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Chapter 8 Chemical Bonding

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Chapter 8 Chemical Bonding Introduction zHow and why to atoms come together (bond) Chapter 8 to form compounds? zWhy do different compounds have such Chemical different properties? Chemical zWhat do molecules look like in 3 dimensions? Bonding 1 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 8 Topics Molecular and Ionic Compounds 1. Types of Bonds 2. Ionic Bonding 3. Covalent Bonding 4. Shapes of Molecules 3 4 Calcite-CaCO3.ent Ionic and Covalent Ionic and Covalent A. In ionic compounds, ions are held together by electrostatic forces – forces between oppositely charged ions. A. In molecular compounds, atoms are held together by covalent bonds in which electrons are shared. 5 6 Figure 8.2 1 8.2 Ionic Bonding z Lewis Symbols help us to focus on the valence electrons – those that can participate in bonding. 7 8 Ionic Compounds Structures of Ionic Compounds z Ions of like charge repel each other in ionic compounds, and opposite charged ions attract. This results in a 3-dimensional regular pattern called a crystal lattice. Figure 8.7 Figure 8.8 Sodium Chloride, NaCl, Crystal Nacl.ent 10 11 Cesium Chloride Calcium Fluoride Crystal Crystal Cscl.ent Figure 8.10 Caf2_.ent 12 13 2 Properties of Ionic Solids 8.3 Covalent Bonding z Why are crystalline ionic solids hard and brittle? z When two nonmetals form z Why are they poor conductors of electricity in the a bond, the bond is solid state? covalent. They are both close to the noble-gas electron configuration, so sharing will allow both to obtain it. z In a covalent bond, each Figure 8.12 shared electron interacts simultaneously with two 14 nuclei. 15 Carbon Dioxide – Covalent bonds The Octet Rule z The atoms of CO2 molecules are held together by zJust as in ionic strong covalent bonds. No bonds connect the bonding, covalent molecules, so CO molecules separate from each molecules, so CO2 molecules separate from each bonds are formed so other into the gas state at room temperature. that each atom can have the noble-gas electron configuration. Noble gases have 8 Figure 8.13 valence electrons, an octet. 16 17 Dry_ice.ent Figure 8.14 Lewis Formulas for the Diatomic Elements The Halogens zDo the atoms in each of these molecules have an octet? z How does hydrogen obtain a noble-gas zWhy do the halogens exist as diatomic electron molecules? configuration? Figure 8.16 Figure 8.15 18 19 3 Multiple Bonds P4 and S8 z How many valence electrons z How do phosphorus does an oxygen atom have? and sulfur obtain an • How many does it need to obtain an octet in P and S ? octet? 4 8 • O2 has a double bond, two pairs of shared electrons z How many valence electrons does a nitrogen atom have? • How many does it need to obtain an octet? • N2 has a triple bond, three pairs of shared electrons Figure 8.17 20 Figure 8.18 21 Valence Electrons and Number of Practice Bonds z How many bonds do each of the following z Draw the Lewis atoms like to form? structures for each of the following: a) H a) C H b) Cl 2 6 b) C2H4 c) O c) C2H2 d) N d) HCN e) C 22 23 Valence Electrons and Number of Steps for Writing Lewis Structures Bonds z Draw a Lewis structure for each of the following 1. Write an atomic skeleton. based on how many bonds each likes to form. 2. Sum the valence electrons from each atom to get Remember to include the nonbonding electrons so the total number of valence electrons. that all have octets (except H). 3. Place 2 electrons (a single bond) between each pair of bonded atoms. a) CO 2 4. Add electrons to the outside atoms to make b) H CO b) H2CO octets. Count electrons, and if you have enough, c) NF3 add to the central atom to give it an octet. d) H2O 5. If you don’t have enough electrons to give the central atom an octet, shift unshared electrons to bonding positions so that the central atom has an octet. This will give multiple bonds. 24 25 4 Writing Lewis Structures Resonance zWrite Lewis structures for each of the z Sometimes Lewis structures do not give an following: accurate picture of the bonding. z For example, there are two Lewis structures that • CO2 represent the SO2 molecule. • SO2 − • NO2 • HNO3 z The actual molecule is a blend of these two: • Both S-O bonds are the same length with partial double bond character 26 27 Resonance Resonance Structures z Like a single Lewis z Which of the following have resonance? structure, each alone structure, each alone z For those that do, how many resonance shows some aspect of the shell. structures do they have? − z Like a set of resonance a) NO3 structures, the pair of b) HNO3 photos together gives a c) CH better representation 4 of the shell. d) H2CO Figure 8.19 28 29 ionic-covalent Exceptions to the Octet Rule Classification of Bonding z Molecules with an odd number of electrons • E.g. NO z Incomplete octets • E.g. BH3 or BF3 z Expanded octets • E.g. SF4 or SF6 30 31 Figure 8.3 5 Polar Covalent Bonds Electronegativity z In polar covalent bonds, z The electronegativity scale tells us which electrons are not shared elements have a greater pull on electrons equally. This result in in a covalent bond. unequal sharing which The more electronegative element can be described as pulls bonding electrons more strongly and obtains a partial negative charge partial electron transfer. and obtains a partial negative charge (δ−). z In a polar bond there are In a polar bond there are The less electronegative element loses partial charges on the electron density in the bond and atoms sharing electrons. atoms sharing electrons. Figure 8.4 therefore obtains a partial positive charge (δ+). 32 33 Electronegativity Values Electronegativity Trends Figure 8.5 34 Figure 8.6 35 Electronegativity Trends 8.4 Shapes of Molecules z The difference in electronegativity between metals and z The shape of a molecule nonmetals is so large, that the electrons are transferred, influences many of its not shared. properties including taste z The greater the electronegativity difference, the more and smell. polar the bond. z Si-F > N-F> O-F >F-F z Sweet substances are often of a shape similar to z What partial charges go that of glucose. It’s –H and –OH groups fit into a on each atom? and –OH groups fit into a taste receptor site on the tongue. 36 37 Figure 8.23 6 Odor Receptors Predicting Shapes of Molecules zA simple model used with Lewis structures allows us to predict shapes of molecules. zValence-Shell Electron-Pair Repulsion Theory (VSEPR theory) is based on the fact that negative charges repel one another. ¾Valence electron pairs (bonding or nonbonding) repel one another and take up positions that maximize their distance and angles between them. Figure 8.24 38 39 VSEPR (CH4) Parent Structures z The methane molecule (CH4) has four pairs of electrons around the central atom (all bonding): H | H―C―H | H z Maximizing their distance gives bond angles of 109.5∞. VSEPR movie Figure 8.25 VSEPR program Figure p. 303 40 41 Parent Structures Table 8.05 zWhen determining the parent structure and bond angles for a molecule or polyatomic ion, each of the following is counted as 1 set of electrons: • Nonbonding electron pair • Single bond • Double bond Multiple bonds don’t repel and separate from each other so they are counted only • Triple bond once even though they contain more than one pair. 42 43 7 Parent Structures and Bond Angles Shapes of Molecules and Ions z Determine the parent structure and bond z The actual shape of a angles for each of the following. molecule includes only the atoms. a) NH3 z To envision the shape b) CO2 c) SO from a parent 2 structure, make the d) BH structure, make the 3 nonbonding electrons e) H O 2 invisible (but they are still there). z What is the shape of a water molecule? Figure 8.27 44 45 H2o.ent Shapes of Molecules and Ions zWhy do CO2 and SO2 have different 2 shapes? 3 46 47 Figure 8.28 Molecular Shapes 4 z Trigonal planar and trigonal pyramidal molecular shapes are much different from one another. Figure 8.26 48 49 8 Describing Shapes of Molecules with More Describing Shapes of Molecules with More than One Central Atom than One Central Atom z Glycine: H2NCH2CO2H z Glycine: H2NCH2CO2H 1. Draw the Lewis Structure. 1. Draw the Lewis Structure. 2. Determine the parent structure, remembering to 2. Determine the parent structure, remembering to consider all sets of electrons, including nonbonding consider all sets of electrons, including nonbonding electrons. electrons. 3. Determine the bond angles at each central atom 3. Determine the bond angles at each central atom (N, C, C, and O) (N, C, C, and O). 50 Figure 8.29 51 What are the bond angles? What are the bond angles? Figure 8.22 Figure 8.21 52 53 Molecular Polarity Molecular Polarity zWhich of these substances is a polar zWhich of these substances is a polar z Molecules with polar bonds can be polar under substance? certain conditions. ¾ A diatomic molecule with a polar bond is polar since there is only one bond. ¾ A molecule or polyatomic ion with polar bonds will be • polar if its geometry causes an asymmetric distribution of partial charge.
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