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500 BCE
The Alchemists
Alchemy was practiced throughout Europe, Africa, China, and throughout Asia. The goal of alchemy was to transmute base metals into other metals, particularly gold. This practice lasted up until the 18th century. -
400 BCE
Democritus
Democritus proposed that everything was made up of atoms, that atoms were indestructible, that they have been and always will be in motion, there is empty space between atoms, and that there is an infinite number of atoms and kind of atoms. -
384 BCE
Aristotle
Aristotle believed that everything was made up of the same four elements. Air, fire, water, and earth. -
1492
Columbus Arrives In America
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Robert Boyle
Robert Boyle helped define what an element is and contributed to the "death" of the four elements. He defined it as any substance that can be broken down into two or more substances is not an element. -
American Revolution
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John Dalton
He proposed that everything was composed of atoms that were indestructible and indivisible. He also proposed that all the atoms in an element were the same but other elements had atoms that differed in size and mass. -
American Civil War
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Dmitri Mendeleev
He found that when all the elements were arranged on a table by an increasing atomic weight it displayed a pattern of properties within these element groups. -
J. J. Thomson
He performed an experiment that found that all atoms contain tiny negatively charged particles, electrons. After, he proposed the plum pudding model of the atom which says that the atom was a ball of "soup" that was positively charged and had electrons embedded inside. -
Robert Millikan
Robert Millikan performed a series of experiments that determined the charge of a single electron. He measured charged water droplets in an electric field and found that the water droplets and a charge that were multiples of a single number. -
Ernest Rutherford
He found that when he shot alpha particles at an atom some of them were violently reflected which led him to the discovery of the nucleus. and also found that the atom is mostly just empty space. -
Hans Geiger
Hans Geiger, along with Ernest Rutherford, invented a counter that measured alpha particles and helped Rutherford's clear theory of the atom. -
Niels Bohr
He proposed a new theory to the atom based on quantum theory that showed that electrons should move around the nucleus in prescribed orbits. -
WW1
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James Chadwick
When he bombarded beryllium atoms with alpha particles, he discovered that the nucleus was composed of a particle that had a neutral charge that was named the neutron and found the approximate mass of the proton. -
WW2
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Hiroshima