Atomic Theory Project

Democritus

400 BCE

Democritus believed that atoms came in different sizes and shapes. He also said that if an atom was cut into smaller and smaller pieces, at some point it would become indivisible.

John Dalton

1803

Dalton said that atoms of the same element are identical and atoms of different elements are not identical. Compounds are formed when atoms of different elements are combined together.

J.J. Thompson

1897

His experiments with the cathode ray tube showed that all atoms contain tiny negatively charged subatomic particles or electrons. He knew that the atoms had an overall neutral charge and therefore believed that there must be a positive charge to counterbalance the negative charge of the electrons.

Ernest Rutherford

1911

Rutherford believed the mass and charge were uniformly distributed throughout the atom. Most of the particles were undeflected and a very small percentage bounced off at large angles. All of the positive charge and the majority of the mass of an atom must be concentrated in the nucleus.

Niels Bohr

1913

Bohr said the electrons orbited the nucleus
in very specific orbits based on their
energies and they could not be found
anywhere in between these orbits. The energy of an electron depends on the size and location of the orbit.

Erwin Schrodinger

1921

He discovered there was no specific location of electrons. Electrons could only be found based in areas of probability called electron clouds. These orbitals do not look like anything Bohr proposed.