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Period: 400 BCE to 450 BCE
Democritus
Democritus, a Greek philosopher, determined that all matter around us is made up of small, tiny, invisible particles that could not be further divided into smaller units. He called these particles "atomos" and believed that they were made of the same material, but although they were made of the same material, they were all different shapes and sizes. He formulated the Atomic Theory of the Universe and was the first person to term the atom that we know today. -
300 BCE
Aristotle
Unlike Democritus, Aristotle, a Greek Philosopher, declared that all matter was made up of the four elements: fire, air, water and earth. He also believed that matter only had four properties: hot, cold, dry, and wet. He believed that matter could also be divided into smaller units, but believed that matter was made up of the elements. -
Period: to
Antoine Lavoisier
Antoine Lavoisier, a French chemist, explained the nature of combustion and regarded measurement as the essential operation of chemistry. He understood that combustion involved oxygen, not phlogiston, and discovered that life was also supported by a process that involved oxygen and was similar in many ways to combustion. He is considered to be the Father of Modern Chemistry and is credited for introducing the metric system. -
John Dalton
John Dalton, an English chemist, performed multiple experiments that led to the acceptance of the idea of atoms. He proposed the first atomic theory -Dalton's Atomic Theory- which states that all matter and elements are made of atoms. He hypothesized that all atoms of one element are the same but atoms of different elements are all different. He also proposed the Law of Multiple Proportions, stating that different atoms form compounds in constant ratios. -
Dmitri Mendeleev
Dmitri Mendeleev, a Russian chemist and inventor, formulated the Periodic Law and arranged the elements in the periodic table based on their atomic mass. He created the symbols for the elements and helped formulate a version of the periodic table based on their atomic weights. -
Period: to
J.J. Thomson
John Dalton discovered the electron, or a negative particle in an atom, by using a cathode ray tube and hypothesized that the atom is made of even smaller particles. He proposed the idea of the Plum-Pudding Model, stating that electrons are dispersed in a positively charged substance. He also discovered the electron's charge to mass ratio and knew that the electrons were dispersed in the positively charged substance like chocolate chips in a cookie. -
Ernest Rutherford
Ernest Rutherford, an English physicist, performed his famous Gold Foil Experiment to determine that a positive charge is not like pudding, but instead concentrated in the nucleus. He coined the term nucleus by theorizing the idea of it and stated that the nucleus was tiny compared to the atom as a whole. He theorized that atoms have a nucleus, but were mostly composed of open space because his positively charged alpha particles varied on how many were deflected from the gold sheet. -
Robert Millikan
Robert Millikan, an American physicist, measured the magnitude of the electron’s charge using oil droplets by dropping them between two metal plates. The plates were then subjected to the downward force of gravity and the upward attraction of an electrical field. By measuring how the oil droplets moved around, he showed that their charge always was a multiple of precisely determined charge. -
Period: to
Niels Bohr
Niels Bohr, a Danish physicist, determined the magnitude of the electron's charge and learned that electrons orbit the nucleus, yet the energy is quantize and located at certain distances from each other. He proposed the idea of the Bohr's Orbit Model and performed an experiment where drops of oil were suspended between two metal plates. After reasoning that the oil droplets, he showed that their charge always was a multiple of a precisely determined charge. -
James Chadwick
James Chadwick, an English physicist, was credited for proving the existence of neutrons by shooting alpha particles through paraffin wax. He theorized that bombarding elements with neutrons can succeed in penetrating and splitting nuclei, creating energy. This led to the discovery of nuclear fission.