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400 BCE
Democritus
He was among the first Greek philosophers to observe that a cone or pyramid has one-third the volume of a cylinder or prism with the same base and height. From his examination of nature, Democritus developed what could be considered some of the first anthropological theories. According to him, human beings lived short lives in archaic times, forced to forage like animals until fear of wild animals then drove them into communities. -
John Dalton
John Dalton's atomic theory was the first attempt to describe all matter in terms of atoms. This theory was based on the law of conservation of mass and the law of constant composition. The first part of his theory states that all matter is made of atoms. The second part of the theory says all atoms of a given element are identical in mass and properties. The third part says compounds are combinations of two or more different types of atoms. A chemical reaction is a rearrangement of atoms. -
Eugen Goldstein
He is known for his work on electrical phenomena in gases and on cathode rays; he is also credited with discovering canal rays. Goldstein concluded that in addition to the electrons, or cathode rays, that travel from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction, from the anode toward the cathode. Because these rays pass through the holes, or channels, in the cathode, Goldstein called them canal rays. -
J.J Thomson
J.J Thomas discovered the electron by experimenting with a Crookes. He demonstrated that cathode rays were negatively charged. In addition, he also studied positively charged particles in neon gas. He then realized that the accepted model of an atom did not account for negatively or positively charged particles. So, he created a model of the atom using plum pudding which represented the atom, raisins which represented the negative electrons, and dough which contained a positive charge. -
Robert A. Millikan
Robert Millikan was an American, Nobel Prize-winning physicist, credited with discovering the value for electron charge, e, through the famous oil drop experiment, as well as achievements related to the photoelectric effect and cosmic radiation. The formula is e=f/na -
Ernest Rutherford
He was invited by J.J Thomson to collaborate on a study of X-rays. Together, Rutherford and Thomson studied the effects of X-rays on the conductivity of gases, resulting in a paper about dividing atoms and molecules into ions. Ernest Rutherford found that the atom is mostly empty space, with nearly all of its mass concentrated in a tiny central nucleus. The nucleus is positively charged and surrounded at a great distance by the negatively charged electrons. -
James Chadwick
Chadwick made the discovery that in the domain of nuclear science: he proved the existence of neutrons – elementary particles devoid of any electrical charge. In contrast with the helium nuclei (alpha rays) which are charged, and therefore repelled by the considerable electrical forces present in the nuclei of heavy atoms, this new tool in atomic disintegration need not overcome any electric barrier and is capable of penetrating and splitting the nuclei of even the heaviest elements.