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465
Democritus 465 BC
Democritus Said everything was composed of atoms. That between atoms, there is empty space; that atoms are indestructible, in motion, that there is no number to describe how many atoms there is. The number of atoms, and kinds of atoms, are different in shape, and size. -
John Dalton
Elements are made of small particles called atoms. Atoms of an element are the same in size and mass, atoms of different elements differ in size, mass, and other properties. Atoms can’t be created or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In chemical reactions, atoms are combined, separated, or rearranged. 1803 -
JJ Thompson
He knew that the rays were made of very light, negatively charged particles which were a universal building block of atoms. He called the particles "corpuscles”. Farther on in science the scientists changed the name to electron. 1897 -
Ernest Rutherford
He discovered alpha and beta rays, created the laws of radioactive decay, and found alpha particles as helium nuclei. Most important, he postulated the nuclear structure of the atom, showing that when alpha particles are fired into gas atoms, a few are deflected, which sets a dense, positively charged central region containing most of the atomic mass. 1911 -
Robert Milliken
His experiment measured the force on tiny charged droplets of oil suspended against gravity between two metal electrodes. Knowing the electric field, the charge on the droplet would be found. 1915 -
Neils Bohr
Bohr was the first to discover that electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the properties of an element.
1922 -
James Chadwick
James Chadwick began working in the Cavendish laboratory. Using polonium as a source of (what he believed were) neutrons. Protons were released by the wax and Chadwick made measurements of the protons’ behavior.The protons behaved in the manner they should of if they had been hit by electrically neutral particles with a mass similar to the proton. Chadwick had discovered the neutron. 1932
