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Download Chapter 3 – Atomic Structure - Mercer Island School District
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Chapter 3 – Atomic Structure Chem 2H Early Models of the Atom • Greek philosopher Democritus, 450 BC – All matter is composed of tiny, invisible particles called atomos. • French accountant Antoine Lavoiser, 1782 – Law of Conservation of Matter – matter is neither created nor destroyed. • French chemist Joseph Proust, 1799 – Law of Constant Composition – compounds contain the same elements always in the same proportions. Early Models of the Atom • English schoolteacher John Dalton, 1803 – Dalton’s Atomic Theory • Each element is composed of tiny atoms • Atoms of an element are identical but differ from those of other elements. • Atoms are neither created nor destroyed. • A given compound always has the same relative numbers and kinds of atoms. • Still no protons, neutrons or electrons… Discovering Atomic Structure • Michael Faraday, 1839 – What makes atoms different? It may be due to electricity. • J.J. Thomson, 1896 – Cathode ray tube. – Discovered and named the electron. – Determined charge of electron: -1.76 x 108 C/g Discovering Atomic Structure • Robert Millikan, 1909 – Oil drop experiment – Electron charge = 1.60 x 10-19 coulombs – Electron mass = 9.11 x 10-28 g • 1890s – 19teensish – Henri Becquerel – Pierre and Marie Curie – Discovery of radioactive elements Discovering Atomic Structure • Ernest Rutherford, 1909 – Gold foil experiment – Discovery of the positively charged nucleus – Empty electron cloud – Suspected protons and neutrons Subatomic Particles Particle Location Charge (C) Mass (g) Mass (amu) Proton Nucleus +1.602 E-19 1.673 E-24 1.0073 Neutron Nucleus 0 1.675 E-24 1.0087 Electron Electron -1.602 E-19 9.109E-28 0.0006 cloud Terminology • Atomic number = number of protons – Hydrogen = 1 – Oxygen = 8 • Ion: atom that has taken on a charge because of the loss or gain of electrons. • Isotope: differing forms of the same element, caused by differing amounts of neutrons. • Atomic mass: average mass of atoms of an element based on percent abundance of existing isotopes. • Mass number = protons + neutrons – Chlorine-35: 17 protons + 18 neutrons Atomic Mass • Calculate the atomic mass of chlorine using the following data: • Cl-35: 34.969amu, 75.53% • Cl-37: 36.966amu, 24.47% Changes in the Nucleus • Strong nuclear force: an attractive force that overcomes the electrostatic repulsion between protons in the nucleus. • Attractions between P-N and N-N exist. • Presence of neutrons adds a net attractive force to the inside of the nucleus. • Elements 1-20 have nearly 1:1 ratio of P to N. • Elements >20 have more neutrons than protons. • Elements >83 have unstable nuclei = radioactive Types of Radioactive Decay Name Identity Charge Penetrating Ability Alpha (α) Helium-4 nuclei 2+ Low, stopped by paper Beta (β) Electrons from the nucleus 1- Medium, stopped by heavy cloth Gamma (γ) Electromagnetic None radiation High, stopped by lead Radioactive Decay • Nuclear equations help radioactive decay make sense • Alpha decay: loss of an alpha particle • Beta decay: decay of a neutron accompanied by gamma release Radiometric Dating • All organic matter contains a small percentage of C-14 • Since Carbon-14 undergoes beta decay with a ½-life of 5730 years, scientists can estimate the age of a fossil, antique, etc. by evaluating the ratios of other carbon isotopes to the C-14 currently in the sample. • This process is called radiometric dating.
