A History of Atomic Theory Atomic Models

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A History of Atomic TheoryAtomic Models
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Lets Take a Trip Through Time!

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It Started with the Greeks
DEMOCRITUS
Lived in Greece 2500 years ago (460-370 BC)
Father of modern science
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Demacritus said All matter is made of atoms
that are tiny, indestructible and indivisible.
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Atoms are small
new rings
old ring
EVIDENCE Old gold rings wear away slowly,
getting thinner and thinner, but you never see
gold atoms on your finger, so gold atoms must be
very small!
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Atoms are indestructible
EVIDENCE Earth has been around long time. Even
though mountains get washed away, rocks continue
to exist. New plants grow where old plants die.
Atoms are indivisible
EVIDENCE None This was HYPOTHESIS made by
Democritus to explain nature as he saw it.
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What did Democritus think atoms looked like?

• Appearance of atoms assumed based on behavior
• Liquids pour
• Solids are hard and rigid

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Liquids pour. Why?
Liquids require a container
If liquid atoms were like little balls they would
roll out when you tip the container
Greek idea of liquid atom
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Solids are rigid. Why?
cocklebur plant
cockleburs stick on clothing and each other
little hooks on cockleburs
Velcro fastener
Velcro hooks and loops
Greek idea of solid atom
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Democritus (Greek) 460-370 BC
All matter is made of atoms that are tiny,
indestructible, and indivisible (cant be
divided) Atomos
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Aristotle had other ideas
Greek banknote and coin picturing Aristotle

• ARISTOTLE Famous Greek philosopher, born 384 BC.
• student of Plato (another famous philosopher)
• teacher of Alexander the Great (who later
  conquered the world)

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Aristotles idea of matter
Aristotle did not believe Democrituss idea of
atoms was correct Aristotle believed all matter
made from four elements Earth Air Fire Water
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Aristotles idea of matter
Aristotle was more famous than Democritus, so
people believed him, even though he was
wrong! Democrituss idea of atomos (atoms) was
lost for nearly 2000 years until John Dalton
brought it back in 1803
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Aristotle 384BC
Thought Democritus was wrong
Matter made of 4 elements Earth Sun Air Water
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Atom idea lost for 2000 years
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John Dalton, New Atom,1803
Born in England, 1766 Studied chemistry,
physics, and color blindness Brought back
Democrituss idea of an indivisible atom
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Color blindness
Can you see a number in this gray box? If not,
you may be color blind. (More males are color
blind than females)
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Daltons Theory of Atoms

• Five parts to Daltons modern atomic theory
• 1
• Elements are made of extremely
• small particles called atoms

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• 2
• Atoms of given element are identical in size,
  mass, and other properties
• atoms of different elements are different in
  size, mass, and other properties
• (found out not exactly correct)

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• 3
• Atoms cannot be subdivided, created, or destroyed
• (later proved wrong)

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So Daltons atoms are kind of like billiard
balls
Atoms combine in whole-number ratios
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• 4
• Atoms of different elements combine in simple
  whole-number ratios to form chemical compounds
• (H2O 21 ratio HO)

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• 5
• In chemical reactions, atoms are combined,
  separated or rearranged
• 2H2O 2Na ? 2NaOH H2

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In a reaction, atoms are rearranged
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Daltons Chemical Symbols
We use different symbols today
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Dalton 1766
Billard Ball

• Atoms (tiny)
• Atoms of same element are identical
• Indestructible
• Combine in whole ratio (H2O)
• In chem rxn. are combined, separated, rearranged
• turned out to not be entirely be ture

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J.J. Thomson

• English physicist (1856-1940) won Nobel Prize in
  1906
• Discovered the ELECTRON in 1897
• studied cathode rays using Crookes tube
• showed atoms were divisible

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Crookes tube
cross-shaped anode
cathode
cross-shaped shadow
Cathode rays (stream of electrons) move from
metal cathode (on left) to the cross-shaped anode
on right and casts a cross-shaped shadow on glass
on right. So Electrons are particles with
negative charge
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Cathode Ray Tube Experiment

Thought cathode rays were streams of particles
smaller than atoms
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electric or magnetic fields deflect beam
of charged particles
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More About Charge
Opposite charges attract, alike charges repel
(push away.
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• JJ Thomsons discovery of (-) charged
  electrons proved that atoms were divisible!

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Thomsons Plum pudding atom
electron
ENGLISH PLUM PUDDING
raisin
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• we move from
• Daltons Billard Ball Model
• to
• Thomsons Plum pudding model

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Thomsons Plum Pudding Model
() charge

evenly spread out
while (-) charge

is in bits
like chocolate
chips
in cookies
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JJ Thomson 1906
Plum Pudding Model
Discovered the Electron using cathode ray tubes
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Ernest Rutherford1871-1937

• Discovered the proton in 1920
• Won Nobel Prize in
• chemistry in 1908
• Discovered that most of
• mass of atom is in nucleus

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Rutherfords Experiment 1911
Rutherfords expt animation
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To test plum pudding model of atom, Rutherford
sent tiny radioactive particles called a (alpha)
particles, like bullets, towards thin sheet of
gold foil.
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Rutherford gold foil experiment
Rutherford expected a particles to pass straight
through, like this most a particles went
straight through didnt bump into anything so
most of atom is empty space
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• But even though most a particles went straight
  through, a few were deflected, like this so the
  particles must have hit something
• really heavy
• () charged

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• The gold foil experiment scattering results meant
  the Plum pudding model of the atom was INCORRECT,
  so Rutherford had to think of new model that fit
  what he saw so
• proposed Nuclear Model

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• Rutherford concluded
• all atoms have positive () nucleus that contains
  most of mass of atom
• atom is mostly empty space (except very tiny ()
  electrons)

electrons () /
The NUCLEAR atom model
nucleus / protons ()
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Rutherford proposed Nuclear atom
Rutherford did not speculate how electrons
arranged around nucleus
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Rutherford 1920
Nuclear Model

• -discovered the proton ( charge)
• Gold Foil Experiment
• Atoms have a nucleus
• Nucleus contain most of the mass
• Atom is mostly empty space

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Problem with Rutherfords Model
To prevent (-) electrons from being attracted to
() nucleus, electrons have to orbit nucleus like
Earth orbits sun But
electrons need energy to orbit nucleus, and
theres no source of energy for this so every
Rutherford atom would die yet Real atoms do not
die
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So how big is the nucleus compared to the entire
atom?

• If atom as big as football stadium, nucleus
  smaller than flea on 50-yard line!
• If atom big as period at end of sentence in
  textbook, it would have mass of 70 cars!

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So how big is an atom?

• Most atoms are 1-2 angstroms across
• An angstrom 1 X 10-10 m
• 6 Billion Cu atoms in a line less than 1 meter!
• Can we see an atom?

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James Chadwick1891-1974

• Worked with Rutherford
• Discovered Neutron 1932
• Nobel Prize in Physics 1935

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Niels Bohr 1885 - 1962
In Bohrs model, electrons can only move between
energy levels in the atom, emitting energy when
they jump from higher to lower levels, absorbing
energy when they jump from lower to higher
levels.
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• Bohr addressed the issue of electrons in the atom
• Nucleus surrounded by electrons orbiting at
  different energy levels
• Electrons have definite orbits

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• Bohrs new atomic model had quantized energy
  levels, meaning the electrons could only move by
  jumping between levels (numbered n 1, n 2, n
  3, etc.)

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electrons absorb energy when they jump out, and
emit (send out) energy when they jump in
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Bohrs Planetary Model

• Electrons travel only in specific orbits
• Each orbit has definite energy
• Inner orbit (n1) least energy
• Outer orbit (n7) most energy
• Atoms emit radiation when e- jumps from outer
  orbit to inner orbit
• Outermost orbits determine atoms chemical
  properties

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Bohr 1930s
Planetary Model

• Electrons travel around the nucleus in orbits
• Gain/Emit energy when moving from level to level

e-
e-
e-

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Erwin Schrödinger Wave Model

• Austrian scientist (1887-1961)
• won the Nobel Prize in physics in 1933
• calculated wave model of hydrogen atom in 1926
• also called cloud model, quantum mechanical
  model, modern model of the atom
• atom model we still use today

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In Schrödingers wave model of atom, electron
behaves as energy wave as well as a matter
particle (Light also behaves as particle and
wave) Einstein had predicted that energy and
matter were related in his equation E mc2
If we could see an electron it might look like
this cloud
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Modern/Wave/Cloud Model

• Electrons energy is quantized (has only certain
  values)
• Electrons in probability zones called orbitals,
  not orbits
• - location cannot be pinpointed
• Electrons are particles waves at same time
• Electrons move around nucleus at speed of light

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Orbitals

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Bohr 1933s
Wave (Cloud) Model

• Electrons behave like waves
• Not located in an exact orbit but rather a zone
  called an orbital

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The Complete Modern Atom

• An atom is
• mostly empty space
• nucleus has most of mass of an atom
• nucleus contains protons neutrons
• electrons are in energy levels around nucleus
• electrons jump between levels, emitting and
  absorbing energy as jump

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The development of atomic theory represents the
work of many scientists over many years
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Next Atomic Theory ?

• Which one of you will develop a better theory and
  win the Nobel Prize?