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 is made of small, indivisible particles called atoma
Proposed that the behavior of substances were due to the type of atom
Alchemists
Great many 'experiments' to try to convert one element to another
Many discoveries, but very secretive
Robert Boyle
Robert Boyle: 1627-1691 Performed experiments with controls and
published detailed results 1661: Published claiming that Aristotle's ideas
were incorrect, and that some 'elements' (like mercury) did not contain the 4 Aristotlean elements (earth, air, fire, water)
Antoine Lavoisier
Lavoisier: 1743-1794 Experimented and established the Law of
Conservation of Mass Investigated combustion and cellular respiration
in terms of energy and mass
Charles de Coulomb
Coulomb 1736-1806 Developed Coulomb's Law, the definition of
electrostatic force
Joseph Proust
Proust: 1754-1826 Controversy with another scientist (Berthollet)
led him to discovery of the Law of Definite Proportions (sometimes called Proust's Law)
A chemical compound always contains the same proportion of elements by mass
There are exceptions, which are called non-stoichiometric (or Berthollide) compounds
John Dalton
Dalton: 1766-1844 Law of Multiple Proportions (1803): elements
combine in mass ratios of small whole numbers. Elements may combine with the same element in different proportions to make a different compound.
Atomic theory contains 4 hypotheses
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.
James Clerk Maxwell
1831-1879 Showed that
electricity and magnetism were related, and were related to atoms
Predicted that accelerating charges would make waves (electromagnetic radiation)
Cathode Ray Tube Experiments
Glass tube with wire at each end; as much air pumped out as possible
Charge passed across tube makes fluorescent glow
William Crookes Tube coated with fluorescent material can be made
to glow in one focused dot Rays travel in straight lines
Ray carries negative charge
Joseph John Thomson 1856-1940 Used a study of the
cathode ray tube to determine the presence of electrons 1897
Suggested the plum pudding model of the atom and the existance of isotopes
Won the Nobel Prize in Physics in 1906
J. J. Thomson’s 4 Experiments Used magnetic field to show cathode rays had
negative charge Used electric charge to show cathode rays were
particles with negative charge Used varying electric currents to determine charge to
mass ratio Either particles very small or Carried huge charge
Evidence suggested particles very small and came from atom
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
Wilhelm Röntgen 1825-1943 Rays produced by
cathode ray tube interaction with other matter could pass through objects, affect photographic plates and cause some materials to fluoresce (X-rays)
Nobel Prize in Physics in 1901
Antoine Henri Becquerel 1852-1908 Investigated possible link
between X-rays and natural phosphorescence
Discovered that the rays emitted by uranium were different from X-rays
Nobel Prize in Physics 1903 (shared with Pierre and Marie Curie)
Marie Curie 1867-1934 Worked with huband
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 1831-1937 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 with dense central nucleus and vast empty areas
Nobel Prize in Chemistry 1938
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
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
Mass and Atomic Number Each element's atoms have a specific number of
protons in the nucleus (Z=atomic number) For a neutral atom, the number of protons is
equal to the number of protons The mass number (A) is the number of protons
plus the number of neutrons (the number of particles in the nucleus).
Some atoms of the same element have different A values. This means they have different numbers of neutrons. They are called isotopes.
Isotope notation Isotope notation is
used to represent an isotope.
Z, A and symbol of element are represented.
Isotope notation can be used to determine the number of protons, neutrons and electrons in an isotope.
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