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350
Aristotle 384BC - 322BC
Aristotle in his study of Physics has said that there are basically five elements
Earth, which is cold and dry; this corresponds to the modern idea of a solid. Water, which is cold and wet; this corresponds to the modern idea of a liquid. Air, which is hot and wet; this corresponds to the modern idea of a gas. Fire, which is hot and dry; this corresponds to the modern idea of heat. Aether, which is the divine substance that makes up the stars and planets. -
470
Democritus Ca.460 - Ca.370BC
His theory is "Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. This piece would be indivisible".
He named the smallest piece of matter “atomos,” meaning “not to be cut.”
To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes. Atoms were infinite in number, always moving and capable of joining together. -
John Dalton 1766 - 1844
He was such a brilliant youth that he became an English shool teacher when barely 12 years old.
The main postulates of Dalton’s atomic theory are as follows.
1.All matter is made of atoms. Atoms are indivisible and indestructible. 2.All atoms of a given element are identical in mass and properties. 3.Compounds are formed by a combination of two or more different kinds of atoms. 4.A chemical reaction is a rearrangement of atoms. 5.Atoms can be neither created nor destroyed. -
Henri Becquerel 1852 - 1908
He discovered radioactivity by accidentally putting crystal in the same drawer as a piece of photographic paper. He was going to do a different experiment, but it was canceled because of bad weather. When it started to clear up, he went to get his tools, He saw that the piece of material, which contained Uranium, could produce an image on the photographic paper even in the absence of light. -
JJ Thomson 1856 - 1940
He identified the negatively charged electron in the cathode ray tube in 1897. He deduced that the electron was a component of all matter and calculated the charge to mass ratio for the electron. His atomic model is plum pudding model.1An atom consists of a sphere of positive charge with negatively charged electron embedded in it.2The positive and the negative charges in an atom are equal in magnitude, due to which an atom is electrically neutral. It has no over all negative or positive charge. -
Marie and Pierre Curie 1867 - 1934
Helped Henri Becquerel with his discovery of radioactivity between 1898 and 1902. Marie says “My experiments proved that the radiation of Uranium compounds can be measured with precision under determined conditions, and that this radiation is an atomic property of the element of Uranium.” All three thought the Atom looked as though it was a planet with many electron rings around them. -
Robert Millikan 1868 - 1953
Robert Millikan determined the unit charge of the electron with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. e = 1.60 x 10-19 coulombs -
Ernest Rutherford 1871 - 1937
He proposed the nuclear atom as the result of the gold-foil experiment. His experiment involved the firing of radioactive particles through minutely then metal foils and detecting them using screens coated with zinc sulfide. His model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. He proposed the existence of the third atomic particle, the neutron, in 1920. -
Neils Bohr 1885 – 1962
He proposed that electronds orbit the nucleus without losing energy, could move only in fixed orbits of specific energies. Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus. -
James Chadwick 1891 - 1974
James Chadwick found the third elementary particle of the atom; the neutron.Chadwick smashed alpha particles into beryllium, a rare metallic element, and allowed the radiation that was released to hit another target: paraffin wax. When the beryllium radiation hit hydrogen atoms in the wax, the atoms were sent into a detecting chamber.nt results showed a collision with beryllium atoms would release massive neutral particles. -
Quantum Theory - up to now
The quantum theory is the most up-to-date theory of the times. Its' model of the atom explains its stability and like Bohr's Model, treats atoms as electrons surrounding a nucleus. The electrons, however, do not have a planetary orbit. the theory describes electron as the wave-particle duality of quantum. it allows electrons to be described as waves, instead of particles
