The Atomic Theory

Democritus (Greece)

460

• Had proposed that all matter is made of infinite indivisible particles which differ from each other in form, position & arrangement http://scienceworld.wolfram.com/biography/Democritus.html

Issac Newton (England)

Jan 1, 1704

• Proposed a mechanical universe with small solid masses in motion
• Also suggested that atoms were held together by attractions that we now call forces http://scienceworld.wolfram.com/biography/Newton.html

John Dalton (England)

Jan 1, 1803

• First suggested the existance of an atom
• His original theories provided us with the weight of atoms which can distinguish one atom from another http://scienceworld.wolfram.com/biography/Dalton.html

George Johnstone Stoney (Ireland)

Jan 1, 1894

• Calculated the electric charge of an atom
• Proposed that electricity was made of discrete negative particles called electrons http://atomictimeline.net/index.php

Joseph John Thomson (England)

Jan 1, 1897

• Showed us the existance of subatomic particles through cathrode rays
• Research on rays led to a means of separating atoms & molecules according to their atomic weights http://scienceworld.wolfram.com/biography/Thomson.html

Max Planck (Germany)

Jan 1, 1900

• Founder of the quantum theory
• Quantum theory has revolutionised our understanding of atomic & sub-atomic processes http://scienceworld.wolfram.com/biography/Planck.html

Hantora Nagaoka (Japan)

Jan 1, 1903

• Created a “Saturnian” model (albeit slightly incorrect) of an atom with flat rings of electrons revolving around a positively charged atom http://atomictimeline.net/index.php

Robert Andrews Milikan (America)

Jan 1, 1909

• Measured the charge of an electron using the oil drop experiment
• Allowed us to measure the charge to mass ratio of an electron http://atomictimeline.net/index.php http://en.wikipedia.org/wiki/Robert_Andrews_Milikan

Ernest Rutherford (Britain/New Zealand)

Jan 1, 1911

• Established that the nucleus was very dense, small & positively charged
• Also assumed that the electrons were located outside the nucleus http://en.wikipedia.org/wiki/Ernest_Rutherford http://atomictimeline.net/index.php

Henry Gwy-Jeffreys Moseley (England)

Jan 1, 1914

• Arranged the periodic table by their atomic numbers
• Used x-ray tubes to determine the charges in the nuclei of most atoms http://atomictimeline.net/index.php

Niels Bohr (Denmark)

Jan 1, 1922

• Developed an explanation of atomic structure that underlies regularities of the periodic table of elements
• Expanded upon Rutherford’s theory by proposing that electrons travel only in successively larger orbits http://www2.lucidcafe.com/lucidcafe/library/95oct/nbohr.html

James Chadwick (England)

Jan 1, 1932

• Used the alpha particles to discover a neutral atom particle with a mass close to a proton – thus discovered the neutron http://atomictimeline.net/index.php