Atomic Theory Timeline

Democritus

300

His theory: Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

Antoine Lavoiser

Oct 31, 1779

French chemist known as the father of modern chemistry; discovered oxygen and disproved the theory of phlogiston (1743-1794)

Law Conservation of Mass

Oct 31, 1789

Law of Conservation of Mass is a relation stating that in a chemical reaction, the mass of the products equals the mass of the reactants.

John Dalton

Oct 31, 1803

) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour).
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Dalton's Atomic Theory

Oct 31, 1803

Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights. He stated his theory in a lecture to the Royal Institution in 1803. The theory proposed a number of basic ideas: All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the

Dmitri Mendeleev

Oct 31, 1869

Russian chemist who developed a periodic table of the chemical elements and predicted the discovery of several new elements (1834-1907)

Cathode Ray Tube

Oct 31, 1879

is a vacuum tube containing one or more electron guns (a source of electrons or electron emitter) and a fluorescent screen used to view images.[1] It has a means to accelerate and deflect the electron beam(s) onto the screen to create the images.

JJ Thompson

Oct 31, 1897

English physicist who experimented with the conduction of electricity through gases and who discovered the electron and determined its charge and mass (1856-1940)
Sir Joseph John Thomson, Thomson

Plum Pudding Atomic Model

Oct 31, 1897

is a model of atomic structure proposed by J.J. Thomson in the late 19th century. Thomson had discovered that atoms are composite objects, made of pieces with positive and negative charge, and that the negatively charged electrons within the atom were very small compared to the entire atom. He therefore proposed that atoms have structure similar to a plum pudding, with tiny, negatively charged electrons embedded in a positively charged substrate. This was later shown to be incorrect.

Robert Millikan

Oct 31, 1909

first came to prominence with his elegant oil-drop experiments of 1909, in which he measured the electric charges on tiny falling water (and later oil) droplets. His study established that any particular droplet's electrical charge is a multiple of a definite, fundamental value — the electron's charge.

Rutherford Model

Oct 31, 1911

The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.

Henry Mosely

Oct 31, 1913

Moseley's discoveries resulted in a more accurate positioning of elements in the Periodic Table by closer determination of atomic numbers.

Bohr Planetary

Oct 31, 1922

Bohr model, introduced by Niels Bohr in 1913, depicts the atom as small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus—similar in structure to the solar system, but with attraction provided by electrostatic forces rather than gravity.

Niels Bohr

Oct 31, 1922

was a Danish physicist who made foundational contributions to understanding atomic structure and quantum theory, for which he received the Nobel Prize in Physics in 1922. Bohr was also a philosopher and a promoter of scientific research.

Quantum Mechanical Model

Oct 31, 1926

The quantum mechanical model is based on mathematics. Although it is more difficult to understand than the Bohr model, it can be used to explain observations made on complex atoms.

Ernest Rutherford

Oct 31, 1926

British physicist (born in New Zealand) who discovered the atomic nucleus and proposed a nuclear model of the atom (1871-1937)

Gold Foil Experiment

Oct 31, 1926

) was an experiment to prove the structure of the atom performed by Hans Geiger and Ernest Marsden in 1909,[1] under the direction of Ernest Rutherford at the Physical Laboratories of the University of Manchester. The unexpected results of the experiment demonstrated for the first time the existence of the atomic nucleus, leading to the downfall of the plum pudding model of the atom, and the development of the Rutherford (or planetary) model.
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Erwin Schrodinger

Oct 31, 1926

was an Austrian physicist who developed a number of fundamental results in the field of quantum theory, which formed the basis of wave mechanics: he formulated the wave equation (stationary and time-dependent Schrödinger equation) and revealed the identity of his development of the formalism and matrix mechanics.

Ernest Rutherford

Oct 31, 1926

British physicist (born in New Zealand) who discovered the atomic nucleus and proposed a nuclear model of the atom (1871-1937)

James Chadwick

Oct 31, 1932

was an English physicist who was awarded the 1935 Nobel Prize in physics for his discovery of the neutron in 1932. He was the head of the British scientists who worked on the Manhattan Project during World War II. He was knighted in 1945 for achievements in physics.

Electron Cloud Model

Oct 31, 1935

The electron cloud model is an atom model wherein electrons are no longer depicted as particles moving around the nucleus in a fixed orbit