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Period: 400 to
History of atom- from then to now
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460
Democritus
The theory of Democritus and Leucippus held that everything is composed of "atoms". They are physically, but not geometrically, indivisible. There are an infinite number of atoms, and kinds of atoms, which differ in shape, and size. -
John Dalton- Atomic Theory
there are five main points to daltons atomic theory. the first is is that elements are made of extremely small particles called atoms. the second would be that atoms of a given element are identical in size, mass, and other properties. the third and fourth is that atoms cannot be subdivided, created, or destroyed and that atoms of different elements combine in simple whole-number ratios to form chemical compounds. -
J.J. Thomson- Electrons
Between 1856-1940, JJ Thomson used a glowing beam,or cathode ray, that traveled from the cathode to the anode. Through further experimentation he discovered that the charge to mass ratio of an electron is Constant. He also found that electrons from different atoms are the same. -
Dmitri Mendeleev- periodical table/atoms
Accorriding to Mendeleev the atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Mendeleev is best known for his work on the periodic table. His first Periodic Table was compiled on the basis of arranging the elements in ascending order of atomic weight and grouping them by similarity of properties. He predicted the existence and properties of new elements and pointed out accepted atomic weights that were in error. -
Eugen Goldstein- Protons
Goldstein discovered rays in a cathode ray tube traveling the opposite direction of the cathode (electron) rays. The positively charged particles are called protons. They have a mass of about 1840 times that of an electron. In the 1870s Goldstein undertook his own investigations of discharge tubes. -
Robert A. Millikan- oil drop
Robert millikan used Thomson's data and his own oil drop experiments to calculate the charge and mass of an electron. However, the actual charge and mass values were unknown. Therefore, if one of these two values were to be discovered, the other could easily be calculated. -
Ernest Rutherford- Gold Foil Experiment
He carried out the Geiger–Marsden experiment, which demonstrated the nuclear nature of atoms by deflecting alpha particles passing through a thin gold foil. Rutherfords conclusion was that atoms are mostly empty space. the other was that the positive charge and most of the mass of the atom are concentrated in a small region. The nucleus is the tiny core of an atom and is composed of neutrons and protons was his last conclusion. -
Niels Bohr
The discoveries of the electron and radioactivity at the end of the 19th century led to different models for the structure of the atom. In 1913, Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is moved only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits. When jumping from one orbit to another with lower energy, a light quantum is emitted. the theory exp. why atoms emitted light in fixed wavelengths. -
James chadwick-neutron
In February 1932, after only about two weeks of experimentation with neutrons,Chadwick sent a letter to Nature titled "Possible Existence of a Neutron". He communicated his findings in detail in an article sent to Proceedings of the Royal Society A titled "The Existence of a Neutron" in May. His discovery was a milestone in understanding the nucleus. -
Enrico Fermi
In 1934, Fermi began his most important work with the atom, discovering that nuclear transformation could occur in nearly every element. One of the elements' atoms he split was uranium. This work led to the discovery of slowing down neutrons, which led to nuclear fission and the production of new elements beyond the traditional Periodic Table.
