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Joyce Wang Atomic Theory • The physical theory of the nature of the structures, behavior, and properties of the atom. Greek Philosophers • From the earliest times, philosophers debated what the world was made of • Democritus was one of the earliest of these philosophers • Democritus thought that the physical world was made of tiny individisble particles called atomos, “indivisible” Greek Philosophers • Plato and Aristotle challenged his view by saying there can be no indivisible particles • Because of their influence, the atomic view of matter faded for centuries • The Aristotelean philosophy dominated Western culture for centuries Aristotle Reemergence of Atomic Theory • Concept of atoms reemerged in Europe during the 17th century, when scientists wanted to explain properties of gases. Ex: air in constant motion • Isaac Newton favored the atomic theory- at this time he was the most famous scientist. • John Dalton’s chemical Atomic Theory: – Each element made of small particles (atoms) – All atoms of same element are identical; atoms of different elements are different and have different properties – Atoms of an element are not changed into different types of atoms by chemical reactions; atoms are not created or destroyed in chemical reactions – Compounds are formed when atoms of more than one element combine; given compound always has the same ratio of atoms Dalton • According to Dalton’s theory, atoms are the basic building blocks of matter. They’re the smallest particles of an element that retain the chemical identity of the element. • His theory explains several simple laws of chemical combination that were known in his time. – Law of constant composition – Law of conservation of mass – Law of multiple proportions Discovery of Subatomic Particles • Until 19th century, everyone believed the atom was only a round sphere • Scientists used instruments called cathode rays to study electrical discharge. • 1897, J.J. Thompson challenged this view when he discovered the electron. • Thompson came up with the plum pudding model J. J . Thompson’s Plum pudding model How did he discover it? • The cathode ray tube The Cathode Ray Tube • If electric current passes through a vacuum, a stream of glowing materials can be seen. These were electrons. • Thompson discovered that this glowing material would bend towards a positively charged electric plate. • Therefore, he concluded that the particles were negatively charged. Robert Millikan • Thompson measured the value of 1.76 x 10^8 coulombs per gram for the ratio of electron’s electrical charge to its mass. • Once the charge-to-mass ratio of electrons were known, Robert Millikan could figure out either the charge or the mass of an electron from the other value. • He performed the “Mikllikan oil-drop experiment” and measured the charge of an electron. • By using this value, he could find the mass of the electron which he found to be 9.10 x 10^ -28 g • The modern value is 9.10939 x 10^ -28 g Rutherford • Rutherford disproved the plum pudding model by discovering a nucleus • Rutherford scattered particles through a thin gold foil and observed the angles they were deflected in. • Some of the particles were deflected right back in the direction in which they had come. • Rutherford explained this by saying that most of the mass in the atom was in a positively charged dense region in the very center of the atom. Rutherford’s Experiment James Chadwick • 1932, the British scientist James Chadwick discovered neutrons. Henri Becquerel & Marie Curie • Becquerel was studying pitchblende, and discovered it emitted high-energy radiation • This spontaneous emission of radiation is called radioactivity • Marie Curie began experiments to isolate the radioactive components Rutherford • Revealed three types of radiation: alpha, beta, gamma radiation. • Alpha particles have a positive charge and are fast moving particles. • Beta particles are also fast-moving but have a negative charge. • Gamma radiation is highenergy radiation and it has no charge or particles. Modern View of Atomic Structure • Atoms are made of protons, neutrons, and electrons. • Atoms have an equal number of electrons and protons, so they have no net electrical charge • Protons and Neutrons reside in the nucleus • Nucleus is very tiny; electrons take up the most volume in the atom. Isotopes, Atomic Numbers, Mass Numbers • All atoms of an element have the same number of protons in the nucleus. • The specific number of protons is different for different elements. The number of electrons of an element should match the number of protons. • Isotopes: atoms of an element that differ in the number of neutrons and therefore in mass. • Atomic number: the number of protons. All atoms in an element have the same atomic number. • Mass Number: Total number of protons plus neutrons in the atom. • Nuclide: Atom of a specific isotope.