History of Atomic Theory

Democritus

460

Scientist: Democritus
Date: 460 BC
Location: Greece
Idea/experiment: Theory of ‘Atoms and Void’ – the existence of matter in space.
The universe was divided into an enormous vacuum of space. The substance of material things were made up of minute unseen particles.

Contribution to understanding: Matter is always changing but the atoms never change or die.

Isaac Newton

Jan 1, 1704

Scientist: Isaac Newton
Date: 1704
Location: England
Idea/experiment: Proposed a mechanical universe with small solid masses in motion.
Contribution to understanding: Anticipated a mechanical universe with small solid masses in motion.

John Dalton

Jan 1, 1803

Scientist: John Dalton
Date: 1803
Location: England
Idea/experiment: Predicted the ‘atomic theory’ with round solid atoms based upon quantifiable properties of mass.
There are three states in kinds of bodies: elastic fluids, liquids and solids.
Contribution to understanding: There are three states in kinds of bodies: elastic fluids, liquids and solids – atomic theory.

G.J. Stoney

Jan 1, 1894

Scientist: G.J. Stoney
Date: 1894
Location: Ireland
Idea/experiment: Electricity was made of negatively charged particles that he called ‘electrons’. Nature gives us a single exact amount of electricity which is independent of the particular bodies acted on.
For each chemical bond which is broken with an electrolyte, a certain amount of electricity crosses the electrolyte which is the same in all cases.
Contribution to understanding: Electrons in electricity.

J.J. Thomson

Jan 1, 1897

Scientist: J.J. Thomson
Date: 1897
Location: England
Idea/experiment: Used a CRT (cathode ray tube) to experimentally determine the charge to mass ratio of an electron. Verified the existence of the ‘electron’ and showed the sub atomic structure of matter.
Contribution to understanding: Proved and confirmed the existence of the ‘electron’ in the atoms – sub atomic structure of matter.

Determined the charge to mass ratio of an electron.

Max Planck

Jan 1, 1900

Scientist: Max Planck
Date: 1900
Location: Germany
Idea/experiment: Established the quantum theory into the mechanics of classic physics in 1900. The theory gave details about why vibrating particles of light could only emit certain forms of energies.
Contribution to understanding: Energy determined in colour or frequency instead of intensity.

Nagaoka

Jan 1, 1903

Scientist: Nagaoka
Date: 1903
Location: Japan
Idea/experiment: The structure of an atom is made up of a many particles of identical masses placed in a circle at equal interval that repel each other with forces.
Contribution to understanding: Electrons are placed in rings around the positively charged particle.

R.A. Millikan

Jan 1, 1909

Scientist: R.A. Millikan
Date: 1909
Location: America
Idea/experiment: Precise determination of the charge of an electron was found by using the ‘falling-drop method’. He also confirmed that this amount was identical for all electrons. This then indicated the atomic structure of electricity.
Contribution to understanding: Showed the atomic structure of electricity.

E. Rutherford

Jan 1, 1911

Scientist: E. Rutherford
Date: 1911
Location: England/New Zealand
Idea/experiment: He assumed the configuration of the atom by using an experiment that included firing radioactive particles. He recognized that the nucleus was very dense, very small and positively charged. He also believed that the electrons were situated outside of the nucleus.
Contribution to understanding: Atoms are made up of mostly empty space. The electrons were situated outside of the nucleus.

H.G.J. Moseley

Jan 1, 1914

Scientist: H.G.J. Moseley
Date: 1914
Location: England
Idea/experiment: He wrote ‘The atomic number of an element is equal to the number of protons in the nucleus’. Therefore, the periodic table based is on the atomic number instead of atomic mass.
Contribution to understanding: The atomic number is equal to the number of protons in the nucleus. Therefore, the periodic table should be based on the atomic number (instead of the atomic mass).

Niels Bohr

Jan 1, 1922

Scientist: Niels Bohr
Date: 1922
Location: Denmark
Idea/experiment: He identified the sub atomic radiochemistry of matter. He then found out that radioactive properties of matter came from the nucleus and chemical properties of matter depended on the frequency and position of electrons surrounding the nucleus.
Contribution to understanding: Sub atomic radiochemistry of matter. Elements in the periodic table should be placed according to atomic number of electrons.

James Chadwick

Jan 1, 1932

Scientist: James Chadwick
Date: 1932
Location: England
Idea/experiment: Chadwick recognized the neutron particles of the atom. The atom was made up of mostly empty space.
Contribution to understanding: Confirmed Rutherford’s theory that the atom was made up of mostly empty space. Identified the neutron particles of the atom.