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500 BCE
Alchemy
Alchemists original goal was to change common elements into gold. However they were unable to do so. They believed that all metal was made of sulfur and mercury. Alchemists also came up with many experimental procedures and lab apparatus. -
400 BCE
Democritus
Democritus was a philosopher from Ancient Greece and began the search for a description of matter. Questioned if matter could be divided to be smaller until the smallest particle is obtained. Named the matter "atomos" which means "not to be cut". He also believed that atoms were hard, small particles made of the same material formed into different shapes and sizes. -
300 BCE
Aristotle
Aristotle was a Greek philosopher that believed that all matter was made up of four elements: air, water, fire, and earth. He also believed that matter only had four properties: hot, cold, wet, and dry. -
Aug 3, 1492
Columbus arrives in America
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Robert Boyle
Robert Boyle replaced Aristotle's idea that all matter was composed of four elements with a more modern idea. Boyle's idea was that an element is a substance that cannot be separated into simpler components by chemical methods. -
Period: to
American Revolution
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John Dalton
John Dalton performed many experiments which led to the acceptance of the idea of atoms. Dalton came up with the first atomic theory since the "death of chemistry" in the prior 2000 years. He also believed that all matter is made of atoms and that atoms are uncuttable and indestructible. His theory also used the idea that atoms of a given element are exactly alike and that atoms of different elements can combine with one another in simple whole number ratios to create compounds. -
Period: to
American Civil War
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Dmitri Mendeleev
Dmitri Mendeleev formulated the table of elements listing them by their atomic weight and grouping them into families by looking at their similar characteristics. This was known as the periodic law. -
J.J. Thomson
J.J. Thomson gave the first hint that atoms were made of smaller particles. He did an experiment using cathode ray tubes and discovered that all atoms contain tiny negatively charged subatomic particles, also known as electrons. Thomson's new model was known as the plum pudding model which had negatively charged electrons embedded within a positively charged "soup". -
Ernest Rutherford
Ernest Rutherford performed an experiment using positively charged particles that were fired at gold foil which proved that atoms are not "pudding" filled with positively charged substances. Rutherford theorized that atoms have a small, dense, positively charged center which he called the nucleus. He also concluded that negatively charged particles are scattered outside of the nucleus at a distance. -
Period: to
Hans Geiger
Hans Geiger tested the plum pudding model by aiming beams of positively charged particles at very thin gold foil which ended up disproving the previous plum pudding model. According to the previous model the particles should have passed through yet many of them changed direction. -
Robert Millikan
Robert Millikan hypothesized that the mass of an electron was at least 1,000 times smaller than that of the smallest atom. Millikan measured the charge of an electron with his oil drip apparatus. -
Neils Bohr
Neils Bohr built on the idea that the mass of an atom is contained mostly in the nucleus of an atom. He theorized that electrons move in definite orbits around the nucleus like the planets around the sun. The orbits or energy levels are located at certain distances from the atom's nucleus. -
Period: to
World War I
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James Chadwick
James Chadwick found out that neutrons do exist. Chadwick also discovered that neutral particles made up approximately half of the mass of an atom. -
Period: to
World War II
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Hiroshima
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Hiroshima