History of Atomic Structure

Democritus

400

(Greece)
Democritus introduced the concept of the atom. He developed the theory that the universe consists of empty space and almost an infinite indivisible number of invisible particles which differ from each other in form, position, and arrangement.

Newton

Jun 14, 1704

(England)
Newton came up with the theory of light.
He proposed the idea that there was a mechanical universe with small solid masses in motion. This theory was projected stating that "matter is formed of solid, massy impenetrable particles”

Dalton

Jul 28, 1803

(England)
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.

Stoney

Jul 28, 1894

(Ireland)
Stoney proposed that electricity was made of discrete negative particles he called electrons. He introduced us to the basic concept of negative charge.

Thomson

Jul 28, 1897

(England)
Thomson discovered tiny negatively charged particles which he called electrons. He discovered them when he subjected gases in glass tubes to high voltages. Thomson was also able to determine the charge to mass ratio (e/m) of the electron =1.759 x 108 coulombs/gram.

Planck

Jul 28, 1900

(Germany)
Planck was the founder of the quantum theory. He was able to deduce the relationship between the energy and the frequency of radiation based on the idea that the energy by a resonator could only take on discrete values or quanta. Through this he discovered a new constant which he called Planck’s constant and later described his own set of units of measurement based on fundamental physical constants. He also created Planck’s Law of Black-body Radiation

Nagaoka

Jul 28, 1903

(Japan)
Nagaoka developed an early
(though slightly incorrect) model of the atom. The model was based around an analogy to the explanation of the stability of the Saturn rings. The model made two predictions that the nucleus is big and that the electrons that revolve around the nucleus are bound by electrostatic forces

Milikan

Jul 28, 1909

(America)
Milikan accurately determined the charge carried by an electron using the “falling-drop method” and proved that this quantity was the same for all electrons.

Rutherford

Jul 28, 1911

(British - New Zealand)
Rutherford was able to probe atoms in a piece of thin gold foil using alpha particles. He established that the nucleus was very dense and small and that it had a positive charge. He also theorised the existence of neutrons which could somehow explain the repelling effect of the positive charges of the protons. This theory was later proved by his James Chadwick, his associate.

Mosely

Jul 28, 1914

(England)
Mosely discovered the concept of radioactive half life. He proved that radioactivity involved the transmutation of one chemical element to another and differentiated and named alpha and beta radiation. He was also the first person to transmute one element into another.

Bohr

Jul 28, 1922

(Denmark)
Bohr developed an atomic model where the atoms where built up of successive orbital shells of electrons. He expanded a past theory made by Ernest Rutherford by proposing that electrons travel only in certain successively larger orbits. He suggested that the outer orbits could hold more electrons than the inner ones and that the outer orbits determine the atom’s chemical properties.

Chadwick

Jul 28, 1932

(England)
Using alpha particles Chadwick made a fundamental discovery in the domain of nuclear science in 1932. He discovered an elementary particle devoid of any electrical charge- the neutron.