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Transcript
The Atom Democritus 400 B.C. Believed that matter was composed of invisible particles of matter he called atoms According to Democritus, atoms could not be broken into smaller particles. Atomic Theory of Matter • The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton. • Using two scientific laws discovered in the late 1700’s, Dalton built his atomic theory Dalton’s Postulates (1803) All matter is composed of extremely small particles called atoms. 1. Law of Conservation of Mass Antoine Lavoiser • The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. 2. Law of Constant Composition Joseph Proust •Also known as the law of definite proportions. •The elemental composition of a pure substance never varies. Dalton’s Postulates (1803) All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. 1 Dalton’s Postulates (1803) Atoms can not be subdivided, created or destroyed. Dalton’s Postulates (1803) Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms. Size of an atom Dalton’s Postulates (1803) The radii of most atoms fall within the range of 5 x 10-11 m to 2 x 1010 m Atoms in compounds combine, separate, or rearrange in normal chemical reactions to form new compounds. Atoms of an element are not changed into atoms of a different element by chemical reactions. Experimentation on matter has revealed to scientist that the atom is divisible into smaller particles called subatomic particles. There are three subatomic particles: J. J. Thomson (1897) Using a cathode ray tube, he determined the charge-to-mass ratio for the electron as: 1.76 x 108 C/g 1. Electrons 2. Protons 3. Neutrons 2 The Atom, circa 1900: Milikan Oil Drop Experiment (1909) • “Plum pudding” model, put forward by Thompson. • Positive sphere of matter with negative electrons imbedded in it. Radioactivity • One of the pieces of evidence for the fact that atoms are made of smaller particles came from the work of Marie Curie (18761934). • She discovered radioactivity, the spontaneous disintegration of some elements into smaller pieces. Discovery of the Nucleus Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. • Using voltage and change in the rate of fall of charged oil drops, he was able to determine the charge on each drop. • From Thompson’s charge to mass ratio, Milikan determined the charge and mass of an electron. Particles of Radioactivity • Three types of radiation were discovered by Ernest Rutherford: particles particles rays Discovery of the Nucleus Since some particles were deflected at large angles, Thompson’s model could not be correct. 3 The Nuclear Atom (1911) • Rutherford postulated a very small, dense nucleus of positive charge with the electrons around the outside of the atom. • Most of the volume of the atom is empty space. Combination of Millikan’s Findings and the Au Foil Experiment Lead to Rutherfords Model Rutherford (~1911) Nuclear Model • Protons were later discovered by Rutherford in 1919. . . heavy central (+) nucleus James Chadwick (1932) Further developed the atomic model by theorizing that alpha and beta radiation results from the decomposition of a neutral particle found in the nucleus, the neutron . . . . . . . . e- “about” nucleus “sea of e-” Subatomic Particles • Protons and electrons are the only particles that have a charge. • Protons and neutrons have essentially the same mass. • The mass of an electron is so small we ignore it. • H atoms - 1 p; He atoms - 2 p • mass He/mass H should = 2 • measured mass He/mass H = 4 4