Structure of The Atom Timeline



460 BCE

A central figure in the development of philosophical atomism and of the atomic theory of the universe.


1766 - 1844

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.



developed an “Electron Cloud Model” in 1926. It consisted of a dense nucleus surrounded by a cloud of electrons at various levels in orbitals.



Discovered the electron. 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

Robert Milikan


He was an American experimental physicist, and Nobel laureate in physics for his measurement of the charge on the electron and for his work on the photoelectric effect. in 1909 Millikan worked on an oil-drop experiment in which they measured the charge on a single electron. His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes.



gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus. Rutherford designed an experiment to use the alpha particles emitted by a radioactive element as probes to the unseen world of atomic structure.



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. When jumping from one orbit to another with lower energy, a light quantum is emitted.



Heisenberg is best known for his uncertainty principle and theory of quantum mechanics which he published at the age of twenty-three in 1925. He was awarded the Nobel Prize for Physics in 1932 for his subsequent research and application of this principle.

de Broglie


French physicist best known for his research on quantum theory and for predicting the wave nature of electrons.

Antoine Lavoisier


He stated the first version of the law of conservation of mass, recognized and named oxygen, and hydrogen. He abolished the phlogiston theory and helped construct the metric system, He wrote the first extensive list of elements, and helped to reform chemical nomenclature. He discovered that, although matter may change its form or shape, its mass always remains the same.