Atomic Theory

780 Words 4 Pages
The atom is the smallest unit of matter. The name “atom” comes from the Greek word atomos, which means “uncuttable.” Atomic theory states that all matter is made up of these small particles, and that these atoms, and the matter made from them, cannot be created nor destroyed, merely reshaped and reformed. Modern chemistry unofficially began in the eighteenth century. French scientist Antoine Lavoisier discovered the law of the conservation of mass c. 1785. This law states that “The total mass remains constant during a chemical change,” (Ebbing and Gammon, 7) or that matter does not simply vanish during a chemical reaction.
Shortly after Lavoisier’s discovery, Chemist John Dalton created the theory that all matter is comprised of tiny, indivisible
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In 1897, British physicist J.J. Thompson performed multiple experiments in accordance with Dalton’s theory. In one study in particular, Thompson used a piece of equipment known as a Cathode Ray Tube (CRT). This machine used two high-voltage electrodes inside a sealed glass tube which, when powered, would emit a greenish light from one end to another. This light is formed by the interaction of the cathode rays and the surrounding glass. The rays traveled down the length of the tube and pass through a hole to form a beam. The beam would bend away from negatively charged plates and towards positively charged ones, demonstrating that the rays held a negative charge. “From such evidence, [Thompson] concluded that a cathode ray consists of a beam of negatively charged particles (or electrons) and that electrons are constituents of all matter” (Ebbing and Gammon, …show more content…
He was able to find the charge of an electron with an experiment involving passing oil through an electrically charged plate with a hole in the center. Using this newfound charge and the determined mass-to-charge value of an electron, “you obtain an electron mass of 9.109 x 10-31 kg, which is more than 1800 times smaller that the lightest atom” (Ebbing and Gammon, 47).
Some years later, in 1911, Ernest Rutherford created his nuclear model of the atom. Rutherford launched positively-charged alpha particles at a metal foil and recorded the results. Most all of the particles passed through the foil with ease, however, a small few were reflected at large angles backwards off of the foil. From this data, Rutherford was able to find that most of the mass of an atom is in the center, or nucleus. This mass is extremely compact, however, “Nuclei have diameters of about 10-15m whereas atomic diameters are about 10-10m” (Ebbing and Gammon,

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