A History of Atomic Theory Atomic Models

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A History of Atomic TheoryAtomic Models
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What is a model ?
MODEL detailed, 3-D representation of an
object, (typically on smaller
scale than original)
model airplane
model car
clothing model
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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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Democritus
Greek banknote with Democritus and drawing of atom
Where did Democritus get his ideas for small,
indestructible, indivisible atoms?
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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
• Mountains wash away but 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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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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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 box? If not, you may
be color blind. (More males are color blind than
females)
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• Start writing -
• Foldable information

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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, other properties
• Atoms of different elements are different in
  size, mass, 7 other properties
• (later found not to be 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 considered to be like
billiard balls
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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 and/or rearranged

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Ex. of a chemical reaction

2H2O 2Na ? 2NaOH H2
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Daltons Chemical Symbols
We use different symbols today
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Matter is electrical!
Michael Faraday (English chemist, born
1791) 1834 learned matter interacts with
electricity Realized electricity had to be made
up of particles that could be counted, but didnt
know what they were (electrons)
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J.J. Thomson
English physicist (1856-1940) won Nobel Prize in
1906 1897 discovered ELECTRON -
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 cross-shaped anode (on
right) cast 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 magnetic fields deflect beam of
charged particles
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What do we mean by charge?

• ELECTRICAL CHARGE
• property of matter has either more or fewer
  electrons than protons
• ELECTRONS are negatively charged (-1)
• PROTONS are positively charged (1)
• MATTER IS NEUTRAL (no charge) () charges equal
  (-) charges so cancel out each other

batteries have () and (-) ends
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More About Charge
Opposite charges attract, Alike charges repel
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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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RULE Matter is NEUTRAL (no charge) so for every
one electron (-) charge there must also be one
() charge
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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 raisins in

plum pudding
(or chocolate
chips in
cookies)
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Ernest Rutherford1871-1937

• 1920 discovered proton
• 1908 won Nobel Prize (Chemistry)
• Discovered that most of
• mass of atom is in nucleus

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

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• gold foil experiment scattering results showed
  Plum pudding model of atom was
  INCORRECT, so Rutherford devised new model that
  fit his results
• proposed Nuclear Model

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• Rutherford concluded
• all atoms have dense, positive () center
• (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 model
did not speculate how electrons arranged around
center (name nucleus came later)
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Problem with Rutherfords Model
To prevent (-) electrons from being attracted to
() nucleus, electrons must orbit nucleus like
Earth orbits sun But
e- need energy to orbit nucleus Rutherford never
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
• 1 Å 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
• 1932 discovered neutron
• 1935 Nobel Prize (Physics)

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Niels Bohr 1885 - 1962
Created quantized atom model, 1915 Worked on
Manhattan Project Won Nobel Prize in 1922
Bohrs model e- only move from one energy level
to another in atom emit energy when jump from
higher to lower levels absorb energy when jump
from lower to higher levels.
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• Bohr addressed issue of atoms electrons
• nucleus surrounded by e- orbiting at different
  energy levels
• e- have definite orbits

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Quantum Mechanical TheoryElectron in a Hydrogen
atom

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• Bohrs new atomic model had quantized energy
  levels
• e- only move by jumping levels (numbered n
  1, n 2, n 3, etc.)

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• electrons
• absorb energy when they jump away from nucleus
• emit energy when they jump towards nucleus

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Bohrs Planetary Model

• electrons travel only in specific orbits
• each orbit has definite energy
• inner most orbit (n1) least energy
• outer most 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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Erwin Schrödinger Wave Model

• Austrian scientist (1887-1961)
• 1933 won Nobel Prize (Physics
• calculated wave model of hydrogen atom (1926)
• atomic model called
• cloud model
• quantum mechanical model
• modern model atom model

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• Schrödingers wave model of atom
• e- behaves as energy wave AND as 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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A Missing Particle The Neutron

• 1932 James Chadwick discovered last major piece
  of atom
• discovered neutron
• (neutral, no charge)
• 1935 won Nobel Prize (Physics)
• neutron weighs about as much as proton while
  electron is much smaller

(1891-1974)
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The Complete Modern Atom

• An atom is
• mostly empty space
• nucleus most of atoms mass
• nucleus contains protons neutrons
• electrons in energy levels around nucleus
• electrons jump between levels, emitting
  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 perhaps develop a better
  theory for the atomic model and win a Nobel Prize
  in the future?