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Period: 460 BCE to 370 BCE
Democritus
Democritus was the first philosopher to theorise about the atom. He hypothesised that if you take an object and cut it into smaller and smaller pieces, eventually you would reach a point where you could no longer cut it anymore. Democritus called this piece atomos. He thought, for example, that water was made of water atoms, bread was made of bread atoms and soil was made of soil atoms. -
Period: 384 BCE to 322 BCE
Aristotle
Aristotle rejected the ideas of Democritus, instead believing that matter on Earth was made up of four elements – earth, air, fire and water – and the amounts of these elements determined how materials behaved. Aristotle had such an influence over people at the time that it took about 2000 years for Democritus’s theory to be re-examined. -
Period: to
John Dalton
Dalton suggested that all elements, which were now arranged in the periodic table, contained atoms, and that atoms of the same element would be identical in size, shape and mass. This theory was called the ‘solid sphere model’ as Dalton believed the atom was a solid sphere. He also stated that compounds were a combination of elements. The question he could not answer, though, was why atoms behaved the way they do. -
Period: to
Joseph John Thomson
Thomson was the first scientist to discover particles smaller than the atom, disproving Dalton’s and Democritus’s Theories. The first subatomic particle to be discovered was the lightest – the electron. By studying rays within a cathode ray tube, Thomson was able to determine that these rays had a mass 1000 times smaller than a hydrogen atom. He concluded that these rays were not rays at all, but very light particles. He named these particles corpuscles, but they are now called electrons.