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THE DEVELOPMENT OF ATOMIC THEORY
THE SCIENTISTS BEHIND THE ATOM
JOHN DALTON
• Sept 1776- Jul 1844
• English chemist
• Best known for his fivepoint atomic theory first
published in 1805.
Five main points of Dalton's atomic theory:
 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.
J.J. THOMSON
• Dec 1856- Aug 1940
• Sir Joseph John "J. J."
Thomson was a British
physicist
• He is credited with
discovering electrons and
isotopes
DISCOVERY OF THE ELECTRON
 Around 1897 Thomson conducted experiments using a
cathode ray tube.
 A cathode ray
tube is a vacuum
with electrodes at
each end. A current
is passed through
the electrodes
which causes electrons to be emitted.
DISCOVERY OF ISOTOPES
• Thomson is also credited with the discovery of the
isotope in 1912.
• An isotope has the same number of protons and
electrons as the original element, but it has a different
number of neutrons
• Every element has isotopes
• For example, Carbon has 3 main isotopes: Carbon-12,
Carbon-13, and Carbon-14
ISOTOPES
• The atomic masses on the periodic table are
‘isotopic averages’
• This means they take all the isotopes’ mass and
average them together.
• To find an isotopic average:
• First multiply the isotope’s mass by its percent abundance
• Do this for all the isotopes
• Then add your answers together
ISOTOPES
• Example:
• C-12
• C-13
• C-14
12 x
94% =
11.28
13
x
5% =
0.65
14
x
1% =
0.14
• Total: 11.28 + 0.65 + 0.14 = 12.07
PLUM PUDDING MODEL
 Thomson believed the atom was composed of electrons
surrounded by a soup of positive charge
 This model is known as “the plum pudding model” - the
idea being that the negatively charged electrons are
like "plums" surrounded by positively charged
"pudding".
PLUM PUDDING MODEL
ERNEST RUTHERFORD
• Aug 1871- Oct 1937
• Ernest Rutherford was a
New Zealand-born
British chemist and
physicist who became
known as the father of
nuclear physics
GOLD FOIL EXPERIMENT
• 1909
• Rutherford believed Thomson’s view of the atom
as ‘plum pudding’ was inaccurate based on his
experiments with alpha particles and gold foil.
• Alpha particle = 2 protons and 2 neutrons
• They can be produced during some forms of
radioactive decay.
GOLD FOIL EXPERIMENT
In his experiment Rutherford would aim a beam
of alpha particles at a piece of gold foil
A screen was placed behind the foil as a
backdrop for the alpha particles to appear
upon.
GOLD FOIL EXPERIMENT
• If Thomson’s plum pudding model was accurate
the tiny alpha particles would pass through the
gold atoms and fly straight into the screen.
• What he noticed, however, was that the alpha
particles did not fly straight, but were deflected,
sometimes at extreme angles.
GOLD FOIL EXPERIMENT
• Some of the alpha particles flew back straight at
the alpha emitter.
• Rutherford explained that to see an alpha
particle reflected off of the gold foil was “as if
you fired a 15-inch shell at a sheet of tissue
paper and it came back to hit you.”
GOLD FOIL EXPERIMENT
• This led to his conclusion
that there was a
positively charged
center to atoms which
contained most of the
atom’s mass. He termed
this region the nucleus.
NIELS BOHR
• 1885-1962
• Bohr believed that
Rutherford’s model of the
atom was incomplete
• If the nucleus is positive and
the electrons are just floating
around then why don’t they
cling to the nucleus?
NIELS BOHR
• Bohr believed the electrons in an atom traveled on
specific rings around the nucleus.
NIELS BOHR
• He believed that
for an electron to
move from its ring
it would need to
gain energy
ERWIN SCHRODINGER
• Aug 1887 – Jan 1961
• In 1926 Schrödinger
used mathematical
equations to describe
the likelihood of
finding an electron in
a certain position.
ERWIN SCHRODINGER
• Unlike the Bohr model,
the quantum mechanical
model does not define
the exact path of an
electron, but rather,
predicts the odds of
finding an electron in
any particular location.
JAMES CHADWICK
• Oct 1891- Jul 1974
• In 1932 he conducted
experiments which proved the
existence of a neutral
subatomic particle similar in
mass to a proton.
• Chadwick named this particle
a Neutron.