ATOMIC THEORY TIMELINE

DEMOCRITUS (370 BC)

• The theory of Democritus said that everything is composed of atoms
• He also said that Atoms are indestructible, there are an infinite amount of atoms, and they are constantly moving.

ANTOINE LAVOISER (1794)

• Law of Conservation of matter: Lavoisier found that mass is conserved in a chemical reaction.

DMITRI MENDELEEV (1869)

• properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.

HENERY MOSELY (1887)

• Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote"The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.

GOLD FOIL EXPERIMEMT(1899)

• scientists discovered that every atom contains a nucleus where its positive charge and most of its mass are concentrated.

PLUM PUDDING MODEL (1904)

• The plum pudding model is an obsolete scientific model of the atom proposed by J. J. Thomson in 1904. It was devised shortly after the discovery of the electron but before the discovery of the atomic nucleus. In this model, the atom is composed of electrons

RUTHERFORD MODEL (1909)

• Called the plum pudding model incorrect and stated why it was incorrect.

NIELS BOHR (1913)

• Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits.

ERWIN SCHRODINGER (1926)

• Created a model that describes the probability that an electron can be found in a given region at a given time

ELECTRON CLOUD MODEL (1926)

• The cloud model represents a sort of history of where the electron has probably been and where it is likely to be going. The red dot in the middle represents the nucleus while the red dot around the outside represents an instance of the electron