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400
Democritus (Greece) B.C.
He hypothesized that all matter is composed of tiny indestructible units, called atoms. This idea seems motivated by the question of how finely one can go on cutting up matter. The atoms in Democritus theory themselves remain unchanged, but move about in space to combine in various ways to form all macroscopic objects. His theory was can be considered the first to say that atoms exist. http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec05.html -
Isaac Newton (England)
His theory was that atoms were held together by forces of attraction. This suggests very natural explanations for various physical chemistry-type phenomena, such as deliquescence and heat of mixing. He also knew that the forces between atoms affected the chemical properties of matter. Newton’s theory helped people understand that atoms are needed in order to make everything work. http://galileo.phys.virginia.edu/classes/252/atoms.html -
John Dalton (England)
He said that the reason an element is pure is because all atoms of an element were identical and that in particular they had the same mass. He also said that the reason elements differed from one another was that atoms of each element were different from one another; in particular, they had different masses. This helped people to classify and differentiate atoms. http://dl.clackamas.edu/ch104-04/dalton's.htm -
George Johnstone Stoney (Ireland)
Proposed that electricity was made possible by negative particles called electrons and that within an atom, there would be electrons. This helped us understand that within the atom, there are other particles and that atoms have a charge (introduced the basic concept of negative charge.). This also meant that there were things that were even smaller than atoms.
http://www.chemteam.info/Chem-History/Stoney-1894.html -
Joseph John Thomson (England)
Thomson confirmed the existence of electrons in atoms. He discovered this through finding that cathode rays could be deflected by an electric field, thus concluding that these rays were composed of very light negatively charged particles which he called ‘corpuscles’, later to be named ‘electrons’. His research proved that Stoney’s hypothesis was correct. http://www.aip.org/history/electron/jjhome.htm -
Max Planck (Germany)
He devised a theory that perfectly described the experimental evidence, but part of it was a radical new idea: energy did not flow in a steady continuum, but was delivered in discrete packets Planck later called quanta. This led to the invention of the quantum theory. This then gave us a better understanding of the interactions of matter and radiation. http://www.thebigview.com/spacetime/quantumtheory.html -
Hantaro Nagaoka (Japan)
Hantaro Nagaoka developed an early but incorrect planetary model of the atom. He based his model of the atom around the rings of the planet Saturn. However his model was not created properly. He explained that the rings are held there due to its massive orbit. Although this model was wrong it still allowed for the discovery of the atoms rings. http://www.chemteam.info/Chem-History/Nagaoka-1904.html -
Robert (Andrews) Millikan (America)
Discovered the accurate determination of the charge carried by an electron, using the oil-drop method; he also proved that this quantity was a constant for all electrons (1910), thus demonstrating the atomic structure of electricity. This meant that people knew that the electrons in any atom have the same charge. http://www.juliantrubin.com/bigten/millikanoildrop.html -
Ernest Rutherford (New Zealand)
Discovered that Thomson’s plum pudding model was not exactly accurate and formulated his own model of the atom. His model had a very small positively charged nucleus, orbited by electrons. He also suggested the existence of ‘neutrons’.Rutherford discovered the basic form of an atom regarding the positions of the protons and electrons. http://www.visionlearning.com/library/module_viewer.php?mid=50 -
Henry Gwyn-Jeffreys Mosely (England)
Showed that each nucleus was characterized by an atomic number, equal to the number of unit positive charges associated with it. By rearranging the periodic table according to atomic number rather than atomic weight, a more systematic arrangement was obtained. http://www.chemistry.co.nz/henry_moseley.htm -
Niels Bohr (England)
Suggested that when an atom is heated, electrons can jump to higher levels, & when the atom cools, they drop down to lower levels, where precise quanta of energy are released as specific wavelengths. This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit. This helps us understand why atoms release light and energy on at certain frequencies. http://www.crystalinks.com/bohr.html -
James Chadwick (England)
Discovered a third subatomic particle, which he named the neutron. Because the nucleus is so tightly packed together, the neutrons helped to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons do not have any electrical charge; they are electrically neutral. This helped us understand why atoms stay together despite the positive charges of the protons. http://atomic-molecular-optical-physics.suite101.com/article.cfm/the_discovery_of_the_neutron
