-
322
Aristotle
Aristotle (384-322 BC) He believed in the four elements of air, earth, water and fire. Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter.This view held for 2000 years primarily because Aristotle was the tutor of Alexander the Great. -
460
Democritus 460 – 370 B.C.
There are various basic elements from which all matter is made
Everything is composed of small atoms moving in a void -
Johann Becher (1635-1682) and Georg Stahl (1660-1734)
Johann Becher (1635-1682) and Georg Stahl (1660-1734) developed the Phlogiston theory. Basically, when something burned, it lost phlogiston to the air a problem with the theory was that burning of metals resulted in an increase in the mass. This problem was solved by assigning negative mass. -
John Dalton 1766-1844
All the atoms of a given element are identical, but they differ from those of any other element -
Antoine Lavoisier (1743-1794)
Antoine Lavoisier (1743-1794)He proposed the Combustion Theory which was based on sound mass measurements. He named oxygen. He also proposed the Law of Conversation of Mass which represents the beginning of modern chemistry. -
Joseph Priestly (1733-1804)
Joseph Priestly (1733-1804) discovered oxygen (which he called "dephlogisticated air") in 1774.Priestly was also an early anti-war activist who favored both the American and French Revolutions. -
Joseph Proust (1754-1826)
Joseph Proust (1754-1826) proposed the the Law of Constant Composition in 1799. This law was very radical at the time and was hotly contested by Claude Berthollet (1748-1822). -
Joseph Gay-Lussac ( 1778-1850)
Joseph Gay-Lussac ( 1778-1850) announced the Law of Combining Volumes in 1808. He showed that at the same temperature and pressure, two volumes of hydrogen gas reacted with one volume of oxygen gas to produce two volumes of water (as a gas). -
J.J. Thompson 1856-1940
Discovered electron 1897 – Cathode Ray Experiment
Plum Pudding model 1904
Electrons in a soup of positive charges
Discovered isotopes 1913 -
Amadeo Avogadro (1776-1856)
Amadeo Avogadro (1776-1856) proposed what is now known as Avogadro's Hypothesis in 1811. The hypothesis states that at the same temperature and pressure, equal volumes of gases contain the same number of molecules or atoms. When this is combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O, respectively. The solution to the atomic weight problem was at hand in 1811. However, Avogadro's Hypothesis was a radical statem -
Ernest Rutherford 1871-1937
Nucleus Theory 1910
alpha particle gold foil experiment
An atom’s mass is mostly in the nucleus
The nucleus has a positive charge (Moseley)
Electrons in fixed orbit -
Niels Bohr 1885-1962
Planetary Model 1913
Nucleus surrounded by orbiting electrons at different energy levels
Electrons have definite orbits
Utilized Planck’s Quantum Energy theory
Worked on the Manhattan Project (US atomic bomb) -
Ernst Schrödinger 1887-1961
Quantum Mechanical Model 1926
Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed
Electrons are particles and waves at the same time
Developed quantum numbers based on theories of Einstein and Planck -
Werner Heisenberg 1901-1976
Quantum Mechanical Model 1926
Electrons are in probability zones called “orbitals”, not orbits and the location cannot be pinpointed
Electrons are particles and waves at the same time
Developed quantum numbers based on theories of Einstein and Planck -
Dimitri Mendeleev (1834-1907)
Dimitri Mendeleev (1834-1907) proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged according to increasing atomic weight and left holes for elements that were yet to be discovered. -
Stanislao Cannizzaro (1826-1910)
Stanislao Cannizzaro (1826-1910), in 1860 at the Karlsruhe Conference, proposed that Avogadro's Hypothesis be accepted and the implications used for a period of five years. At the end of this five year period, a new conference would be called to discuss any problems that might develop.
