Atomic Structure
Chemical Discoveries
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Democritus - a Greek philosopher that
proposed that matter is composed of
small indivisible particles called atoms.
Alchemy
Alchemy dominated the next 2000
years of chemical history.
 Alchemists were often mystics
obsessed with turning cheap metals
into gold.
 Several elements were discovered
during this period.
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Robert Boyle (1627-1691)
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The first “chemist” to perform truly
quantitative experiments.
He measured the relationship between the
pressure and volume of air.
Established the accepted
definition of an element.
An Englishman.
Joseph Priestly (1733-1804)
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An English clergyman and scientist.
His interest in combustion reactions lead to
the discovery and identity of oxygen and
several other gases.
Antonie Lavoisier
(1743-1794)
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Known as the Father of Chemistry.
He brought chemistry out of the
misconceptions of the 18th century into
more modern ideas such as the importance
of accurate measurement and the
composition of the atmosphere.
He formulated the Law of Conservation of
Mass.
He was a Frenchman.
John Dalton
(1766-1844)
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An English teacher,
meteorologist,
and chemist.
His interest lead to a study of gases and the
Law of Partial Pressure.
He developed the Law of Multiple
Proportions
He proposed the first meaningful atomic
theory.
Dalton’s Atomic Theory
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John Dalton performed
experiments to study the
ratio in which elements
combine in chemical
reactions
He then formulated a
hypotheses and theories
to explain his
observations
Daltons Atomic Theory
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All elements are composed of indivisible
particles called ATOMS
Atoms of the same ELEMENT are identical
ATOMS of any one ELEMENT are different from
those of any other element
Atoms of different elements can physically mix
together or chemically combine to form
compounds
Chemical reactions occur when atoms are
joined, separated or rearranged
Atoms of one element are never changed into
atoms of another by a chemical reaction
What did Dalton base his theory
on?
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Law of Conservation of Mass (Antoine Lavosier,
1789)
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Law of Definite Proportion, (Proust's Law)
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Atoms are neither created or destroyed (under normal
chemical reactions)
Atom ratio is fixed, so mass must be constant.
Law of Multiple Proportions (Dalton)
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Applies where two elements, A and B, form more than
one compound.
How Big is an Atom
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Imagine a penny, made up of copper (Cu)
If we ground up the penny into dust, the
dust would still have the same properties
as copper
If you placed atoms of copper in a row,
100 000 000 atoms would make a line 1
centimeter long
1
2
3
4
5
How Big is an Atom?
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A penny contains around 2.4 x 1022 atoms
of copper (Cu)
Can we see atoms?
To see atoms we need a scanning
tunneling electron microscope or an ion
beam microscope
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A micrograph of a NickelMolybdenum (Ni4Mo) bimetallic compound, each dot
is a single atom
The specimen is in the form
of a needle that is over 1000
times sharper than an
ordinary household sewing
needle. In fact, the end of
the needle is so sharp that it
cannot be seen by the naked
eye or even a standard
optical microscope
http://www.ornl.gov
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Atomic Diameter Units
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The typical atom has a diameter on the order of
1 - 3 x 10-10 m
Angstrom
(Å)
(non-standard)
10-10 m
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Nanometers
(nm)
10-9 m
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Picometers
(pm)
10-12 m
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Typical atom diameter = 1 - 3 Å, 0.1 – 0.3 nm or
100 – 300 pm
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What’s Wrong with DAT?
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Most of Dalton’s theory is accepted today
However, We now know that atoms ARE
divisible
Atoms contain sub-atomic particles
ELECTRONS
PROTONS
NEUTRONS
Electrons
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Discovered in 1887 by J. J. Thomson
Thompson initially called electrons CATHODE RAYS
Negative
Positive
Vacuum
High Voltage
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Thomson then added electrical plates above
and below the tube
He found that positively charged plates
ATTRACTED the ‘ray’, negatively charged
plates repel
Cathode ray made up of tiny negatively
charged particles
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1900 – Electron defined as a small
charged particle approximately 1/2000 the
mass of a Hydrogen atom
1916 Robert A. Millikan determined the
charge carried by an electron and the
charge to mass ratio
Look up Milikan’s Oil Drop Experiment
The Electron Today
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Symbol
Relative mass
(compared to a proton)
Actual mass
Relative electrical charge
Actual electrical charge
e1/1840
9.11 x 10-28g
1-1.602 x 10-19 C
Protons
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1866 E. Goldstein found evidence of
positively charged particles traveling in the
opposite direction to electrons in a
cathode ray tube
He named them CANAL RAYS
He deduced that canal rays were positively
charged particles we now know as
PROTONS
Proton Summary
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Symbol
Relative mass
Actual mass
p
1
1.67 x 10-24g
(1840 x me)
Relative electrical charge
1+
Actual electrical charge
+1.602 x 10-19 C
Neutrons
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1932 James Chadwick discovers the neutron
Symbol
Relative mass
Actual mass
Relative electrical charge
Actual electrical charge
n
1
1.67 x 10-24g
(1840 x me)
0
0C
Rutherfords Experiment
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1911, University of Manchester, England
A beam of a-particles (a Helium nucleus He –
2e-) aimed at a gold foil
Foil surrounded by fluorescent screen which
flashed when hit by an a-particle
Experiment done in a vacuum
Vacuum
Flourescent Screen
Au Foil
a-source
c. Rutherford Appleton Laboratories
Rutherford’s Results
Conclusions
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Rutherford concluded that the atom is mostly
empty space
However, the atom contains a nucleus where
the mass and positive charge of the atom
are concentrated
We now know that the nucleus contains
PROTONS and NEUTRONS
Electrons ORBIT the nucleus but are not
part of it
Review
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What do you think of these statements?
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All atoms are identical
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Chemical Reactions occur when atoms of one
element change into atoms of another
An atom has protons, neutrons and electrons in
the nucleus