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Atomic Theory
Past and Present: pieces of a puzzle
The First Atomic Hypothesis
• Democritus (460 – 370 BC):
– Greek philosopher
– Speculated that matter is
composed of atoms which move
through empty space
– Atoms are solid, homogeneous,
indestructible and indivisible
– Different types of atoms have
different sizes and shapes.
– “Atomos” = indivisible
The Wrong Direction
• Aristotle (384 – 322 BC):
– Aristotle rejected Democritus’ ideas
and said empty space could not exist.
Instead, he believed that matter is only
made of earth, fire, air and water.
– Very influential philosopher of the
time whose ideas were accepted.
– His denial of the existence of atoms
went unchallenged for nearly 2000
years!!
The Early Atomic Hypotheses
• Boyle (1627-1691):
– In 1650, without proof, Boyle
stated that matter was
composed of atoms. He
discovered a gas law.
• Newton (1642-1727):
– In 1670, without proof,
stated that matter was
composed of atoms. He
discovered calculus and
g____.
Law of Conservation of Mass
• Lavoisier (1760):
– Founded modern chemistry
– Discovered 33 elements and
organized them into 4 categories
(Gases, Metals, Nonmetals,
Earths-chalk,clay,magnesia)
– Discovered combustion
– Discovered the Law of
Conservation of Mass
• Matter cannot be created nor
destroyed.
• Mass of reactants equals the mass
of products.
Law of Conservation of Mass
Mass is conserved during chemical reactions!
Law of Conservation of Mass
• In a chemical reaction atoms separate,
combine or rearrange, but, atoms are not
created, destroyed or divided in the process.
• The number of atoms of each type is the same
before or after the reaction. (see previous slide)
• The mass of the reactants is the same as the
mass of the products.
John Dalton’s Atomic Theory
• Dalton (1766-1844):
• John Dalton, a practicing Quaker
left school at age 11 and returned
at age 12 as a teacher in England
• In 1803 after studying the works
of Democritus and Boyle and
others, and performing numerous
experiments, he proposed his
atomic theory
• It was the first atomic theory
based on scientific evidence.
John Dalton’s Atomic Theory
1. Matter is composed of extremely small particles called atoms
2. Atoms are indivisible and indestructible
3. Atoms of a given element are identical in size, mass and
chemical properties
4. Atoms of a specific element are different from those of
another element
5. Different atoms combine in simple whole-number ratios to
form compounds
6. In a chemical reaction atoms are separated, combined or
rearranged
Law of Multiple Proportions: when 2 elements combine to
form different compounds in small whole number ratios
(predicted by Dalton’s theory)
(NH4)2Cr2O7 (orange)

 CrO3 (red)
Cr2O3 (green)
2.167 g : 1.000 g
1.083 g : 1.000 g
2.167 g : 1.083 g
2
:
1
Dmitri Mendeleev
• 1834-1907
He organized the 56 elements that were
known in 1869 according to similarities
and masses and noticed the periodicity
(repeating pattern) in their properties.
He was puzzled by inconsistencies like
Tellurium and Iodine (he thought Iodine
should be heavier) and attributed it to
inaccuracies in atomic mass
measurements of the time.
Because of this periodicity, he could
predict where unknown elements (not
yet discovered) would appear on the
periodic table.
• Scientists still thought the atom was the smallest
particle of matter for several decades until……….
Discovery of Electrons
• J.J. Thomson (1856-1940):
– In 1897, Thomson (English physicist)
conducted the Cathode Ray Tube
Experiment (CRT). (led to the invention of TV).
• Thomson discovered that cathode
rays were streams of negatively
charged particles.
• He later determined the charge to
mass ratio of an electron and found
the mass to be much smaller than a
H atom (lightest known atom).
• Thomson realizes Dalton is incorrect
and there are particles smaller than
an atom.
• He proposes the Plum Pudding
Model of the atom (a uniform,
positively charged sphere with e-)
Cathode Ray Tube
Henry Gwyn Jeffreys Moseley
1887-1915
• Henry Moseley
• British scientist
• discovered that atoms
contain a unique positive
charge in the nucleus which
exactly identifies the
element
• He named this positive
charge the atomic #
Electron’s Charge & Mass is Determined
Millikan (American physicist:18681953)
In the early 1910s, using the Oil Drop
Apparatus, Millikan discovered the
charge of an electron.
*
“so good was Millikan’s experimental setup
and technique that the charge he measured
almost 100 years ago is within 1% of the
currently accepted value” (Glencoe)
* Knowing the electron’s charge and
using Thompsons’ charge/mass ratio,
Millikan calculated the electron’s mass
at 9.1x10^-28g.
Millikan Oil Drop Experiment
The Gold Foil Experiment
The nucleus is discovered.
• The question “what accounts for the
mass of an atom if the electron mass is
so small?” was still unanswered.
• Rutherford (1871-1937):
– In 1911, Rutherford shot alpha particles
from Thorium at a thin layer of gold foil to
test Thompson’s model of the atom.
• Most went straight through
• Some deflected a little
• A few were deflected a lot (cannon ball off TP)
– Conclusion:
• Atoms are mostly empty space
• Atoms have a very small dense nucleus that is
positively charged. (<--This dot (if a nucleus)
would weigh as much as 70 automobiles!)
The Gold Foil Experiment
Neutron Discovered
• Chadwick ( a coworker of Rutherford)
– In 1932 he discovered the nucleus contains
another particle called the neutron by
bombarding Li, Be, and B with alpha
particles.
+
4 α
2
---------------> Li Target ------------> 10n
-
– The neutron has no charge and has a mass
1840 times heavier than that of an
electron
• Neutron Mass = 1.675 x10-24 g
• Neutron Charge = 0
Niels Bohr’s Atom
• Danish physicist working in
Rutherford’s laboratory in 1913
proposed a quantum model of the H
atom.
• Electrons orbit the nucleus in
orbits, like a solar system; multiple
energy levels
Planetary Model
Quantum Mechanical Model –
like a shot gun blast
.
•
•
•
•
•
Erwin Schrödinger 1887-1961
Born in Vienna, Austria
University of Berlin professor
Nobel Prize 1933
Based on waves of light and
probability.
Modern Atomic Structure
• Electron Cloud:
– Electrons located outside the nucleus
– Volume of atom is mostly empty space
• Most atoms’ radii = 1 x10-10 m = 1Ǻ
• Nucleus:
– Protons and neutrons are located in the nucleus.
– Most of the mass of the atom is in the nucleus.
• Nucleus size = 1 x10-15 m
Atomic Models
Atomic Size – So how big is it?
• A cubic cm of air (size of a
sugar cube) has 4.5 x 10^19
atoms!
• 45,000,000,000,000,000,000
atoms
• How many atoms would it
take to fill a universe?
http://www.youtube.com/watch?v=0fKBhvDju
y0&feature=player_embedded
IBM’s Scanning Tunneling
Electron Microscope
Quarks, Leptons, Hadrons………..
• No model can accurately represent
every known aspect of the atom!
• Youtube video: Exploring the Atomic Composition of Steel
http://www.youtube.com/watch?v=fz5gzQJq-0s
• Ted Talk (size of an atom)
http://ed.ted.com/lessons/just-how-small-is-an-atom