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Chapter 19 Winter 2011

Chapter 19 Winter 2011 - Chapter 19 Atoms Molecules and...

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Unformatted text preview: Chapter 19 Atoms, Molecules, and Extended­Bonding Substances (oh, my!) Did you read all of Chapter 19 for today? a) b) Yes No Chemical matter is classified into a number of types Elements vs Compounds Elements: only one kind of atom Compounds: two or more kinds of atoms Which are elements? Which are compounds? Chemical Bonds The constituent atoms in molecules are held together by “bonds”. A bond is usually a pair of electrons. • Strong Bonds: Metallic, Ionic, Covalent • Weak Bonds: van der Waals, Hydrogen Quantum Model Explains Them All! • Chapters 20­23 will feature each of the bonding types In our visual models we often use sticks to represent bonds, and balls to represent atoms How does bonding work? Atoms give up, obtain, or share electrons and in the process combine to form the substances around us. Bonding involves only the electrons in the outermost, unfilled orbitals: the valence electrons. All other inner electrons don’t matter as far as bonding is concerned! All bonding involves atoms sharing or exchanging electrons in a “stable” way. Stability means: • To fall to a lower energy state and thus be more tightly bound. • To completely fill an orbital set. Writing chemical formulas for molecular matter Identify atoms in a molecule Give the number of each atom type Examples • H2O • S8 • CH4 Write down the chemical formula for these two molecules Hydrogen peroxide Bleach and disinfectant Glycine One of the 23 essential Amino acids Chemical Formulas for Network/Extended Matter Atoms of Atoms metallic Mg metallic 1. Identify kinds of atoms 2. Give the relative number of each atom type Sodium chloride NaCl O Na Cl Si Quartz SiO2 Molecules have shapes How many ways can you arrange Carbon and Hydrogen? Guess the shape of Benzene • C6H6 Planar Tetrahedral Linear Friedrich August Kekulé Friedrich discovered benzene’s structure discovered The shape of a molecule is determined by molecular orbitals When H2 was formed from H, energy was released as heat. So in H2 each electron needs more energy than before to escape. H has a different discrete spectrum than H2. This allows astronomers to tell the temperature of hydrogen gas. H H H2 The electrons sit lower in the energy well than before Molecules belong to families -Hydrocarbons What elements are present? Methane -- CH4 Propane -- CH3CH2CH3 (or C3H8) (or Formulas? All react to form CO2 and H2O Octane -- CH3CH2CH2CH2CH2CH2CH2CH3 (or C8H18) (or methane, propane, octane belong to hydrocarbon family all three are used as fuels What formula represents this family of molecules? A. (CH2)n B. CnH2n+2 C. CnHn+2 D. C2nHn CH4 C3H8 where n is an integer starting with 1 C4H10 Another molecular family – Organic acids CH3CO2H -- Acetic acid (gives vinegar its taste) What groups of atoms are common to all three molecules? CH3(CH2)10CO2H Lauric acid – in coconut milk CH3__ CH2 __ __ CO2H (carboxyl) CH3(CH2)14CO2H -- Palmitic acid (palm oils, and animals) family of organic acids all three are in foods we eat Amino Acids, the building blocks for protiens NH2CH2CO2H NH2CHCH3CO2H NH2C10H9NHCO2H What elements are present? Formulas? What groups of atoms What are common to all three molecules? three __ NH2__ (amine) (amine) family of amino acids – acid group + amino group -- also in foods we eat __ CO2H (Carboxyl) CO (Carboxyl) Deducing molecular formulae and structures: Time-of-Flight Mass Spectrometer detector Number of fragments accelerator plates positive fragments Length of flight path kinetic energy = ½(mass)(speed)2 If all fragments are given the same kinetic energy, which ones get to the detector first? time Deducing molecular structure with a mass spectrometer Deducing molecular structure with a mass spectrometer Masses of molecular fragments Mass Spectroscopy Mass Spectroscopy C9O4H8 Aspirin Parent peak: Mass of the molecule 180 amu Motion within a molecules Bonds are not rigid • lengths change • Bond angles bend – angle opens and closes The various types of stretches and bends occur with different energies Energy depends on how stiff the bond is and the atomic masses that are moving. A simplified view of infrared spectroscopy Absorption IR spectrum (continuous with dark bands) continuous IR spectrum Detector Infrared Source Analogous visible continuous spectrum Sample Analogous visible absorption spectrum IR spectroscopy can be used to deduce chemical formulas and structures IR Vibrational Spectroscopy fingerprint region C9O4H8 Aspirin C=O Energy of vibration Different molecular groups vibrate in different regions A portion of the IR spectrum for each molecule is shown. Formulate a hypothesis about what portion of the molecule gives rise to the sharp set of peaks labeled P? A. B. OH group Hydrocarbon part P P PA P A How do we deduce chemical formulas and structures? Crystallography ...
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