Atomic Structure Timeline
History of Atomic Theory
Essential Questions
• What does it mean when my science
teacher says stuff cannot be created or
destroyed?
• What is matter made up of? And who
figured it out?
Evolution of
Modern Atomic
Theory
Democritus (400 B.C.)
• Proposed that matter was
composed of tiny indivisible
particles
• Not based on experimental
data
• Greek: atomos
The Early Atom
• Aristotle succeeded Democritus
and did not believe in atoms.
Instead, he thought that all
matter was continuous. It was
his theory that was accepted for
the next 2000 years.
Alchemy (next 2000 years)
• Mixture of science and mysticism.
• Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.
Lavoisier 1777
• French chemist, who is considered
the founder of modern chemistry.
• Lavoisier clarified the concept of an
element as a simple substance that
could not be broken down by any known
method of chemical analysis
• He devised the Law of Conservation of
Mass
Basic Laws of Matter
• Law of Conservation of Mass- mass is
neither created nor destroyed during
ordinary chemical reactions or physical
changes.
CH4 + 2O2 → 2H2O + CO2
16g + 64g → 36g + 44g
Antoine Lavoisier
stated this about 1785
Alka Seltzer in Water
• Ziploc bag
• Alka seltzer tablet
• Water
Using the reaction
between the tablet
and the water, prove
that the Law of
Conservation of
matter is true.
Antoine Lavoisier and his wife, MarieAnne
"It took them only an instant to cut off that head, and a hundred years may not
produce another like it." Joseph-Louis Lagrange
Proust (1794)
• Proust stated the Law of Definite
Proportions.
• The law states that the ratio of elements
in a compound is always the same.
Basic Laws of Matter
• Law of Definite Proportions – no matter how much
salt you have, it is always 39.34% Na and 60.66% Cl by
mass.
Example: Sodium chloride always contains
39.34% Na and 60.66% Cl by mass.
2NaCl
100g
116.88g
→ 2Na + Cl2
→ 39.34g + 60.66g
→
? + ?
Joseph Louis Proust
stated this in 1794.
John Dalton (1808)
• Dalton add his own - the Law of multiple
proportions
• He proposed a theory to summarize and
explain these three laws
I was a school
teacher at the
age of 12!
Basic Laws of Matter
• Law of Multiple Proportions- Two or more
elements can combine to form different
compounds in whole-number ratios.
Example
John Dalton
proposed this
in 1803.
John Dalton
Dalton’s Four Postulates
1. Elements are composed of small indivisible
particles called atoms.
2. Atoms of the same element are identical.
Atoms of different elements are different.
3. Atoms of different elements combine together
in simple whole number ratios to create
compounds.
4. In a chemical reaction, atoms are rearranged,
but not changed.
John Dalton’s Elements
Was Dalton right?
Elements are
composed of small
indivisible particles
called atoms.
Subatomic particles –
electrons, protons,
neutrons, and more
Atoms of the same
element are identical.
Atoms of different
elements are different.
No, isotopes are atoms
that have the same
number of protons but
a different number of
neutrons
Was Dalton right?
Atoms of different
Yes! He was right!
elements combine
together in simple
whole number ratios to
create a compound.
In a chemical reaction, Yes! He was right!
atoms are rearranged,
but not changed.
Essential Questions
• So, Dalton figured out the atom, how did
we figure out what the inside of an atom
looks like?
How Big (Small) Is the Nucleus
of an Atom?
• If the total atom was the size of a
football field, then the nucleus would be
the size of an apple in the middle of the
field.
How Small is an Atom?
The width of a human hair = 1,000,000
carbon atoms across
One bucket full of water contains more
atoms than there are bucketfuls of water
in the Atlantic ocean
Discovery of the Electron
• In the late 1870’s many experiments
were performed in which electric current
was passed through gases at low
pressures due to the fact that gases at
atmospheric pressure don’t conduct
electricity well.
• These experiments were carried out in
glass tubes called cathode-ray tubes or
Crookes tubes for the man that
developed them.
Crookes Tube
CRT
J. J. Thomson (1903)
• Cathode Ray Tube
Experiments
– beam of negative particles
• Discovered particles
smaller than the atom!
J.J. Thomson (1903)
• Proved that the atom was divisible and that
all atoms contain electrons.
• This contradicted Dalton’s Atomic Theory.
• This allowed a new model of the atom.
J. J. Thomson (1903)
Plum-pudding Model
– positive sphere (pudding)
with
negative electrons (plums)
dispersed throughout
http://www.youtube.com/watch?
v=GzMh4q-2HjM
Robert Millikan (1910’s)
• American physicist
• Determined the mass and charge of an
electron
• Oil drop experiment
• http://www.youtube.com/watch?v=XMfY
Hag7Liw
Oil Drop Experiment
• Millikan dropped negatively charged microscopic
oil particles into a chamber containing metallic
plates and viewed them with a microscope.
• By applying voltage to the metallic plates,
Millikan created an electric field.
 He was able to suspend the
oil droplets by adjusting the
electric field to the
appropriate strength and
direction to overcome
gravity.
Oil Drop Experiment
• Knowing the mass of the droplets and
the strength of the electric field
necessary to suspend them, he was
able to calculate the charge of the
electron.
• He noticed that the charge was always
a whole-number multiple of 1.602 X10-19
Coulombs.
Ernest Rutherford (1911)
• Gold Foil Experiment
• Discovered the nucleus
– dense, positive charge in
the center of the atom
• Nuclear Model
Gold Foil Experiment
Gold Foil Experiment
• As expected, most of the alpha particles
passed straight through with little or no
deflection.
• However, 1/8000 of the positively charged
alpha particles were deflected, some back
at the source.
Gold Foil
Experiment
Gold Foil Experiment
• From this experiment, Rutherford
discovered that there must be a very
densely packed positively charged
bundle of matter within the atom which
caused the deflections.
• He called this positive bundle the
nucleus.
• He tried this experiment with other
metals and found the same results.
Gold Foil Experiment
• The volume of the
nucleus was very small
compared to the volume
of the atom.
• Therefore, most of the
atom was composed of
empty space.
“It was about as believable as if
you had fired a 15-inch shell at
a piece of tissue paper, and it
came back and hit you.”
-Ernest Rutherford
Ernest Rutherford (1911)
• Nuclear Model
• http://www.youtube.com/watch?v=XBq
Hkraf8iE
Let’s take a break
• Work with a partner to complete the
“Rutherford’s Gold Foil Experiment”
Evolution of
Modern Atomic
Theory
Almost there!
- - +
-
Niels Bohr (1913)
• Met with J.J. Thomson
but didn’t impress him
• Worked with Rutherford
and liked his model of the
atom
• Incorporated idea of
quantum mechanics into
the Rutherford model
Niels Bohr (1913)
• Introduced the idea of electrons traveling
around the nucleus in orbits or energy
levels.
• Like the planets, there are multiple orbits
like concentric circles around the nucleus
• Planetary Model
Niels Bohr (1913)
• He used Bright-Line Spectrum
– tried to explain presence of
specific colors in hydrogen’s
spectrum
• Built upon Max Planck’s
concepts of quantized energy
Erwin Schrödinger (1926)
• Treats electrons as waves
• Tells us the probability of
finding an electron at any
given location at any given
moment
• Electron cloud model
– Atomic orbital: region around
the nucleus where electrons
are likely to be found
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)
• dots represent probability of finding an enot actual electrons
James Chadwick (1932)
• Studied under Rutherford
and built upon his model
• Discovered neutrons
– neutral particles in the nucleus
of an atom
• He projected alpha particles (from a
decaying polonium) towards beryllium
target.
• The particles released are uncharged
and have the same mass as protons.
• He called them neutrons.
James Chadwick (1932)
Neutron Model
• revision of Rutherford’s Nuclear Model
Models of the Atom Timeline
Let’s recap with a video (10:52)
http://www.youtube.com/watch?v=NSAgL
vKOPLQ
Dogs teaching Atoms
• http://www.youtube.com/watch?v=_M9k
hs87xQ8&list=FLTWKpM0RNaHUX5D
RoXcf7qw&index=2
Atomic Timeline Poster Project
• Divide into 7 groups and make a poster
that includes:
– Year of discovery
– Name of model
– Name of scientist (s)
– Picture of the model
– Picture of equipment used in discovery
– Key information about model
Poster project
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Group 1 – Dalton (pg 104-105)
Group 2 – Thompson (pg 108 – 110)
Group 3 – Millikan (pg. 109)
Group 4 – Rutherford (pg. 111-113)
Group 5 – Niels Bohr (handout)
Group 6 - Schrödinger (pg. 152-155)
Group 7 – Chadwick (pg. 113-114)