Atomic Theory Timeline

John Dalton

Dec 2, 1808

Scientist from Cambridge, England.
He believed that elements were made up of atoms and proposed 4 new ideas about the atom:
1. All substances are made up of atoms which are small particles that cannot be created, divided, or destroyed.
2. Atoms of the same element are exactly alike and atoms of different elements are different.
3. Atoms join with other atoms to form different substances
4. In chemical reactions, atoms are combined, separated, or rearranged.

Sir William Crookes

Dec 2, 1855

Sir William Crookes invented a glass tube with 2 electrodes connected to a battery. The tube made him be able to see the pathway where the electrons traveled.

Dmitri Mendeleev

Dec 2, 1869

Dmitri Mendeleev was a professor. When he couldn't find any text books for his students to use, he made his own. He decided to organize the elements that had been discovered by their properties. This was the first periodic table. It organized the elements by their atomic weight and valance (valance electrons are those in the outer shell).

J.J. Thomson

Dec 2, 1897

J.J. Thomson experimented on the Crookes' tube and held a magnet to the side of the tube and tried to direct the beam of light.Thomson took Crookes’ tube and used a magnet to bend the ray. He concluded that the ray must have had a negative charge if it was attracted to the positive side. He called his model the “Plum Pudding Model” He called it this because he thought that there were positive (protons) and negative (electrons) particles tightly packed together.

Ernest Rutherford

Dec 2, 1911

Ernest Rutherford proposed 3 things:
1. An atom has a dense center called the nucleus that has a positive charge!
2. Most of an atom is empty space!
3. The nucleus has a large mass compared to the electrons

Niels Bohr

Dec 2, 1913

Niels Bohr said that electrons move in paths at certain distances around the nucleus and that electrons can jump from a path on one level to a path on another level. He also believed that electrons move in circular orbits within specific energy levels.

Erwin Schrödinger

Dec 2, 1926

The probability function pf the Cloud Model describes a cloud-like region where the electron is likely to be found. It can not say with any certainty, where the electron actually is at any point in time, yet can describe where it ought to be.

James Chadwick

Dec 2, 1932

He discovered neutrons using the Joliot-Curie Experiments. He based on his theory on their experimental evidence.

Democritus

Dec 1, 2000

460-370 BC.
Democritus concluded that there is a limit to how far you can divide matter. The smallest part that could not be broken down any smaller was called an atom. He theorized that:
1. Atoms are small hard particles
2. Made of a single material formed into different shapes and sizes
3. They are always moving
4. They form different materials by joining together.

Aristotle

Dec 1, 2000

350 BC.
Aristotle was a philosopher who believed that there were 5 elements: earth, wind, water, fire, and ether. Many people believed his "5 element theory" that it became a common belief.