Atomic Theory
Let’s Take a Trip Through Time!
Democritus
460 – 370 B.C.
• There are various basic
elements from which all
matter is made
• Everything is composed
of small atoms moving in
a void
• Some atoms are round,
pointy, oily, have hooks,
etc. to account for their
properties
• Ideas rejected by leading
philosophers because
void = no existence
First Concept of an Atom
John Dalton
1766-1844
• Each element is composed of
extremely small particles
called atoms
• All the atoms of a given
element are identical, but they
differ from those of any other
element
• Atoms are neither created nor
destroyed in any chemical
reaction
• A given compound always has
the same relative numbers and
kinds of atoms
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small particles
called atoms.
2. All atoms of a given element are identical, having the
same size, mass and chemical properties. The atoms of
one element are different from the atoms of all other
elements.
3. Compounds are composed of atoms of more than one
element. In any compound, the ratio of the numbers of
atoms of any two of the elements present is either an
integer or a simple fraction (Law of Definite Proportion).
4. A chemical reaction involves only the separation,
combination, or rearrangement of atoms; it does not
result in their creation or destruction.
6
Dalton’s Model
Law of Multiple Proportions
8
Law of Conservation of Mass
16 X
+
8Y
8 X2 Y
9
Modern Atomic Theory
• “Atoms cannot be subdivided, created, or destroyed.”
– Atoms can be destroyed via nuclear reactions but
not by chemical reactions.
• “Atoms of a given element are identical* in size,
mass, and other properties;”
– There are different kinds of atoms (differing by
their masses) within an element that are known as
"isotopes“.
J.J. Thomson(Discovery of Electron)
1856-1940
• Discovered electron
1897 – Cathode Ray
Experiment
• Plum Pudding model
1904
– Electrons in a soup of
positive charges
• Discovered isotopes
1913
Discovery of Electron
• J. J. Thomson(1897)
– British physicist at
Cambridge University
• Cathode Ray Experiment Hypotheses
– Cathode rays are charged particles (which he
called "corpuscles").
– These corpuscles are constituents of the
atom.
• Observation:
-the rays bended with a magnet
-the electrometer measured a large amount of negative
charge
Cathode Ray Tube
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
13
The Electron
•
•
•
•
•
J.J. Thomson 1897 - CRT experiment
Negative Charge
eActual Mass: 9.11 x 10-28g
Relative mass information - discovered later
1/1840 the mass of proton
Plum Pudding Model
Millikan’s Experiment
Measured mass of e(1923 Nobel Prize in Physics)
e- charge = -1.60 x 10-19 C
Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
16
Ernest Rutherford
1871-1937
• Nucleus Theory 1910
– alpha particle gold foil
experiment
• An atom’s mass is
mostly in the nucleus
• The nucleus has a
positive charge
(Moseley)
• Electrons in fixed orbit
Rutherford Gold Foil Experiment
• Bombard the gold foil with alpha particles,
positively charge particles w/ 4x of the mass of H
• Only expect
slight deflection
• Evidence:
Wide-angle
deflection
Click for Interactive Demonstration
Experiment Conclusion
• atoms positive charge is concentrated in the
nucleus
• Volume of a nucleus is very small compared
with the total volume of an atom
• The nucleus is heavier than electron
• Suggested that the atom might resemble a tiny
solar system, with a massive, positively
charged center circled by only a few electrons
• proton (p) has opposite(+)charge of electron (-)
• mass of p is 1840 x mass of e- (1.67 x 10-24 g)
Rutherford Model
James Chadwick
1891-1974
• Worked with Rutherford
• Interpreted work of the
Curies
• Discovered Neutron
1932
• Nobel Prize in Physics
1935
Chadwick’s Experiment (1932)
(1935 Noble Prize in Physics)
H atoms - 1 p; He atoms - 2 p
mass He/mass H should = 2
measured mass He/mass H = 4
a + 9Be
1n
+ 12C + energy
neutron (n) is neutral (charge = 0)
n mass ~ p mass = 1.67 x 10-24 g
22
The Neutron
•
•
•
•
•
Chadwick 1932 - nuclear bombardment
No charge
n0
Actual Mass: 1.67 x 10-24g
Relative Mass: 1
Niels Bohr
1885-1962
• Planetary Model 1913
– Nucleus surrounded by
orbiting electrons at
different energy levels
– Electrons have definite
orbits
• Utilized Planck’s
Quantum Energy theory
• Worked on the Manhattan
Project (US atomic bomb)
Bohr’s Model
Bohr Model for Nitrogen
Ernst Schrödinger 1887-1961
Werner Heisenberg 1901-1976
• Quantum Mechanical
Model 1926
– Electrons are in probability
zones called “orbitals”, not
orbits and the location
cannot be pinpointed
– Electrons are particles and
waves at the same time
– Developed quantum
numbers based on theories
of Einstein and Planck
Orbitals
Quantum Mechanical Theory
Electron in a Hydrogen atom
Atomic Theories Timeline
Name
Time
Frame
Key
Points
Historical
Events
Democritus
Dalton
Thomson
Rutherford
Bohr
Schrödinger
Heisenberg
Atomic Theory
JJ
Thomson
Democratus
400 BC
1803
John
Dalton
1904
Niels
Bohr
1910
Ernest
Rutherford
1913
1926
Schroedinger
/ Heisenberg