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Early Atomic Theory and Structure Dalton’s Model of the Atom 1. Elements composed of small, indivisible particles (atoms). 2. Atoms the same element are identical (mass & size). 3. Atoms of different elements have different mass and size. Dalton’s Model of the Atom 4. Atoms of different elements combine in simple, whole number ratios to form compounds. 5. Atoms of 2 elements may combine in different ratios to form different compounds. The Law of Constant Composition Every compound contains 2 or more elements combined in a definite proportion by mass. This proportion is the same for every example of that compound. The Law of Constant Composition For example, every molecule of ibuprofen (C13H18O2) is 75.69% C, 8.79% H, and 15.52% O, by mass. These can be used as conversion factors: 75.69 g C = 100 g ibuprofen 8.79 g H = 100 g ibuprofen 15.52 g O = 100 g ibuprofen The Law of Multiple Proportions Atoms of 2 or more elements may combine in different ratios to produce more than one compound. The Nature of Electric Charge Positive and Negative Opposites attract, likes repel. May be transferred q1q 2 F= k 2 r Subatomic Particles Proton Neutron Electron Proton Mass = 1.673 x 10-24 g Charge = +1 Symbol = p (or p+) Neutron Mass = 1.675 x 10-24 g Charge = 0 Symbol = n (or n0) Electron Mass = 9.110 x 10-28 g Charge = -1 Symbol = e— Thomson’s Plum Pudding Model of the Atom Rutherford’s Gold Foil Experiment Thomson’s Model vs. Rutherford’s Experiment The Nuclear Atom Most of the mass and all of the positive charge is in a very small region in the center of the atom, the nucleus Very little of the mass, and all of the negative charge occupies most of the volume of the atom, outside of the nucleus. The Nuclear Atom The nucleus contains protons and neutrons The electrons surround the nucleus In a neutral atom, the number of electrons is equal to the number of protons. The Atomic Number (Z) The atomic number of an element is the number of protons in the nucleus. The number of protons in the nucleus is what determines which element any given atom is. Isotopes Have the same number of protons, but different number of neutrons. This means they are the same element, but have different masses. Mass number = p + n A Z Atomic number = p E Elemental symbol Atomic Mass Use the atomic mass unit (amu) The amu is defined as exactly 1/12 the mass of a 12C atom. 1 amu = 1.6606 x 10-24 g The average atomic mass of the elements is on most periodic tables. Average Atomic Mass (AAM) AAM = f1m1 + f2m2 + … m1 = mass of isotope 1 f1 = fractional abundance of isotope 1 m2 = mass of isotope 2 f2 = fractional abundance of isotope 2 . . . Average Atomic Mass Rubidium has 2 naturally occurring isotopes: Rb-85 which has a mass of 84.9118 amu and a natural abundance of 72.17%, and Rb-87 which has a mass of 86.9092 amu and a natural abundance of 27.83%. Calculate the AAM of Rubidium. 85.47 amu Average Atomic Mass Silver (Ag) has 2 naturally occurring isotopes, Ag-109 and Ag-107 which has a natural abundance of 51.84% and a mass of 106.905 amu. What is the isotopic mass of Ag-109? 108.9 amu