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Chem 1050
Chapters 2 & 3
Atomic Structure
What is matter made of?
(History of the Atom)
• Democritus – Greek philosopher
– said that the universe made up
of an infinite number of
indestructible atoms moving in
empty space
• Pretty profound for someone
who could not see an atom!
But as politics go:
• Aristotle said Matter was
continuous, infinitely
indivisible and made up of 4
elements…
• Earth, Air, Fire, Water
History of the Atom
• And chemistry as an earnest
and respectable science is
often said to date from 1661
when Robert Boyle of
Oxford published The
Sceptical Chymist
Dark Ages
• What happened with science
during this time?
Late 1700’s
• Antoine Lavoisier – came up
with law of conservation of
mass
• Called the father of modern
chemistry – quantified it
Industrial Revolution
• What made science leap ahead
during this time?
• Proust and Dalton – law of
definite proportions
• Henry Cavendish – broke down
H2O2 into H2O
John Dalton
• 1803 English Quaker
• Atomic Theory
• 1. All matter is composed of
extremely small particles called
atoms.
John Dalton
• 2. All atoms of a given element
are alike but atoms of one
element are different from
atoms of another.
• 3. Compounds are formed when
atoms of different elements
combine in fixed proportions.
John Dalton
• 4. A chemical reaction involves
rearranging of atoms. No atoms
are created or destroyed in a
chemical reaction.
John Dalton
• English schoolmaster, and a
Quaker, born 1766
• Published in 1808 1st
carefully reasoned arguments
to show that atoms must exist
John Dalton
• There in a short chapter (5
pages out of 900) in a book
entitled A New System of
Chemical Philosophy, Dalton
established his reputation.
John Dalton
• Dalton’s simple insight was that
at the root of all matter are
exceedingly tiny, irreducible
particles. Dalton’s contribution
was to consider the relative
sizes and characters of these
atoms and how they fit together.
John Dalton
• Most of his theory is still
acceptable
• But atoms are not all the same,
more on that later.
• The great Caltech Physicist
Richard Feynman once
observed that if you had to
reduce scientific history to one
important statement, it would
be “All things are made of
atoms.” They are everywhere
and constitute everything.
Look around you. It is all
atoms!
• And not just the solid things
like walls and furniture, but the
air in between. And they are
there in numbers that you really
cannot conceive! At sea level,
00C, one cm3 of air (a space the
size of a sugar cube) will
contain 45 billion billion
molecules. Every cm3 has
them!
• Atoms, in short, are very very
abundant!
• And they are very durable
• You are likely part of
Shakespeare and Genghis Khan,
but not Elvis yet!
• Why? 
• No one actually knows how
long atoms can survive, but it is
probably in the order of 1035
years!
• (a number I am happy to put in
scientific notation!)
Definition: Atom
• Neutral system of negatively
charged electrons moving
around a dense, positively
charged nucleus
• That definition is 20th century
stuff!
Atom
• Well, for the next century after
Dalton made his proposal, the
idea of the atom remained
entirely hypothetical and many
scientists doubted the existence
of atoms
Mid 1800’s
• 1869 to be exact a Russian
chemist by name of Dmitri
Ivanovich Mendeleev invented
the periodic table.
• It was organized in rows called
periods and in columns called
groups and organized by mass
• Why?
Mid 1800’s
• He left gaps in the table on
purpose. He knew there were
missing elements.
• Example
• Sc ……Ge
Late in the 1800’s:
• Experimenters used discharge
tubes (glass tubes with gases
and metal electrodes) – studied
conduction of electricity in
gases
• J.J. Thomson, of Cambridge,
England also used discharge
tubes in his studies
J.J. Thomson
• He put meaning into the nature
of the beams
• He called them corpuscles…
• And later called them electrons
J.J. Thomson
• Thomson found out the
properties of the e- beams
were… independent of the
materials used in the cathode
ray tubes
• What does that mean?
J.J. Thomson
• He concluded that are a
universal and fundamental part
of all matter
• Thomson announced the
discovery of the e- in 1897
• 1906 Nobel prize awarded
e
J.J. Thomson
• (demo)
• His experiments were
forerunners of…
• TV, X-rays, Oscilloscopes and
• Electron microscopes
Example of a gas discharge tube
J.J. Thomson
• He was unable to determine the
mass of e- but showed the
following ratio:
• e/m = 1.759 x 108C/g
• SI unit of charge is C, coulomb
Robert Millikan
• Univ. Chicago, 1909-1912
• Conducted a famous
experiment…called the
• Oil-drop experiment
• Millikan calculated the mass of
e-
Robert Millikan
• Me = 9.107 x 10-28g
• Think about this mass vs. the
mass of the proton and neutron!
Model of Millikan’s Oil Drop
Experiment
Also in the late 1800’s:
• In 1895, Wilhelm Roentgen
discovered X-rays at the
University of Wurtzburg in
Germany.
• And in that same year,
• Henri Bacquerel discovered
radioactivity.
Also in the late 1800’s:
• This allowed Eugen Goldstein
in 1886 to study positive rays
• Goldstein worked with a
positive ion, a helium atom
stripped of 2 e• Called alpha particles
Also in the late 1800’s:
• Alpha particles were used in a
crucial experiment – which
provided us with the modern
concept of the atom.
• J.J. Thomson concentrated on e• Mystery was inner atom
J.J. Thomson
• J.J. Thomson’s theory on the
makeup of the atom – “plum
pudding model”
• (diagram on board)
Earnest Rutherford
• Ernest Rutherford born in New
Zealand and educated in
England and Canada (Univ.
Manchester)
• Bombarded gold foil with alpha
particles
Earnest Rutherford
• In 1910, he and his assistant,
Hans Geiger (remember what
he is known for?), watched for
changes in direction as alpha
particles passed through the
gold foil.
Gold Foil Experiment
• Most had no deflection,
consistent with Thomson’s
model, but…
• Some had massive
deflections!…
Gold Foil Experiment
• Rutherford said it was as if he
had fired a 15 inch shell at a
sheet of paper and it rebounded
into his lap!
Gold Foil Experiment
• After considerable reflection, he
realized there could be only one
possible explanation:
• The particles that bounced back
were striking something small
and dense at the heart of the
atom…
Gold Foil Experiment
• While the other particles sailed
through unimpeded.
• Rutherford realized that the
atom was mostly empty space,
with a very dense nucleus at the
center.
Model of Rutherford’s Gold
Foil Experiment
Diagram showing alpha deflections
in Plum Pudding vs nucleus theories
Gold Foil Experiment
• This was a most gratifying
discovery, but it presented one
immediate problem:
• By all the laws of conventional
physics, atoms shouldn’t exist!
• Why?
Rutherford
• Thomson’s model was wrong
and was abandoned in favor of
the nuclear atom
• nuclear atom – small massive
positively charged nucleus
surrounded by negatively
charged electrons
Rutherford
• He published his findings in
1911, most influential of
scientific achievements!
• He was awarded the Nobel
prize in Chemistry in 1922, a
year after Einstein.
Nuclear Atom
• (+) charge on proton must = (-)
charge on e
• Therefore…
• The number of e- in a neutral
atom must = number of protons
in its nucleus
Nuclear Atom
• This is called the atomic
number, symbolized by Z (in
college)
Nuclear Atom
• Rutherford suggested in 1920
that each nucleus should
contain electrically neutral
particles to account for missing
mass…
• And why the nuclei didn’t blow
up!
Nuclear Atom
• He saw that the protons must be
offset by some type of
neutralizing particles, which he
called neutrons
• The idea was simple and
appealing, but not easy to
prove.
Nuclear Atom
• Rutherford’s associate, James
Chadwick, devoted 11 intensive
years to hunting for neutrons
before finally succeeding in
1932.
• Chadwick was awarded the
Nobel Prize in Physics in 1935.
Nuclear Atom
• As it turned out historically, it
was a good thing it took a long
time for the discovery of the
neutron as it was essential for
the development of the atomic
bomb.
Nuclear Atom
• Had it been discovered in the
1920s, its noted, it is very likely
the atomic bomb would have
been developed first in Europe,
undoubtedly by the Germans.
• Science of chemistry deals with
electrons
• Where are they in your body?
• Where are the protons?
• Where are the electrons?
Mass Number
• Atomic mass unit (amu) defined
as 1/12 mass of carbon-12 atom
• A precise instrument called a mass
spectrometer has been used since
1920 to determine masses of
atoms.
Mass Number
• Practical purposes:
• 1 proton = 1 neutron =1 amu
• Cathedral vs the fly example
+
• p = 2000 lb = 1 ton
• n0 = 2000 lb = 1 ton
• e-1 = 1 lb
Mass Number
• Therefore mass number = total
number of protons + neutrons
Isotopes
• All atoms of a given element
have the same atomic number
but they may have different
mass numbers because of
different numbers of neutrons
Isotopes
• Atoms with same atomic
number but with different mass
numbers are called isotopes
• [Apples in a grocery store
example]
• Apples are alike – why?
• What makes them different?
Atomic Mass
• It is reasonable to assume mass
of an atom is concentrated in
nucleus
• [remember the Cathedral and
fly example]
Atomic Mass
• Remember that 1 proton = 1
neutron = 1 amu
• But counting just the amu
ignores the existence of
isotopes
Atomic Mass
• Atomic mass is a weighted
average of masses of all
isotopes of a given element
• So, you are asking how do we
calculate isotopes?
•
• [Example on board]
End of Ch 2 & 3
