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The Structure of the Atom 1 Early Theories of Matter 2 460-370 BC Democritus Greek Philosopher Named atom »smallest unit of matter »means indivisible 3 4 1807 John Dalton: Atomic Theory Revived and revised Democritus’ ideas and began developing the modern atomic theory 5 Dalton’s Atomic Theory all elements are composed of tiny indivisible particles called atoms atoms of same element alike. Each element is different from atoms of other elements atoms of different elements combine in simple whole number ratios to form compounds chemical reactions occur when atoms are rearranged 6 7 Picture shows: •Conservation of Mass •Element combing in simple whole number ratios 8 Subatomic Particles & the Nuclear Atom 9 Discovering the Electron 10 1879 William Crookes investigated electrical discharge in gases cathode ray tube Cathode rays are streams of negatively charged particles. The particles are found in all forms of matter 11 12 1897 J.J. Thomson determined nature of cathode ray determined charge to mass ratio of electron Found that atoms were divisible Dalton & Democritus were wrong 13 1901 J.J. Thomson positive beam experiments plum pudding model of atom or chocolate-chip cookie dough model of the atom 14 15 1909 Robert Milliken determined charge of electron oil drop experiment with Thomson’s charge to mass ratio: able to determine the mass of e Mass of electron = 9.1x10-28 grams 16 1911 Ernest Rutherford discovered nucleus gold foil experiment disproved plum pudding model small dense central part of atom = nucleus (+) charge 17 18 19 20 1920 Rutherford Refined concept of nucleus Concluded that nucleus contained positively charged particles called protons 21 1932 James Chadwick identified neutron same mass as proton no charge 22 23 24 How Atoms Differ 25 NUCLEUS 1. 2. 3. 4. Protons with (+) charge Neutrons with no charge. Protons & neutrons have about the same mass. (+) charge is responsible for most of mass of atom (dense central part). 26 ELECTRONS move around nucleus responsible for most of volume of atom (-) charge negligible mass 27 ATOMIC NUMBER & MASS NUMBER 28 ATOMIC NUMBER # of protons in nucleus it identifies the element elements in Periodic Table are listed in increasing order of atomic # if atom is neutral: the # of protons equals # of electrons 29 MASS NUMBER sum of protons & neutrons in nucleus written as part of name: must be given to you Neon-20 mass #20 p + n = 20 atomic #10 p = 10 n = 10 30 mass # p+n Symbol atomic # p ? # neutrons 31 Oxygen-17 mass # 17 atomic # 8 p + n = 17 p =_8_ 9n 32 To calculate electrons for an ion you must look at the charge written in the upper right corner To determine the number of electrons: » If the charge is positive then subtract that number from the number of protons. » If the charge is negative then add that number to the number of protons 33 ISOTOPES Atoms of the same element are not all identical - may differ in # of neutrons Isotopes - atoms of the same elements (same # of protons), but different mass # (different # neutrons) and therefore different masses 34 12 13 C 6 14 C 6 ? # neutrons 6 7 C 6 8 35 ATOMIC MASS 36 ATOM smallest unit of an element that can exist alone and still have the properties of that element 37 Atomic Mass Average Atomic Mass - weighted average of the masses of the naturally occuring isotopes » relative mass based on carbon-12 as the standard » Carbon-12 is defined as having a mass of exactly 12 amu atomic mass unit (amu) - 1/12 of the mass of a carbon-12 atom 38 Weighted Average Example 50% test 30% Lab 20% Daily 70 80 90 = 50%(70) + 30%(80) + 20%(90) = .5(70) + .3(80) + .2(90) = 35 + 24 + 18 = 77 39 Isotopes of Hydrogen H-1 H-2 H-3 protium deuterium tritium 1.0078 2.0140 3.0160 99.985% 0.015% ------- 40 Calculating Average Atomic Mass Multiply the percent (as a decimal #) by the mass and then add each together. = 99.985% (1.0078 amu) + 0.015% (2.0140 amu) = .99985 (1.0078 amu) + .00015 (2.0140 amu) = 1.0076488 amu + 0.0003021 amu = 1.00795 amu 41 Example chlorine - 35 75.8% chlorine - 37 24.2% Will the average atomic mass be closer to 35 or 37? (35 because higher %) 75.8%(35) + 24.2%(37) = 35.5 amu 42