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Atomic Structure Timeline
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Democritus (400 B.C.)
• Proposed that matter was
composed of tiny indivisible
particles
• Not based on experimental
data
• Greek: atomos
Alchemy (next 2000 years)
• Mixture of science and mysticism.
• Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.
John Dalton (1807)
• British Schoolteacher
– based his theory on others’
experimental data
• Billiard Ball Model
– atom is a
uniform,
solid sphere
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 proportions to create a compound.
4. In a chemical reaction, atoms are rearranged,
but not changed.
Henri Becquerel (1896)
• Discovered radioactivity
– spontaneous emission of
radiation from the nucleus
• Three types:
– alpha () - positive
– beta () - negative
– gamma () - neutral
J. J. Thomson (1903)
• Cathode Ray Tube
Experiments
– beam of negative particles
• Discovered Electrons
– negative particles within
the atom
• Plum-pudding Model
J. J. Thomson (1903)
Plum-pudding Model
– positive sphere
(pudding) with
negative electrons
(plums) dispersed
throughout
Ernest Rutherford (1911)
• Gold Foil Experiment
• Discovered the nucleus
– dense, positive charge in
the center of the atom
• Nuclear Model
Ernest Rutherford (1911)
• Nuclear Model
– dense, positive nucleus surrounded
by negative electrons
Niels Bohr (1913)
• Bright-Line Spectrum
– tried to explain presence
of specific colors in
hydrogen’s spectrum
• Energy Levels
– electrons can only exist in
specific energy states
• Planetary Model
Niels Bohr (1913)
Bright-line spectrum
• Planetary Model
– electrons move in circular
orbits within specific
energy levels
Erwin Schrödinger (1926)
• Quantum mechanics
– electrons can only exist in
specified energy states
• Electron cloud model
– orbital: region around the
nucleus where e- are likely
to be found
Erwin Schrödinger (1926)
Electron Cloud Model (orbital)
• dots represent probability of finding an enot actual electrons
James Chadwick (1932)
• Discovered neutrons
– neutral particles in the
nucleus of an atom
• Joliot-Curie
Experiments
– based his theory on their
experimental evidence
James Chadwick (1932)
Neutron Model
• revision of Rutherford’s Nuclear Model
1964
Murray Gell-Mann & George Zweig
Murray Gell- Mann was an American physicist who received a Nobel Prize for his
theory on elementary particles. He also found that all the elements of an atom are held
together by quarks. To find this, he blasted high speed electrons into a hydrogen atom.
George Zweig proposed the existence of quarks. He thought of them as aces, because he
guessed there were four quarks in every atom.
Now, in the same year, both these guys proposed the idea of quarks. They tested
electrical charges, and that how the numbers for quarks came about.
http://www.profes.net/rep_imagenes/Monograf/NF1990a.jpg
http://panda.unm.edu/images/people/faculty/gell_mann_murray.jpg
History of the Atom - Timeline
1766 – 1844
Antoine Lavoisier
Thomson
makesJ.J.
a substantial
the
number discovers
of contributions
electron
and
to the
field of
proposes the
Chemistry
Plum Pudding
Model 1871
in 1897
– 1937
Niels Bohr
proposes
the Bohr
Model in
1913
1887 – 1961
James
Chadwick
discovered
the neutron
in in 1932
1700s
1800s
1900s
460 – 370 BC
0
Democritus
proposes
the 1st atomic
theory
1743 – 1794
Erwin
John Dalton
Ernest Rutherford
Schrodinger
proposes performs
his
the Gold Foil
describes
1891 – 1974
atomic theory
Experiment
in
in 1909
the electron
1803
cloud in 1926
1885 – 1962
1856 – 1940
Progression of the Atomic Model
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The structure of an atom, according to:
Electron Cloud
Democritus
James
Ernest
Erwin
Neils
Schrodinger
Chadwick
Rutherford
Bohr&
J.J.
Thomson
John Dalton