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Democritus 440 BC
Democritus created the atom. The word "atom," from the Greek means "not able to be divided." It was said that all atoms are small, hard paticles not big enough to be cut. -
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Aristotle 382 BC
Aristotle did not agree with Democritus. He beleived that a particle could never stop being cut. Many people believed this theory for a long time. -
Dalton
John Dalton wanted to know why elements combine in certin proportions based on mass to form compounds. He did many experiments to test this theory. His results said that elements combine in certin proportions because they are made of single atoms. He published his theory in 1803. -
Thomson
J.J. Thomson discovered that Dalton's theory ws incorrect. Atoms have small particles inside, meaning they could be separated into even smaller parts. He began experiementing with cathode-ray tubes. Thomson discovered possitively charged plates and negative electtric charges. He also said every atom has these negative charges, which are now called electrons. -
Rutherford
Ernest Rutherford, a former student of Thomson's decided to test his theory. He started an experiment to study the parts of an atom. After his experiement, he concluded that atoms must be concidered mostly empty space, with a tiny part made of highly dense matter. -
Rutherford
Rutherford revised the atomis theory. He proposed that the center of an atom was a tiny, extremly dense, possitivly charged part called the nucleus. Rutherford calculated that the diameter of the nucleus was 100,000 times smaller then the diameter of the gold atom. -
Bohr
Niels Bohr studied the way the atoms react with light. Bohr's resuls lead him to propose that electrons move around the nucleus in certin paths or energy levels. The electrons can jump from a path on one level to a path on another level. -
Schrodinger and Heisenberg
These two men did especially important work that added to our modern understanding of the atom. For an example, electrons do not run on paths, that do not exactly have a specific path. According to the current theory, there are regions inside the atom where electrons are likely to be found. These regions are called electron clouds.
