Evolution of the Atomic Model

Resources

Evolution of the Atomic Model (Democritus to the Quantum Atomic Model)

• Chemistry 9, Moodle Presentations
• Youtube
• http://atomictimeline.net/

A Timeline by Estel Anahmias - 9B

Please click on the photos and videos to enlarge them.

1891 - 1974

1871 - 1937

1856 - 1940

1791 - 1867

1754 - 1826

460 - 370 BC

James Chadwick

Ernest Rutherford

1926 - Today

J. J. Thomson

Michael Faraday

Joseph Proust

Democritus

Faraday studied the effect of electricity on solutions and came up with the term "electrolysis" as a splitting of molecules with electricity. In addition, he coined the term "ion".

He developed laws of electrolysis.

He did a few experiments and discovered the possibility of the existence of negatively charged particles called "electrons" within the atom.

Proust published the Law of Definite Proportions in 1799 which states that a compound is composed of exact proportions of elements by mass, no matter how the compound was created. So, according to his law, every pure chemical compound consists of elements in a definite proportion. This law is currently valid as well.

James Chadwick discovered neutrons in 1932. He went onto measuring their mass.

Democritus was a Greek philosopher that said all matter was made up of indivisible particles called "atomos".

He also said that;

• Atoms could not be destroyed.
• Atoms were specific to the material they made up.
• All kinds of different atoms existed but they were made of the same things.
• The physical characteristics of materials depended upon the shape of atoms, how they moved and the space between them.
• The rearrangement of the atoms would be enough in order to change a substance.

J. J. Thomson discovered the existence of electrons in 1897. He also measured the ratio of the charge of electron.

He is famous for his "Plum Pudding Model"(1904) that symbolizes his idea of an atom. The plums in the pudding represent the electrons which he believed were randomly spread in an atom. The pudding represents a positively charged cloud. However, this was a hypothesis therefore was not based on any experimental evidence, and is an incorrect theory.

Rutherford proved Thomson's atomic model wrong by testing it with the gold foil experiment in 1911.

He discovered protons in 1919.

Gold Foil (Alpha-Scattering Experiment):

He shot alpha particles at a thin sheet of gold foil in order to observe how the particles scatter.

His results were;

• Nearly all of the particles passed through with no deflection at all.
• A small amount of the particles were deflected at large angles.

The conclusion he reached was;

• The atom's volume is mostly empty space.
• The atom's positive charge as well as it's mass is in the atom's center, within a tiny core. He named this core as the "nucleus".
• The electrons are around the outside of an atom.

Quantum Atomic Model

De Broglie, Heisenberg, Schrödinger reached the conclusion that we are unable to detect the exact place of electrons and their orbits and we can only calculate the probability of finding the electron around nucleus, which is the electron cloud model.

These scientists said that electrons exist only in certain discrete energy levels which are described by quantum numbers.

Heisenberg

De Broglie

Niels Bohr

Robert Millikan

George Johnstone Stoney

John Dalton

Antoine Laurent Lavoisier

Schrödinger

Robert Millikan determined the electric charge of electrons between 1908 and 1917.

Dalton claimed that all atoms are indivisible and indestructible, just like Democritus had in the past.

He created the "Billiard Ball Model".

He also postulated that;

• All atoms of the same element are identical in properties such as; mass, shape, size, etc.
• Compounds are the combination of two or more different kinds of atoms.
• A chemical reaction is simply the rearrangement of the atoms.

Lavoisier created the Law of Conservation of Mass which states that matter is neither created or destroyed and mass always remains the same during chemical reactions. This law is still applicable to today's world of chemistry.

He also found out that;

• Atmospheric air includes nitrogen, oxygen and carbon dioxide.
• Water is a compound and is made of hydrogen and oxygen.
• Oxygen has a massive role in combustion, calcinations, oxidation and the formation of acids.

Bohr changed Rutherford's model by stating that electrons cannot move freely around the nucleus.

He said that electrons move in circular orbits around the nucleus. These orbits have specific energy levels and are all placed in a different distance from the nucleus. They are also called as shells.

Moreover, Bohr explained the Hydrogen Emission Spectrum. He wasn't able to explain the atomic spectrum of other elements.

Bohr's model said that electrons are subatomic particles and have very small mass but very high velocity.

In 1894, Stoney said that electricity was made of negatively charged particles called "electrons."

1868 - 1953

1855 - 1962

1826 - 1911

1766 - 1844

1743 - 1794