Atomic Theory

Events

Democritus's Billiard Ball

400 BC

Greek philosopher asked the question:" Could matter be divided for ever or was there a limit?". Democritus named the smallest piece of matter atomos(not to be cut). He claimed it would be an indivisible solid ball, infinite, and small and different shaped.

John Dalton's Atomic Theory

1803 - 1808

John Dalton proposed the atomic theory. He believed atoms were solid spheres base upon properties of mass. 1800's chemist, Dalton performed a number of experiments. Dalton's five part theory stated:
-matter is composed of atoms which can't be divided, created, nor destroyed
-atoms of the same element have identical properties
-atoms of different elements have different properties
-atoms of different elements combine in simple whole number ratios to form compounds
-atoms are never created, destroyed, or changed

J.J. Thomson's Plum Pudding

1897

Joseph John Thomson was positive that atoms were neutral. Thus he concluded that for an atom to be neutral the number of electrons and protons were equal. Thomson developed the Plum pudding model. The negatively charged electrons were embedded in a sphere of positive charge so the charges were balanced. He also used a CRT to determine the charge to mass ratio of an electron(1.759*108 coulombs/gram).

Robert Millikan and the Electron

1909 - 1910

Robert Millikan used a series of experiments to come up with basic info for the electron. First, he measured the course of charged water in an electric field. His results showed that the charge on the water droplet is a multiple of electric charge. Later he conducted the Oil-Drop experiment replacing water with oil. His results were that the electron charge is 1.602*10-19 and the electron mass is 1/1836 amu.

Rutherford's Gold Foil Experiment

1911

In 1911, Ernest Rutherford performed the gold foil experiment. He shot protons at a thin sheet of gold foil. Most protons went straight through. Others bounced back. His results:
-positive repeals positive
-atoms have a small dense positively charged nucleus
-atoms are mostly empty space
-nucleus is dense

Neil Bohr's Model

1913

Neil Bohr placed electrons in energy levels. He proposed that electrons move in orbit around the nucleus. He explained the closer an electron is to the nucleus, the lower amount of energy it has. The Ground state is the lowest energy level. The Excited state is the highest energy level. The difference in electron energy levels is called a quantum.

Heisnberg and Schrodinger's Electron Cloud Model

1920 - 1935

The Quantum Mechanical Model
-Heisenberg proposed Principle of Indeterminacy.
-Heisenberg described atoms by their spectral lines.
-Schrodinger conducted his though experiment of Schrodinger's Cat and discovered the atom has wave states.
-Schrodinger viewed electrons as continuous clouds.
Together that concluded that...
-electrons aren't in orbit
-electrons bounce in a cloud
-(Electron Cloud Model)

Chadwick Discovers the Nuetron

1923 - 1932

Chadwick and Rutherford studied the transmutation of elements by bombarding them with alpha particles. He identified the proton and found its consistency in the nucleus of other atoms. He inferred that radiation as being composed of particles of mass equal to that of the proton but without electric charge(neutrons). Thus he discovered the neutron.

Bibliography

Approx. present

"atom". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 21 Oct. 2016
https://www.britannica.com/science/atom/Conductors-and-insulators.

"atomic theory". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 21 Oct. 2016
https://www.britannica.com/science/atomic-theory.

"Erwin Schrodinger". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 22 Oct. 2016
https://www.britannica.com/biography/Erwin-Schrodinger.

Buescher, Lee. "Atomic Structure Timeline." Atomic Timeline. Lee Buescher, 2004. Web. 21 Oct. 2016. Foundation, Chemical Heritage.

"Joseph John “J. J.” Thomson | Chemical Heritage Foundation." Chemical Heritage Foundation. Chemical Heritage Foundation, 22 July 2015. Web. 22 Oct. 2016.
"James Chadwick". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 22 Oct. 2016
https://www.britannica.com/biography/James-Chadwick.

"John Dalton". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 21 Oct. 2016
https://www.britannica.com/biography/John-Dalton/Atomic-theory.

"Robert Millikan". Encyclopædia Britannica. Encyclopædia Britannica Online.
Encyclopædia Britannica Inc., 2016. Web. 22 Oct. 2016
https://www.britannica.com/biography/Robert-Millikan.