4.01 Evolution of the Atomic Theory

4.01 Evolution of the
Atomic Theory
The Events Leading to the Discovery of
the Building Block of Matter
Dr. Fred Omega Garces
Chemistry 152
Miramar College
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Idea of the Atom
How did man come up
with the idea of the
atom ?
It took ~2400
years from when it
was conceived to
the time
experimental
evidence prove of
the atom
existence.
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History of Chemistry and development
of the atomic model
History of Atomic Structure
The timeline of the progression of the development of the
Modern Atomic theory.
Atomic Theory Timeline
J. Dalton
J. Proust
Empedocles
Aristotle
A. Lavoisier
R. Boyle
Democritus
Leucippus
Zeno
Aristotle
Democritus
Boyle
Lavoisier
Dalton
R. Bacon
-5 0 0
3
-2 5 0
0
250
500
750
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1000
1250
1500
1750
2000
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Matter according to ... Greek Philosophers
492-375 BC: Empedocles
Earth-Air-Fire-Water
(supported by Aristotle)
400 BC: Zeno
(also supported by Aristotle)
Matter can be infinitely divided
400 BC: Leucippus
470-375 BC: Democritus (student of Leucippus)
Proposed the idea of the atom
Matter -indivisible particles called atoms
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Next 2000 Years
Alchemy (pseudo science) lead into gold
experiments (black magic) took detailed notes
(lab notebook conceived) Systematic metallurgy
(extracted metals from ore)
Roger Bacon (1214-1292) An
English and alchemist, Bacon
became known as the founder of
experimental science. He believed
that doing experiments for
yourself rather than just accepting
what other people tell you was the
way to learn about nature. His
most important work was the Opus
Matus, in what he wrote about the
scientific method of learning. He
did many experiment which showed
how rainbow are made by the
effect of water drops on sunlight
and how lenses could be used to
help people with weak sight.
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Atomic Structure Revolution
1500 Robert Boyle The Skeptical Chemist
He performed detailed experiment with gases; this
initiated the break down of the Greek's model of matter.
1700 Antoine Lavoisier
Father of modern chemistry
(Law of conservation of mass)
He made very careful measurements and concluded
that mass is neither created nor destroyed. The
thought at the time was then when a substance was
burned, the ashes were lighter than the starting
material, but Lavoisier did the experiment in an
enclosed container and determined that the mass did
not change. Subsequent experiments with other
reactions was consistent with the conservation of
mass findings.
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1790-1800 Joseph Proust
(Law of definite proportion or Law of constant
composition) A given compound always contain exactly
the same proportion of elements by mass.
i.e., H2O 2:16 or H : O 1 : 8 by mass always true
whether there is 18 g or 139 g H2O
For, H2O2 2:32 the H : O is 1 g H : 16 g O
In other words,
Water found in SD or the Arctic or Jupiter’s moon
will all have 1 g hydrogen to 8 g oxygen ratios.
Likewise glucose found in sugar cane or sweet beets
have 6 g carbon to 1 g hydrogen to 8 gram oxygen
ratios (12 x 6 C: 1 x 12 H: 16 x 6 O). i.e.,The
relative numbers and kinds of atoms are constant.
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1800 John Dalton
J. Dalton: Founder of the Modern Atomic Theory
(Law of Multiple proportion)
When two elements form a series of
compounds, the ratio of the masses of the element that combine with 1
gram of the first element can be reduce to small whole numbers.
Example: : Mass of nitrogen that combines with 1.0 gram of oxygen
*Compound A
1.750 g g A/B = 1.750/0.8760
= 2 / 1
Compound B
0.8750 g g B/C = 0.8760 / 0.4375
= 2 / 1
Compound C
0.4375 g g A/C = 1.750 / 0.4375
= 4 / 1
Compounds consist of
atoms which
combines in whole
number ratios.
*N2O g N2O
NO g NO
NO2 g N1/2 O
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1808 Dalton's Atomic Theory :
A new system of Chemical Philosophy
• All matter is made of atom.
• In a chemical reaction atoms are neither
created or destroyed but only arranged
differently.
• Atoms of the same element are identical.
• Atoms of different elements are not the same.
• Compounds are formed from two or more
atoms. A given compound always has same
relative number and type of atoms.
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... what’s next ??
Around this period, the model of the atom
was gaining acceptance by scientist.
Questions still unanswered What is an atom?
How is it put together?
What are the parts of an atom?
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Early experiments to characterize the atoms
The parts: Electrons - Thomson (1904), Millikan (1909)
Protons - Goldstein (1886) Rutherford (1911)
Neutrons - Sir James Chadwick (1932)
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Faraday
Electrolysis
~1832
Balmer
Discharge Tubes
~ 1830 - 90
Thomson
e/m
~1897
Millikan
e
~1909
Contributions to the Modern Atomic Theory
Mendeleev
Periodic Table
~1870
Michelson
C
~1885
Becquerel, Curie
Radioactivity α,β γ
~1897
Planck
Quantum
~1900
Rutherford
Nucleus
~1911
Einstein
E=hv, E=mc2
~1905
Bohr
Shell Model
~1913
Aston
Isotopes
~1909
Stern, Gerlach
Electron Spin
~1921
Schrodinger
Wave Model,
Pauli Exclusion,
Hund's Rules
~1926
deBroglie
Matter Waves
~1924
Heisenberg
Uncertainty Principle
~1927
Chadwick
Neutron
~1932
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The modern Atom: a flow chart
of major theoretical and
experimental contributions
(who, what, and when) leading to
our understanding of the atom.
Scientific discovery is a
complex human endeavor. The
developments leading to our
understanding of the modern
atom, dramatically depict
scientific discoveries not as
individual efforts, but as the
collective work of the scientific
community.
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Early experiments to Characterize the atoms
1820's Sir Humphrey Davy / Michael Faraday
Matter contains negative charge particles
(electrons). Later, cathode rays were
discovered and in the late 1800's cathode rays
was investigated further. (1832)
http://video.google.com/videoplay?docid=4144497206440839046
1900 John J. Thomson
Cathode rays were electrically charge particles. Cathode Ray
tube with electron stream where deflected by positive and negative
plate: e / m = -1.76•108 C/g Proposed the "plum pudding model"
theory of the atom; based on Coulomb's Law: F = k ( Q1 Q2) / r.
Atoms contains protons and electrons.
1909 R.A. Millikan
Oil drop expt.: Determine the
electron’s charge-value.
e- = -1.60•10-19 C
m = 9.11 •10-31 kg
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Arrangement of subatomic particles
1911 Ernest Rutherford: Gold thin foil experiment.
Reflection of alpha radiation on gold foil proved that the
atom consisted of a very dense nucleus.
• Based on the particles deflection pattern,
Rutherford calculated the fraction of the atomic
volume occupied by the positive nucleus of about 1
part in 1014. The mass is so dense that a nucleus
the size of a pea would have mass greater that
250 million tons.
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Characteristics of Atomic Parts
The parts: Subatomic particles and their properties
Particle
Mass
*Rel Mass
(amu)
Relative
Charge
e -;
electron
9.109•10 -31 Kg
1/1837
- 1.602•10 -19 C
p+;
proton
1.673•10 -27 Kg
1
+ 1.602•10 -19 C
1.675•10 -27 Kg
1
0
n;
neutron
* Relative mass: 1 amu = 1 proton
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Summary
These were a few of the experiments which lead to the discovery
and a better understanding to the parts of the atom.
Three sub-atomic particles : electron, protons, and neutrons
combine in various numbers and arrangement to make the different
109 elements (and counting) in the periodic table.
The picture of the atomic architecture is now complete (or is it ?).
From the time of Dalton g
Thomson g Rutherford g
Bohr g Schrodinger, our
model of the atom has
undergone many
modifications. The model is
not finish however.
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String Theory
String theory is an active research framework in particle physics that
attempts to reconcile quantum mechanics and general relativity general
relativity.[1] It is a contender for the theory of everything (TOE), a manner
of describing the known fundamental forces and matter in a mathematically
complete system. The theory has yet to make testable experimental
predictions, leading some scientists to claim that it cannot be considered a
part of science.
http://superstringtheory.com/.
http://www.pbs.org/wgbh/nova/elegant/
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