Chapter 13 Structure and Shape PDF

Chapter 13 Structure and Shape Edward I. Peters Mark S. Cracolice The University of Montana Copyright ©2021 Cengage Learning. All Rights Reserved. Goals (1 of 3) (Section 13.2) Draw the Lewis diagram for any molecule or polyatomic ion made up of main grou...

Chapter 13 Structure and Shape Edward I. Peters Mark S. Cracolice The University of Montana Copyright ©2021 Cengage Learning. All Rights Reserved. Goals (1 of 3) (Section 13.2) Draw the Lewis diagram for any molecule or polyatomic ion made up of main group elements. (Section 13.3) Describe the electron-pair geometry when a central atom is surrounded by two, three, or four regions of electron density. (Section 13.4) Given or having derived the Lewis diagram of a molecule or polyatomic ion in which a central atom is surrounded by two, three, or four regions of electron density, predict and sketch the molecular geometry around that atom. Copyright ©2021 Cengage Learning. All Rights Reserved. Goals (2 of 3) Draw a wedge-and-dash diagram of any molecule for which a Lewis diagram can be drawn. (Section 13.5) For a molecule with more than one central atom and/or multiple bonds, draw the Lewis diagram and predict and sketch the molecular geometry around each central atom, and draw a wedge-and-dash diagram of the molecule. (Section 13.6) Given or having determined the Lewis diagram of a molecule, predict whether the molecule is polar or nonpolar and indicate the positive and negative poles on its Lewis diagram Copyright ©2021 Cengage Learning. All Rights Reserved. Goals (3 of 3) (Section 13.7) Distinguish between organic compounds and inorganic compounds. Distinguish between hydrocarbons and other organic compounds. On the basis of structure and the geometry of the identifying group, distinguish among alcohols, ethers, and carboxylic acids. Copyright ©2021 Cengage Learning. All Rights Reserved. Drawing Lewis Diagram (1 of 2) Step 1: Calculate the total number of valence electrons. Adjust for charge on ions Step 2: Place the least electronegative atom(s) in the center of the molecule or ion. Step 3: Draw a tentative diagram. Join atoms by single bonds. Add unshared pairs to complete octet around all atoms except hydrogen. Copyright ©2021 Cengage Learning. All Rights Reserved. Drawing Lewis Diagram (2 of 2) Step 4: Calculate the number of valence electrons in tentative diagram and compare it with the actual number of valence electrons. If diagram has more electrons, remove a lone pair from the central and terminal atoms and replace them with an additional bonding pair. Repeat process till the number matches. Step 5: Check the Lewis diagram. Hydrogen atom must have only one bond, and all other atoms should have a total of four electron pairs. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (1 of 6) Draw the Lewis diagram for ammonia NH3. Solution: 5(N) + 3 ×1 (H) = 8 Since a hydrogen atom is never central, the nitrogen atom must be central. Eight electrons in the tentative diagram match the number available. The diagram is complete. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (2 of 6) Draw the Lewis diagram for the hydrogen carbonate ion, HCO3. Solution: 1(H) + 4(C) + 3 × 6(O) + 1(charge) = 24 Carbon is the central atom, since carbon is less electronegative than oxygen and hydrogen is always a terminal atom. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (3 of 6) There are 26 valance electrons. 4 × 2 (bonds) + 9 × 2 (lone pairs) = 26 All atoms check Hydrogen has one electron pair, and all other atoms have four. Enclosing Lewis diagram in square brackets indicating the charge. Copyright ©2021 Cengage Learning. All Rights Reserved. Resonance Structures and Isomers Resonance structures: Two or more equivalent Lewis diagrams for a molecule or ion that are created by changing only the positions of the electrons, in which the actual species is an average of the resonance structures. Resonance hybrid: The actual molecule is an average of the resonance structures. Isomers: Two compounds having the same molecular formulas but different structures. Copyright ©2021 Cengage Learning. All Rights Reserved. Electron-Pair Repulsion: Electron- Pair Geometry The arrangement of electron pairs around a central atom in a molecule or ion. Valence shell electron-pair repulsion theory (VSEPR) Electron pairs repel each other in real molecules. They distribute themselves in positions around the central atom that are as far away from one another as possible. Electron-pair angle: The angle formed by any two electron pairs and the central atom. Tetrahedron: Four identical equilateral triangles for its faces Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.5 Electron-Pair Geometry Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.6 Electron-Pair Geometries for Two to Four Electron Pairs (1 of 3) Regions of Electron Density: 2 Electron-Pair Geometry: Linear Bond Angle: 180° Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.6 Electron-Pair Geometries for Two to Four Electron Pairs (2 of 3) Regions of Electron Density: 3 Electron-Pair Geometry: Trigonal planar Bond Angle: 120° Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.6 Electron-Pair Geometries for Two to Four Electron Pairs (3 of 3) Regions of Electron Density: 4 Electron-Pair Geometry: Tetrahedral Bond Angle: 109.5° Copyright ©2021 Cengage Learning. All Rights Reserved. Table 13.2 Electron-Pair and Molecular Geometries Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.9 Molecular Geometries Based on Four Regions of Electron Density around the Central Atom Copyright ©2021 Cengage Learning. All Rights Reserved. Draw a Wedge-and-Dash Diagram Type 1: When two atoms are in the same plane as the page, connect them with a solid line of uniform width. Type 2: When an atom is behind the plane of the page, connect it to the central atom by a dashed line. Increase the width of the dashed line as it moves away from the central atom. Type 3: When an atom is in front of the plane of the page, connect it to the central atom by a wedge-shaped line. Increase the width of the wedge-shaped line as it moves away from the central atom. Copyright ©2021 Cengage Learning. All Rights Reserved. Predict Molecular Geometries (1 of 2) Step 1: Draw the Lewis diagram. Step 2: Count the regions of electron density around the central atom, both bonding and lone pairs. Step 3: Determine electron-pair and molecular geometries. Two regions of electron density—both geometries are linear with bond angle 180°. Three regions of electron density—electron-pair geometry is trigonal planar with bond angle 120°. All regions are bonding—molecular geometry is trigonal planar. Two regions of electron density bonding, one lone pair—angular or bent molecular geometry. Copyright ©2021 Cengage Learning. All Rights Reserved. Predict Molecular Geometries (2 of 2) Four regions of electron density—electron-pair geometry is tetrahedral with bond angle 109.5°. All regions are bonding—tetrahedral molecular geometry Three regions of electron density bonding, one lone pair— molecular geometry is trigonal pyramidal. Two bonding regions and two lone pairs—angular or bent molecular geometry Step 4: Sketch the wedge-and-dash diagram that matches the mental picture in step 3. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (4 of 6) Draw the wedge-and-dash diagram for nitrogen trichloride, NCl3. Name the electron-pair and molecular geometries Solution: The wedge-and-dash diagram of NCl3 The wedge-and-dash diagram reflects tetrahedral electron-pair geometry with one lone pair of electrons. Copyright ©2021 Cengage Learning. All Rights Reserved. The Geometry of Multiple Bonds The number of regions of electron density that surround a central atom determines the electron-pair geometry around that atom. A region of electron density can be a single, double, or triple bond, or a lone pair. As predicted by VSEPR theory, each region of electron density is distributed as far away from other regions of electron density as possible. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (5 of 6) Determine the molecular geometry of tetrafluroethylene, C2F4. Sketch a wedge-and-dash diagram of the molecule. Solution: The electron-pair and molecular geometries are trigonal planar around each carbon atom. Copyright ©2021 Cengage Learning. All Rights Reserved. Active Exercise (6 of 6) The Lewis diagram and the wedge-and-dash diagram are the same, with the exception of the lone pairs on the fluorine atoms. Copyright ©2021 Cengage Learning. All Rights Reserved. Polarity of Molecules Polar molecule: Molecule which has an asymmetrical distribution of charge, resulting in positive and negative poles. In an electric field, polar molecules tend to line up with: More electronegative atoms pointing toward the plate with positive charge Less electronegative atoms pointing toward the plate with negative charge Copyright ©2021 Cengage Learning. All Rights Reserved. Polarity Conditions for nonpolarity of molecules Central atoms have no lone pairs. All bonded atoms are identical. CCl4, CH4 When molecules are not symmetrical around an axis of symmetry Result in polar molecule CHCl3, CH2Cl2, CH3Cl Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.16 Polarity of Triatomic Molecules Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.17 Orientation of Polar Molecules in an Electric Field Copyright ©2021 Cengage Learning. All Rights Reserved. Organic and Inorganic Organic chemistry: Chemistry of carbon compounds Carbon atom—right size and right strength at bond formation help form stable molecules but weak enough to undergo reactions Inorganic chemistry—chemistry of all chemical compounds except hydrocarbons and most of their derivatives Carbonates, cyanides, oxides of carbon, and a few other carbon-containing compounds are exceptions that are often classified as inorganic. Copyright ©2021 Cengage Learning. All Rights Reserved. Hydrocarbons Compounds that are made up of only hydrogen and carbon. Alkane: Each carbon atom forms four single bonds. Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.19 The Isomers of Butane and Pentane Copyright ©2021 Cengage Learning. All Rights Reserved. Alcohols and Ethers Hydroxyl group: The part of an alcohol The chemical properties of an alcohol are the chemical properties of the hydroxyl group. Ether: A compound that has two hydrocarbon groups bonded to an oxygen atom. Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.21 Alcohols Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.24 Ethers Copyright ©2021 Cengage Learning. All Rights Reserved. Carboxylic Acids Compounds that contains the carboxyl group, The names end with -ic acid Are weak acids and ionize only slightly in water Copyright ©2021 Cengage Learning. All Rights Reserved. Figure 13.27 Carboxylic Acids Copyright ©2021 Cengage Learning. All Rights Reserved.