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Review of Ancient Models of Matter
 Leucippus and Democritus - the
idea of the “atom” – the indivisible
particle
 Empedocles and Aristotle - the 4element model
 Aristotle, Aquinas and The Church
– the 4-element model persists for
over 2000 years.
 The Alchemists – develop new
chemical lab equipment and
techniques but do not apply the
Scientific Method
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Enter the Scientists
 The great breakthroughs made by Galileo,
Copernicus and Isaac Newton made way for a
revolution in pure science in Europe.
 Great scientists such as Boyle, Lavoisier, Priestly
and Scheele began to make strides in the field of
Chemistry.
 Scientists such as Francis Bacon and Rene
Descartes developed the process we know as the
Scientific Method.
 The first scientist to apply scientific methodology to
the study of matter and atoms was John Dalton
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John Dalton
 Dalton was a schoolteacher and later a
professor of Mathematics and Natural
philosophy (Science)
 His strongest interests were in the field of
Meteorology (Dalton’s Law of Partial
Pressures) but he also made contributions in
Chemistry
 He began to make measurements of the
masses of elements and created a simple
table of elements.
 He then went on to conduct experiments with
chemical reactions and determined the
masses and ratios of elements in the newly
formed compounds.
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Dalton’s Chemical Formulae
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Dalton’s Atomic Theory
 In 1803, Dalton suggested an atom is a
solid featureless sphere.
 Billiard Ball Model of the Atom
 Dalton made two assertions about
atoms:
1) atoms of each element are all
identical to one another but different
from the atoms of all other elements
2) atoms of different elements can
combine to form more complex
substances.
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Five main points of Dalton's atomic theory
1)
2)
3)
4)
5)
Elements are made of extremely small particles called
atoms.
Atoms of a given element are identical in size, mass, and
other properties; atoms of different elements differ in
size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple wholenumber ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or
rearranged.
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Problems with Dalton’s Model
 Atoms exhibit electrical properties –
charged atoms, static electricity, etc…
 The true nature of atoms could not be
determined since atoms were extremely
small and no known methods of
testing individual atoms was known
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J.J. Thomson
 In 1897, J.J. Thomson conducted experiments
using a cathode ray tube.
 Thomson’s was able to create a stream of
particles that flowed through the tube and
fluoresced when they hit a detectoe screen
 Using magnets Thomson was able to bend the
flow of particles.
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JJ Thomson’s Model of the Atom
 The particles were called “electrons” because of their
electrical properties
 Electrons were determined to have very small masses
and a negative charge (they were repelled by the
negative force of a magnet)
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J.J. Thomson’s Model of the Atom
 Atoms contain particles called electrons
 Electrons have a small mass and a negative
charge
 The rest of the atom is a sphere of positive
charge
 Electrons are embedded in this sphere, so that
the resulting atoms are neutral or uncharged
 Thomson compared the (negative) electrons to
plums embedded in a positively charged
pudding
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J.J. Thomson’s Model of the Atom
 Hence the Plum Pudding Model or Raisin Bun
Model
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Ernest Rutherford
 In 1895, the German scientist Roentgen
discovered X-rays and started a rush of work to
study the electromagnetic radiation emitted by
radioactive elements.
 In the early part of the 20th century, many
scientists such as the Curies, Becquerel, Geiger,
and Rutherford studied these invisible rays.
 Scientists began using these minute waves to
study the properties of atoms.
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Ernest Rutherford
 Rutherford decided to test the model of his
mentor, Thomson.
 In 1911, Ernest Rutherford conducted his famous
gold foil experiment.
 He aimed alpha particles (positively charged
particles) at a thin sheet of gold foil
 PREDICTION: Particles would pass straight
through the gold foil
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The Gold Foil Experiment
PREDICTED
OBSERVATION: A small number of
alpha particles bounced almost
straight back from the gold foil
OBSERVED
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Ernest Rutherford’s Model of the Atom
 An atom has a tiny, dense, positive
core called the nucleus (which
deflected the alpha particles and
contains protons)
 The nucleus is surrounded mostly by
empty space, containing rapidly
moving negative electrons (through
which the alpha particles passed
unhindered)
Ernest Rutherford
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Ernest Rutherford’s Model of the Atom
 Rutherford’s atom had a tiny, dense, positive
nucleus orbited by electrons. Most of the atom’s
volume is empty space
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James Chadwick
 In 1932, Chadwick conducted
experiments that suggested
that at least half of the mass
of the nucleus was
unaccounted for.
 Chadwick’s research
suggested the existence of a
neutral particle in the nucleus
with similar mass to the
proton.
James Chadwick – the real
Jimmy Neutron
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Updated Model of the Atom
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Modern Development of the
Model of the Atom
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A Problem with the Planetary Model
 Classical Newtonian Physics suggest that any
object travelling in a curved path (even if it is
moving at a constant speed) is said to be
accelerating.
 An electron accelerating around the nucleus
would continuously emit electromagnetic
radiation and lose energy
 Therefore, it would eventually fall into the
nucleus and the atom would collapse
 However, not consistent with real-world
observations – atoms are stable
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Learning Checkpoint
Assigned Questions
p. 15 UC # 1, 2, 3, 4, 5, 6
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