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History of the Atom
Democritus of Abdera
Democritus: 460-370 BCE Greek mathematician and philosopher Co-author (with Leucippus) of the idea that all
matter made of small parts atoma
Proposed that the behaviour of substances was due to the type of atom
Aristotle
Greek Philosopher: 384-322 BCE Not an ‘atomist’ Proposed all matter made up of 5 elements:
Fire Air Water Earth Aether
Robert Boyle
Robert Boyle: 1627-1691 Performed experiments with controls and
published detailed results 1661: Published claiming that Aristotle's ideas
were incorrect Some 'elements' (like mercury) did not contain the
Greek elements (earth, air, fire, water)
John Dalton
Dalton: 1766-1844 Law of Multiple Proportions (1803): elements
combine in mass ratios of small whole numbers. Small whole numbers reflect the atoms involved,
therefore there are atoms Atomic theory contains 4 hypotheses
Dalton’s Atomic Theory
1. Elements are made of small particles called atoms. (Dalton viewed atoms as small spheres of differing characteristics.)
2.All atoms of an element are identical. All atoms of one element are different from atoms of other elements.
3.Compounds are made of atoms of more than one element. The ratio of the elements is a simple fraction.
4.A chemical reaction involves separation, combination or rearrangement of atoms.
Joseph John Thomson 1856-1940 Used a cathode ray
tube to determine the presence of electrons 1897
Suggested the plum pudding model of the atom and the existence of isotopes
Won the Nobel Prize in Physics in 1906
Robert A. Millikan 1858-1953 Used the 'falling drop
method' to determine the charge of the electron (-1.6022 x 10-19 C) and mass of electron as 9.10 x 10-28 g
Investigated photoelectric effect and spectroscopy of elements
Won the Nobel Prize in Physics in 1923
Marie Curie
1867-1934 Worked with husband
Pierre to study radioactivity
Isolated elements polonium and radium
Shared Nobel Prize in Physics 1903, won Nobel Prize in Chemistry 1911 for work with radioactive elements.
Ernest Rutherford 1871-1937 Investigated release and
measurement of different forms of radiation from elements (α, β, γ)
Gold foil experiment with Geiger and Marsden led to new model of an atom Model:ruther14.swf
Nobel Prize in Chemistry 1908
Model of the atom in the early 1900s
Nucleus: has positive charge, very small, but very dense Proton: same charge as electron, but opposite.
Mass=1.67262 x 10-24 g Vast empty space around nucleus contains
electrons
Where are the electrons? Niels Bohr: 1885-1962
Collaborated with Rutherford
Used emission spectra to propose Bohr model of atom 1913
Electrons found in specific ‘orbits’ around nucleus
Most of atom is empty space
Nobel Prize Physics 1922
James Chadwick 1891-1974 Discovered the
neutron and worked on fission of atoms
Discovery of neutron (mass = 1.67493 x 10-24 g) explained the mass problem of many atoms
Nobel Prize in Physics 1935
Current Model Based on the previous work, as well as
Einstein, Heisenberg, Born, Schrödinger and others
Nucleus contains p+ and no
Made of smaller particles called quarks uud, ddu (baryons)
Electron cloud e- are in areas of probability, predicted by wave
equations leptons