History of Atomic Theory

Democritus (460-370 BCE)

442 BCE

Lived in Greece, he developed theory that stated that all matter consists of atoms with empty space between them, & atoms were indestructible & unchangeable. He cut a seashell in half and stated that you could keep cutting it in half until a certain point when it could no longer be cut. Atoms were unique to the material that they made up. Developed a model that was solid and showed differing atoms with empty space between them. He produced the first evidence based atomic theory.

John Dalton (1766-1844)

1803

Lived in England & developed his own atomic theory that stated that all matter was composed of atoms and that all atoms of an element were identical and different elements had atoms of differing size and mass. Also stated that all compounds were composed of combinations of atoms in specific ratios. Last, atoms cannot be created or destroyed. Experimented with gases to develop first evidence based atomic theory. Also studied color-blindness and defined an atom to be ball-like.

J.J. Thomson (1856-1940)

1897

British physicist that lived in England. Discovered the electron in 1897 and then proposed the Plum pudding model of the atom in 1904. He is most known for his Cathode Ray Tube Experiment in which electrons flowed off of the cathode toward the anode. This created a beam of negatively charged particles, which he discovered to be electrons. He also studied and taught at Cambridge University as well as worked with Ernest Rutherford.

Cathode Ray Tube Experiment

1897

Conducted by J.J. Thomson in 1897. This involved applying high voltage across two electrodes at one end of the tube, causing a beam of particles to flow. Thomson placed two oppositely charged electric plates around the tube to test the properties of particles. He found that the cathode ray was deflected away from the negatively-charged electric plate. He also used two magnets, which deflected the cathode ray. This led him to the discovery of the electron, which he proved with this experiment.

Robert Millikan (1868-1953)

1909

American physicist that lived in the United States. He is responsible for the oil drop experiment in which he replaced water with oil in order to determine the electric charge carried by a single electron. He is credited with the discovery of the size & mass of a single electron. He put charge on a tiny oil droplet, and measured the strength of an electric field on the droplet. He discovered the charge of the electron and that it is constant for all electrons. He won the Nobel Prize in 1923.

Oil Drop Experiment

1910

This experiment was conducted by Robert Millikan. Involved finding the size and charge of an electron. He created an electric chamber with two charged metal plates on each side. An atomizer would spray a mist of oil droplets, which would begin falling. When they fall, the electric field recruits electrons from the air that attach themselves to the droplets. Then, a light source illuminates the droplets, making the rate of their fall measurable. This is how he found the size & mass of an electron

Ernest Rutherford (1871-1937)

1911

British physicist that lived in England. Found that atom is mostly emptyspace and discovered the central nucleus. He conducted the gold foil experiment in which he passed small particles through a sheet of gold and found that a small percentage were deflected. This led him to see that the mass of an atom was concentrated at the center, which proved the existence of a nucleus. He developed the planetary model of the atom where electrons orbit the nucleus.

Gold Foil Experiment

1911

This experiment was conducted by Ernest Rutherford and involved shooting small, alpha particles at a thin sheet of gold. Through doing this, he found that a small percentage of the particles were deflected. However, the majority of the particles passed through the sheet. This led Rutherford to conclude that the mass of an atom was concentrated at its center, the nucleus.

Neils Bohr (1885-1962)

1913

Danish physicist that lived in Denmark. Proposed a theory based on quantum theory that energy is transferred in certain amounts. A light quantum is emitted when jumping from one orbit to another with lower energy. Suggested that electrons would move around the nucleus but only in certain paths. Worked under J.J. Thompson and Ernest Rutherford in years prior to his theory. Bohr's theory could also explain why atoms emitted light in fixed wavelengths.

Erwin Schrodinger (1887-1961)

1926

Austrian physicist that lived in Austria. He developed the atomic cloud theory stating that the nucleus was surrounded by a cloud of electrons. This started when he was displeased in the quantum condition of Bohr's theory. He came up with his own idea for the model which was called the Quantum Mechanical Model of the Atom, and is presented as a nucleus surrounded by an electron cloud. He received a Nobel Prize for his work and accomplishments.

James Chadwick (1891-1974)

1932

British physicist who lived in the UK and received the Nobel Prize for discovering the neutron. He made this discovery by using kinematics and was able to determine the velocity of protons in the atom. He used conservation of momentum techniques to determine that the mass of the neutral radiation was almost exactly the same as the mass of a proton. Also, he inspired the government to begin serious atomic bomb research with his final draft of the MAUD. Developed quantum mechanical model.