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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)

• Planetary Model

– electrons move in circular

orbits within specific

energy levels

Bright-line spectrum

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 e-

not 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

http://www.profes.net/rep_imagenes/Monograf/NF1990a.jpg

http://panda.unm.edu/images/people/faculty/gell_mann_murray.jpg

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.

Democritus proposes

the 1st atomic theory

460 – 370 BC

History of the Atom - Timeline Antoine Lavoisier

makes a substantial number of contributions

to the field of Chemistry

1766 – 1844

John Dalton proposes his

atomic theory in 1803 1743 – 1794

0

1856 – 1940

J.J. Thomson discovers the electron and proposes the Plum Pudding Model in 1897

1871 – 1937

Ernest Rutherford performs the Gold Foil

Experiment in 1909

1885 – 1962

Niels Bohr proposes the Bohr Model in

1913

1887 – 1961

Erwin Schrodinger

describes the electron cloud in 1926

1891 – 1974

James Chadwick

discovered the neutron in in 1932

170

0s

180

0s

190

0s

Progression of the Atomic Model

The structure of an atom, according to:

Democritus & John Dalton

J.J. Thomson Ernest Rutherford Neils Bohr Erwin Schrodinger James Chadwick

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+ Electron Cloud+ Electron Cloud