Evolution of Atomic Theory

Democritus

442 BCE

Democritus’s model stated that matter consists of invisible particles called "atomos," the Greek word for indivisible. Democritus believed that the atoms differed in size and shape, were in constant motion in a void, collided with each other; and during these collisions, could rebound or stick together. Therefore, changes in the matter were a result of dissociations or combinations of the atoms as they moved throughout the void (empty space).

Dalton

1803

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.
Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence of Dalton's theory remains valid.

Thomson

1904

J. J. Thomson performed experiments demonstrating that cathode rays were unique particles, rather than waves, atoms or molecules, as was believed earlier. He discovered the electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. In Thomson’s model, the atom is composed of electrons (which Thomson still called “corpuscles.”)

Rutherford

1909

Rutherford's model shows that an atom is a mostly empty space, with electrons orbiting a fixed, positively charged nucleus in a set, predictable paths. In 1909, Rutherford conducted his famous gold foil experiment. In the experiment, he bombarded a piece of gold foil with positively charged alpha particles, expecting them to travel straight through the foil. Instead, many alpha particles ricocheted off of the foil, suggesting that there was something positive these particles were colliding with.

Niel Bohr

1913

In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation. Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy.

Schrödinger

1926

Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom. 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. Where the cloud is most dense, the probability of finding the electron is greatest.