divided into two, each half being attracted to the plate of the opposite polarity. The part of the ray’s particles that was negative, and thus attracted to the positive plate, he called electrons. The fact that he discovered this with the technology he had at the time is astounding. As he was unsure where the positive charged that balanced the atoms came from, Thomson proposed that atoms are electrons that are surrounded in a mass of positive energy. He named this model after a famous English desert, plums in a pudding.

5. Eugen Goldstien and James Chadwick, 1902 and 1932 These two scientist discovered the other particles in an atom. Goldstien discovered protons (the positive particles) in 1902, and Chadwick discovered neutrons (the neutral particles) in 1932.

6. Ernest Rutheford, 1911 In 1911, a student of Thomson named Ernest Rutheford discovered the nucleus. To do this, he simply aimed a ray at a thin gold sheet. Most of the ray passed through, but some of it was reflected. Amazed, he concluded that the plum- pudding model was incorrect, and that there must be a solid core at the center of each atom, made of protons and neutrons. He called this the nucleus. While technology around now was more advanced, he still managed to find this out with limited means to do such.

7. Niels Bohr, 1927 In 1927, Niels Bohr proposed a new model for the atom. He theorized that electrons in an atom must whiz around the nucleus in definite paths, and that electrons could jump between paths, if there was room on that path (which, incidentally, is where photons, the source of light, comes from).

8. New Wave Model Bohr’s model was largely correct, but recent discoveries have proven part of his model wrong. Apparently, electrons have no definite paths as they travel around the nucleus, just general areas where they will probably travel. These areas are called electron clouds; these clouds, along with the nucleus, form today’s atomic model.

9. The End