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Chapter 4
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History of Atomic Theory and Structure
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https://youtu.be/thnDxFdkzZs
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Democritis (460-370 BC)
 First to say that matter is not infinitely divisible
 Ideas
▪ Atoms are tiny, solid, homogenous, indestructible, and
indivisible
▪ Different kinds of atoms have different sizes and shapes
▪ Size, shape, and movements of atoms determines the
properties of matter
▪ Matter is made of atoms, which move through empty
space
 Couldn’t explain how atoms were held together
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Aristotle (384-322 BC)
 One of Democritus’s biggest critics
 Rejected Democritus’s theory because it didn’t fit
with his own understanding of nature
 Ideas
▪ Empty space cannot exist
▪ Matter is made of earth, fire, air, and water
 Influence was so great that the theory of atoms
was rejected by everyone for almost 2000 years!
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John Dalton (1766-1844)
 Ideas (Dalton’s Atomic Theory)
▪
▪
▪
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All matter is made up of atoms
Atoms are indivisible and indestructible
Atoms of an element are identical
Atoms of one element are different from those of another
element
▪ Unlike atoms combine in simple whole-number ratios to form
compounds
▪ In a reaction, atoms are separated, combined or rearranged
▪ Law of Conservation of Mass: Matter cannot be created or destroyed,
only changed in form
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J.J. Thomson (1856-1940)
 First person discover a subatomic (smaller than an
atom) particle – the electron!
 Ideas
▪ Atoms are made up of smaller parts
▪ Some parts are negative
▪ Since atoms are neutral, the rest of the atom must be
positively charged
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The Plum Pudding Model (1904)
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In 1897, Thomson and his team showed that
atoms were made up of smaller parts through
his experiments with a cathode ray
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Robert Millikan (1868-1953)
 Discovered the charge and mass of an electron
▪ Charge is [1-]
▪ Mass is 9.1 x 10-28g or 1/1840 the mass of a hydrogen atom
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In 1909, Millikan devised the oil-drop
experiment and found the charge and mass
of an electron
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Lord Ernest Rutherford (1871-1937)
 Discovered the nucleus and its positive charge
 Ideas
▪ Atoms are mostly empty space
▪ There is a positively charged nucleus at the center of the
atom made up of protons
▪ The negatively charged electrons orbit the positively
charged nucleus randomly
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The Planetary Model (1911)
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Alpha particles (α) are fired at an extremely
thin sheet of gold
Most α-particles passed through, but some
were deflected
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Niels Bohr (1885-1962)
 Studied the behavior of electrons and discovered
electron shells
 Ideas
▪ Electrons orbit the nucleus in fixed, circular orbits
▪ The paths for these orbits are the same distance away
from the nucleus for every kind of element
▪ Electrons can move back and forth from one shell to the
next shell
 He discovered his model by measuring the
frequency of light given off by energized particles
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The Shell Model
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Louis de Broglie (1892-1987)
 Predicted that matter (especially electrons) could behave
like particles and waves
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Werner Heisenberg (1901-1976)
 Formed a new model of the atom using quantum
mechanics
 Heisenberg’s Uncertainty Principle
▪ Determined it is impossible to know both the
momentum and location of an object simultaneously
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Erwin Schrödinger (1887-1961)
 Formed a new model of the atom using
de Broglie’s wave model
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In 1926-27, Heisenberg and Schrödinger,
working independently, came up with new,
similar models for the atom
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James Chadwick (1891-1974)
 Discovered the neutron, which helped explain
isotopes’ different masses
 Decided the unit of measurement of atomic
weight was the atomic mass unit or amu
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Developed in 1932, the atomic model finally
includes neutrons!
Thomson
Millikan
Rutherford
Bohr
de Broglie
Heisenberg
Schrödinger
Chadwick
1909
1911
1913
1923
1927
1926
1932
Democritus
Dalton
1897
~400 BC 1808