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Chapter 3 History of the Atom 1 Element Symbols Rule 1 First letter is capitalized Ex: Rule 2 Second letter is lower case Ex: Rule 3 Some are Latin names Ex: Rule 4 Some elements are named for places, scientists, greek gods… Ex: 2 3 Foundations of Atomic Theory __________________________________________ Mass is neither created nor destroyed. HgO Hg + O 433.2 g 401.2g + 32g 4 Dalton’s Atomic Theory (1808) 1. All matter is made of indivisible and indestructible atoms. 2. All atoms of the same element are identical in their physical and chemical properties. 3. Atoms of different elements have different properties. 4. Atoms of different elements combine in simple whole-number ratios to form compounds. 5. Atoms cannot be subdivided, created, or destroyed when they are combined, separated, or rearranged in chemical reactions. 5 Changes to Dalton’s Theory Are atoms actually indivisible? Are all atoms identical? 6 The Atom the smallest unit of an element that retains that elements properties Made of protons, neutrons, and electrons Can you see inside an atom? How did they know it contains protons, neutrons and electrons? 7 Cathode Ray Tube When connected to electric current the remaining the gas forms a _________________________. The beam always started at the _____________electrode and flowed to the _______________ electrode. The electrode is named by what type of particle it attracts Cathode: Negative (-) Anode: Positive (+) 8 Cathode Ray Conclusions 1. The beam was attracted to a _____________magnet. 2. The beam must be made of ______________particles. 9 JJ THOMSON DISCOVERED A NEGATIVE PARTICLE CALLED THE: 10 11 Plum Pudding Model Radioactivity Becquerel discovered uranium ______________ would expose photographic plates in the dark The properties of an element changed as it gave off radiation Curie Discovered radium and polonium The radioactive emissions of ________, _________ and ____________ rays were identified. 12 13 (Uranium compound) Types of Radioactive Decay Decay Type Reaction Symbol Charge Strength Particle Can be stopped by… Alpha Beta Gamma 14 Radioactive Decay 15 Ernest Rutherford’s Gold Foil Experiment Set up Gold Foil with a detection sheet around it. Set up radioactive source emitting alpha particles. 16 ____________________shot at gold foil. Rutherford’s Gold Foil Experiment 17 Rutherford: It’s like shooting a cannon at a piece of tissue paper and having it bounce back at you! _______particles went through the gold foil SOME particles ____________back 18 Gold Foil Conclusions 1. 2. 3. 19 Rutherford’s Model of the Atom + NUCLEUS – EMPTY SPACE 20 Niels Bohr – 1913 Developed a new diagram of the atom Electrons can only be at certain energies Electrons must gain a specific amount of energy to move to a higher level, called a quantum ** 21 Bohr’s Model of the Atom 22 Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons AX Z Isotopes: 23 Do You Understand Isotopes? How many protons, neutrons, and electrons are in 146 C? How many protons, neutrons, and electrons are in 116 C? 24 Hyphen Notation The mass number is written with a hyphen after the name of the element. Element – Mass# Uranium – 235 Calcium – 40 25 Ion: an atom, or group of atoms, that has a net positive or negative charge. cation – _________________________________ If a neutral atom loses one or more electrons it becomes a cation. Na Na+ anion – __________________________________ If a neutral atom gains one or more electrons it becomes an anion. Cl 26 Cl- Do You Understand Ions? + 27 3 How many protons and electrons are in 13 Al ? 2- ? Se How many protons and electrons are in 78 34 27 Relative Atomic Mass Carbon is the standard for all masses on the periodic table. Carbon: 6 p and 6 n = 12 amu Atomic Mass Unit Periodic table lists weighted average atomic masses of elements (like a GPA or final grade calculation) 28 Calculation AVERAGE Atomic Mass 75% 20% 5% 133Cs 132Cs Steps: 1. 134Cs 2. 3. 29