Atomic
Structure
An Introduction to Atomic Theory:
Empirical Evidence of the ATOM
Instructional
Goals
• Define Dalton’s atomic theory
• Differentiate between historical atomic
models
• Describe the tentative nature of science.
Overview
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Leucippus, Democritus and Aristotle
John Dalton
J.J. Thompson
Ernst Rutherford
Neils Bohr
Modern Quantum Mechanical Model of
the atom
Leucippus 480-420 BC
• He was a natural philosopher.
• He thought matter to be finite.
• His student was Democritus.
• Only the quotes others have of
Leucippus and Democritus been
found.
Democritus 460-371 BC
He believed matter is
finite.
Finite means having
boundaries or limits,
like “No refills.”
Democritus’ Hypothesized:
If matter is cut again
and again, eventually a
single indivisible part
will remain. This part
called atomos is Greek
for indivisible. His
idea of matter gave us
the word ATOM.
According to Aristotle
• Matter was continuous
or infinitely smaller .
• Waterfalls produce
smaller and smaller
particles we call mist.
• Four basic elements”
Earth, Fire, Water and
Air.
Aristotle 384-322 BC
• His works were
rediscovered in
Western Europe
around 1200 AD.
• Aristotle’s theory
about the atom was
widely accepted and
promoted by the
Catholic Church for
generations.
John Dalton 1766-1844
• Was an English school teacher who
independently studied the nature of matter.
• Recorded observations of his experiments.
• Proposed the first atomic theory based on
empirical evidence.
Chemical Heritage John Dalton
11:36 Minutes of Chemical History
http://maple.lemoyne.edu/~giunta/dalton.html
http://www.tannerm.com/index.htm
Dalton’s Theory
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All elements are composed of submicroscopic
indivisible particles called atoms.
Atoms of the same element are identical.
Atoms of different elements can mix together,
or chemically combine with one another to
form compounds.
Chemical reactions occur when atoms are
separated, combined, or rearranged, but
atoms of one element do not change into
another element.
Dalton’s Model of the atom
History Summary
Philosophers
Democritus
Aristotle
Scientist:
Dalton
Philosophy
Highlights
Matter is finite:
composed of tiny
invisible, indestructible
particles.
His ideas agreed
with later theories,
but lacked
experimental
support.
Matter is infinite:
His idea or matter
composed of basic
was not supported by
elements: earth, air,
later experiments.
water, and fire.
Scientific theory- Matter
Performed
finite: is composed of
experiments to
indivisible particles
support his theory.
called “atoms.”
Wilhelm Roentgen 1895
• Covers Crooke’s tube with thick black
paper and a ZnS screen nearby begins to
fluoresce! The hand placed behind the
screen shows a pattern of his BONES!
• He names the mysterious “X” rays
publishes a paper and receives Nobel prize
in physics in 1901.
Roentgen’s Discovery
• Wilhelm’s x-ray of his
wife’s hand.
• Later realized it was
educational and useful in
medical diagnosis.
• X-rays and gamma rays
are indistinguishable.
Henri Becquerel 1896
• Uranium salt crystal left to “charge” in
sunlight were then exposed to a photographic
plate covered by black paper.
• Result: U.V. Light produced fluorescence in
the salt and it is captured using a
photographic plate.
• a shadow is seen due to fluorescent light
reacting with emulsion .
• Cloudy weather prevented the normal
charging for several days.
• Surprise! subsequent exposure!
A copper cross placed in between the uranium
salts and the black paper covered
photographic plate reveals the mysterious "U"
rays have not penetrated the cross but have
affected the photographic emulsion!
The discovery of the Electron
• J.J. Thomson (1856-1940) pumped out most of the
air in the cathode ray tube and used results from
(commonly abbreviated CRT) experiments to
discover the electron.
• Cathode Ray Tube explanation and animation
• Best ever vintage demos
You Tube Cathode Ray Tube
J.J. Thompson’s Experiments
Thomson devised a cathode ray tube with a paddle
wheel built inside. When the high voltage
electricity was turned on the paddle wheel began
to rotate and move away from the cathode and
towards the anode.
J.J. Thompson’s Experiments
Thomson devised another cathode ray tube with positive
and negatively charged plates. Thomson observed that
the particles were being repelled by the negatively
charged plate and attracted to the positive plate.
http://lausd.k12.ca.us/~kmcmahon/Atoms%20%26%20Period%20Table/page_id_4745.html
Magnetic vs Electric field
deflection ratio
J. J. Thomson's experiment resulted
in a value for e/m (the ratio of the
charge of the electron to the mass of
the electron).
He used a combination of a magnet
and charged plates to figure out the
ratio.
Millikan’s Oil Drop Experiment
http://webphysics.davidson.edu/applets/pqp_preview/cont
ents/pqp_errata/cd_errata_fixes/section4_5.html
Thompson’s Ions Mine
In the dusty lab’ratory,
‘Mid the coils and wax and twine,
There the atoms in their glory.
Ionize and recombine.
Chorus
O my darlings! O my darlings!
O my darling ions mine!
You are lost and gone forever
When just once you recombine.
In a tube quite electrodeless,
They discharge around a line,
And the glow they leave behind them
Is quite corking for a time.
Chorus
…In the weird magnetic circuit
See how lovingly they twine,
As each ion describes a spiral
Round its own magnetic line.
J.J. Thompson’s Electron discoveries
• negative charge
• mass but 2,000
times smaller than a
hydrogen atom.
• Travel 20,000
miles/second so
slower than light
• Nobel prize 1906
Changing the tube by
adding gas or using
different kinds metal for
the cathode or anode
leads Thompson to
believe electrons are
part of all forms of
matter. Critics think
electrons are useless!
J.J. Thompson’s Model
Atom is like
raisin bread.
The atom’s
“dough” is
positive so
the electrons
stick like
raisins.
This is also referred to as the plum pudding model.
Rutherford’s Hypothesis
1911 Hans Geiger and Ernest Marsden experiment suggested by Rutherford/
Rutherford thought if J.J.’s model was correct, alpha particles
would not be deflected through a thin piece of gold foil.
ZnS
screen
Ernst Rutherford 1871-1937
• Thompson’s research or graduate
student.
• Discovers radioactive particles have a
half life or a set time when half of them
will have decayed.
• Gave Marsden and Geiger idea of gold
foil and told them what to do with data
they got.
• First to glimpse “nuclear energy.”
Animation of Gold foil
• Rutherford animation of gold foil
• Modern Rutherford Experiment
What he expected
Why? Because, he thought the mass
was evenly distributed in the atom.
He thought if the mass
was evenly distributed
in the atom…
alpha particles would pass straight through.
Rutherford’s Experiment
Most of the alpha
particles went straight
through.
But some were
deflected; the atomic
model had to change.
How he explained it
•The atom is
mostly empty
space
• A small dense,
positive piece is at
the center of the
atom.
• Alpha particles are
deflected by it if
they get close
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Rutherford’s
Model of the Atom
Click here for
simulation
Summary
Have your neighbor
• explain the reason we
no longer accept the
plum pudding/raisin
bread model of the
atom
Because less than 1% of
the alpha particles were
deflected, so 99% must
have passed through
empty space.
•Describe the Rutherford
model of the atom.
•What is the nucleus?
Nucleus is a dense center of mass
surrounded by empty space
•What is in the nucleus?
Protons and neutrons
•How big is the nucleus?
1/10,000 of an atom
•Where are the electrons?
Interactive Review
• The atom
• Quarks
Atomic Structure in 14 minutes
Sir James Chadwick 1891-1974
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Graduated university 1911 (20 yr)
Prisoner of War in 1914-1918
Doctoral advisor: Ernst Rutherford
Chadwick graduated in 1921 (30yr)
Knocked particles out of the
nucleus of beryllium
• New particles did not bend with
charged plates
• 1935 Nobel prize for discovery of
neutrons in 1932. (Cancer weapon)
• WWII helped invent atomic bomb
Atomic Emission Spectra
Periodic Table of Atomic Emission and Absorbtion Spectra
• Spectrum= Complete rainbow
• Spectra = discrete wavelengths of light
Neils Bohr 1885-1962
• Danish Soccer player
• Dislexic, but marries in
1912
• Colleague of Rutherford,
Einstein
• Nobel prize in 1922
• Bohr- Einstein debates
Neils Bohr 1885-1962
• Atoms have energy levels that contain
electrons.
• Electrons must have a specific or quanta of
energy to change levels.
• His model is the first quantum model of the
atom.
• He later helped with Manhattan Project and
proposed sharing nuclear secrets with
Russians.
• Received highest Order of the Elephant
http://www.colorado.edu/physics/2000/quantumzone/index.html
Bohr Model for Hydrogen
• Hydrogen Line Spectra
• Hydrogen emission spectra You Tube
• PhET LAB Simulation of Atomic
Spectra of Hydrogen
Neils Bohr’s Theory
• The dense nucleus is composed of positive protons
• Electrons travel in elliptical orbits of definite energies
• A force of attraction draws electrons to the nucleus
• Centrifugal force pulls them outward
• Electron energy increases with distance from the
nucleus
Bohr Model
Problems with Bohr’s Model
• The electrons must radiate
energy when they move
• Loss of energy should propel
them closer to the nucleus
• All atoms therefore must
collapse
• The model does not predict
reality- atoms don’t collapse
• Hydrogen is the only atom the
model seems to explain
• We use it to observe patterns
in the periodic table.
The Quantum Model
• Is a mathematical model of the atom as
opposed to a simple visual model
• Predicts electron positions in terms of
probability clouds
• Describes electron positions and energies,
as approximations
• Answers questions raised by previous
theory’s
The modern theory of the atom
Atoms –
the
building
blocks of
matterare mostly
empty
space!
The nucleus
concentrates
atomic mass, and
has the least
volume.
Protons have a
positive charge.
Neutrons are
neutral.
Electrons are negatively charged particles
found in thin shells or clouds at definite
distances from the nucleus.
Mass number =
the number of
p+ = no and their
mass.
Current Animated Atomic Model
Atomic Model Summary
Dalton’s
Bohr’s
Thompson’s
Quantum
Mechanical
Model
Rutherford’s
For Review
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How did Democritus idea of matter differ from
Aristotle’s?
What is John Dalton’s Atomic Theory?
What evidence demonstrated electrons had
charge?
Describe the evidence for a small dense nucleus.
Describe the modern quantum model of the
atom.
Explain, using examples, the tentative nature of
science.
How small is an atom?
Jon Bergman Just how small is an atom?
Relative Mass
Atoms are compared according to how much
mass they have. For example, it takes 12
hydrogen atoms to equal the mass of 1
carbon atom.
Element 114
• Island of Stability - Glen Seaborg's Journey
Vocabulary
Relative Atomic Mass
Mass Number
Atomic Mass Unit
Average Atomic Mass
Isotope
Electron
Neutron
Proton
http://web.jjay.cuny.edu/~acarpi/
NSC/3-atoms.htm
Video lessons
So a relative mass unit was invented, the
AMU, or atomic mass unit.
Carbon has 12 times the mass of hydrogen,
or we say hydrogen weighs 1 amu and
carbon weighs 12 amu.
12
X 1 amu hydrogen
= 1
X
12 amu carbon
So if carbon has a mass of 12 amu that also
means it has a counted number of 12
subatomic particles in its nucleus: 6
neutrons and 6 protons. So 12 amu is also
called the MASS NUMBER.
To find the number of protons, look up the
atomic number. To find the number of neutrons,
subtract the atomic number from the mass
number.
Vocabulary Terms
Mass number = protons plus neutrons = Z
(Z or Mass number = #p+ added to #no)
Atomic number = number of protons = A
Nuclear symbol
Z
A
14
6
carbon-14
Isotopes click here
Atoms of the same element can have different
numbers of neutrons; the different possible
versions of each element are called isotopes. For
example, the most common isotope of hydrogen
has no neutrons at all; there's also a hydrogen
isotope called deuterium, with one neutron, and
another, tritium, with two neutrons.
Hydrogen
Deuterium
Tritium
http://www.colorado.edu/physics/2000/isotopes/
Atomic Theory and the
Periodic Table
• Atoms are the smallest particles of an
element that display the properties of that
element
• There are 92 naturally occurring elements
• Recently scientists found theoretically
predicted elements 114 and 118 and____?
• Modern Atomic Theory is helping to
complete the Periodic Table of Elements
How does this story illustrate
the definition of science?
Science is a tentative, truth
seeking, self correcting
relevant way of learning.
An Introduction to the
Elements
Reactivity
http://www.privatehand.com/flash/elements.html
References
• http://inst.augie.edu/~jkbjerga/hist.html
• http://www.tannerm.com/index.htm
• http://www.watertown.k12.wi.us/hs/teachers/buescher/ato
mtime.html
• http://dbhs.wvusd.k12.ca.us/Greeks.html
• http://www.tannerm.com/atoms/chem_elements/list.htm
• http://www.tannerm.com/atoms/periodic_table/Periodic.ht
m
• http://www.liv.ac.uk/Chemistry/Links/links.html
• http://windows.engin.umich.edu/cgibin/tour_def?link=/sun/Solar_interior/Sun_layers/Core/ato
m_model.html&sw=false&cd=false&fr=f&edu=mid