History of Atomic Models

Empedocles

470 BCE

Empedocles suggested that there were four elements: Water, Earth, Air, and Fire. It seemed logical because when things caught fire, moisture is released, air can be felt coming up from it, and the ashes show the earth that it contained. Classifying all matter as only being made of four elements certainly simplified our view of the world.

Democritus

400 BCE

His model was just a round solid ball. Democritus didn't know about a nucleus or electrons, all he knew was that everything is made of atoms. he reasoned that if the stone were to be continually cut into smaller and smaller pieces then; at some point, there would be a piece which would be so small as to be indivisible. He called these small pieces of matter "atomos," the Greek word for indivisible.

Aristotle

250 BCE

Aristotle said he didn't believe in the theory of Atoms. Aristotle strongly disagreed with Democritus. He felt that there was no smallest part of matter and that different substances were made of earth, fire, air, and water. Aristotle did not have an atomic model due to the fact that he thought atoms did not exist.

John Dalton

1803

Dalton resurrected Democritus' idea of atoms. He included in this theory that all matter is made of atoms, that atoms cannot be created nor destroyed.
Dalton's Atomic Theory:
1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms.

J.J. Thomson

1897

Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube. He demonstrated that cathode rays were negatively charged.He also studied positively charged particles in neon gas.Thomson discovered the electron, which led him to create the "plum pudding" atomic model. In this model, he thought that the atom was mostly positive, and negative electrons wandered around the atom. The "plum pudding" model influenced other scientists to make better atomic models.

Ernest Rutherford

1909

He proved that Thomson's plum pudding structure incorrect. .He created the nucleus, and said that instead of the positive matter being the whole atom, it was just in the middle. He said the atom was mostly empty space and that the electrons surrounded the positive nucleus. This model influenced one of his own students to perfect the atomic model later on.

Niels Bohr

1913

This model is patterned on the solar system and is known as the planetary model. He created energy levels in the atom, where only a certain amount of electrons could fit on one energy level of the atom. Bohr also used Planck's ideas in order to create quantum mechanics, his new concept regarding energy. This model is still used to this day.

Erwin Schrodinger

1926

This atomic model is known as the quantum mechanical model of the atom. Unlike the Bohr model, the quantum mechanical model does not define the exact path of an electron, but rather, predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by an electron cloud.The probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud.This model introduced the concept of sub-energy levels.

Werner Heisenberg

1927

Werner Heisenberg was a German scientist that proposed the uncertainties of the Quantum model. He said that you can't know the exact velocity and momentum of the electron at the same time, which means you can't know the exact location of the electron. This principle proves error in Bohr's model because of the uncertainty of the location of an electron.