Transcript Document

Early Greek atomists
Leucippus
(~480 - 420
B.C.)
http://cont1.edunet4u.net/cobac2/down/dow
n05.html
• All matter is made of tiny,
indivisible particles called
“atoms”
• Change is caused by atoms
moving through empty
space (a “void”)
• Atoms are therefore
“fundamental”
Democritus (470 - 380 B.C.)
www.livius.org/a/ 1/greeks/democritus.jpg
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The mechanistic philosophy of the 1600s
Descartes, Boyle, Newton & others imagined a “clockwork”
universe - perfectly predictable
2
Meanwhile, many new elements being found
3
How to make sense of all these
elements?
Scientists like “a place for
everything, and
everything in its place.”
And no more places and
things than necessary.
4
Dmitri
Mendeleev
(1834-1907)
“Creator of the
Periodic Table”
(but there were earlier
attempts by others)
5
Mendeleev’s
early notes
for the
Periodic Table
(1869)
6
Characteristics of Mendeleev’s Table
• Organized 60+ known elements…
- by similar chemical properties in each
vertical family (group)
- by roughly increasing atomic weight within
each horizontal row
• Used to predict existence of new elements
(of 10, found 7; other 3 do not exist)
7
Mendeleev’s
table, as originally
published
• Formatted
sideways
compared to
modern table
• ? instead of a
name: element
was predicted to
exist but not
known yet
8
Prediction of the properties of an unknown
element below Silicon
*
Property
Observed
for Si
Predicted Observed
for eka-Si
for Sn
Atomic
mass
28
72
118
72.6
Density
(g/cm2)
2.33
5.5
7.28
5.35
Formula
of oxide
SiO2
Eka-SiO2
SnO2
GeO2
Formula
of
chloride
SiCl4
Eka-SiCl4
SnCl4
GeCl4
eka: “one beyond”
Observed
for Ge
9
News flash: a new type of matter is
discovered
J. J. Thomson (1897)
• experimented with “cathode
rays”
• decided that they are charges of
electricity carried by particles of
matter
Schematic of actual
1897 apparatus
(vacuum inside):
10
Cathode-Ray Tubes – ever seen one?
http://www.howstuffworks.com/tv4.htm
11
Thomson’s conclusions
• “We have, in the cathode rays,
matter in a new state...a state
in which all matter...is of one
and the same kind; this matter
being the substance from which
all the chemical elements are
built up."
but...
• “What are these particles? Are they atoms, or
molecules, or matter in a still finer state of
subdivision?” - J. J. Thomson
http://www.aip.org/history.electron/jjrays.htm
12
How big are “electrons”?
• Thomson calculated the mass-to-charge ratio for
cathode ray particles: it was over 1000 times smaller
than of a charged hydrogen atom
• This fact suggested:
- either cathode rays carried a huge charge,
- or they had very small mass
13
Answer: very, very small
• Robert Millikan measured the
charge of a cathode ray particle
in 1910.
• From that & Thompson’s massto-charge ratio, he could
calculate the mass: ~1800
times lighter than a hydrogen
atom
14
Thomson’s “plum pudding” atom model*
• tiny “corpuscles”
of negative charge
• surrounded by a
sort of “cloud” of
positive charge
* Never had plum pudding? Think of a blueberry muffin.
15
More pieces of the atom
Ernest Rutherford
• Thomson’s student
• Gold Leaf Experiment
(1910-11) – actually
conducted by Hans Geiger
and undergraduate Ernest
Marsden
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The gold leaf experiment
• fired positively-charged alpha particles at very thin gold foil
– they caused flashes of light when they hit the screen
• counted flashes and measured the angle of deflection
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Gold leaf experiment: prediction
By Thomson’s model,
mass and + charge of gold
atom are too dispersed to
deflect the positively-charged
alpha particles,
so...
particles should shoot straight
through the gold atoms.
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like this:
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What actually happened:
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What’s going on?
Most alpha particles went
straight through, and
some were deflected,
BUT
a few (1 in 20,000) reflected
straight back to the source!
“It was quite the most incredible event that has ever happened
to me. It was almost as incredible as if you had fired a fifteen inch
shell at a piece of tissue paper and it came back and hit you.”
21
Rutherford’s Model of the Atom
• small, dense, positivelycharged nucleus
surrounded by “mostly
empty” space
in which the electrons must
exist.
+
• positively charged particles
called “protons”
• like tiny solar system
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The nucleus repels alpha particles
+
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Now we understand why the
periodic table works
• The order of the elements is determined by their
atomic number (= the number of protons)
• The atomic mass of the elements is determined
by the number of protons and neutrons.
• The chemical properties of the elements are
determined by the number of electrons in their
outer (valence) shells
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Why do 2 Group I atoms combine
with 1 oxygen (R2O)?
25
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So: is this what atoms are like?
No!
Calculations soon showed
that a “Rutherford atom” would
last less than one minute
Electrons would radiate away
energy and spiral down into the
nucleus.
27
A new understanding of the atom
from spectroscopy
When elements are heated,
they give off light of a
particular wavelength
(or color)
Sodium Potassium Lithium
28
Hydrogen’s emission “fingerprint”
Observation:
when heated with electricity
hydrogen gives off light
of specific wavelengths
The line-emission spectrum of hydrogen gas
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Niels Bohr
(1885-1962)
Danish physicist
Bohr wondered why
hydrogen emitted spectral
lines, and not just a
continuous band of light
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Bohr’s Model of Atom (1913)
H's electron
r1
r2
The first three allowed energy levels,
at distances r1, r2, and r3 from nucleus.
r3
H's nucleus containing 1 proton
• Bohr assumed that electrons can orbit ONLY at
certain distances from nucleus
• this model permits electrons to exist for a long time
without giving off radiation
• Bohr’s model enabled him to predict the number and
wavelength of hydrogen’s emission lines
31
Electron orbits are distinct
(“quantized”) in Bohr’s model
“Quantum
leaps”
from one level to
another
Trefil & Hazen. The Sciences: An integrated approach. 2 nd ed. Fig. 7-6.
32
Protons & neutrons are not fundamental
Protons and neutrons are composed of UP and
DOWN quarks, held together by gluon particles
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