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450
Democritus
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Period: 450 to
Atomic Theory
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John Dalton
Just like Democritus, John Dalton's atomic theory states that elements are composed of indivisible particles called atoms (postulate 1). His theory differs, however, because his theory goes into atoms of different types of elements. For example, all carbon atoms are the same, but a carbon atom and a lithium atom are different (postulate 2 - all atoms of the same element are identical). We know that they are different because they have distinctive atomic weights. -
John Dalton
Democritus also did not talk about the chemical reactions of different types of atoms. Dalton states that atoms can physically blend together or chemically bind together to form compounds, which are in whole- number ratios (postulate 3). These reactions occur when atoms are divided, combined, or rearranged. However, atoms of one element do not change into atoms of another (postulate 4). -
John Dalton
The concept that atoms combined in whole number ratios was merely an assumption. There was no scientific evidence near his time to show which elements chemically reacted to form specific compound molecules.
We know now that a part of Dalton's theories were wrong. Atoms are now known to be divisible, made up of protons, neutrons, and electrons. We also now know that all atoms of an element are not precisely identical, since we know that isotopes of said element have varying atomic masses. -
Eugene Goldstein
Goldstein was working on an experiment dealing with cathode rays and the electrical charges of the particles moving through a discharge tube. The negatively charged particles moved towards the anode (the positively charged end). He noticed that there were rays traveling in the opposite direction, which he called canal rays. This discovery proved the existence of the proton. -
J.J. Thomson
Shortly after the discovery of the Proton, J. J. Thomson discovered the electron (which he called a 'corpuscule' at the time). Performing a similar experiment as Goldstein's, he shot a ray of charged particles through a cathode ray tube. This ray traveled from the cathode (the negatively charged end) towards the anode (the positively charged end), and deflected off of a negatively charged plate to the positively charged plate. -
J.J. Thomson
J. J. Thomson also invented the Plum-Pudding model -
Albert Einstien
Until 1905, many thought that light was made up of waves. In Einstein's paper written in 1905, he proposed that light was a quanta of energy - called photons. These photons are emitted from electrons as they exit the excited state. This is called the photo-electric effect.