Chemistry: Atoms First Second Edition Julia Burdge & Jason Overby Chapter 7 Molecular Geometry, Intermolecular Forces, and Bonding Theories M. Stacey Thomson Pasco-Hernando State College Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

ABx 7.1 Molecular Geometry Molecular shape can be predicted by using the valence-shell electron-pair repulsion (VSEPR) model. A is the central atom surrounded by x B atoms. x can have integer values of 2 to 6. ABx

2 electron domains (on central atom) The VSEPR Model The basis of the VSEPR model is that electrons repel each other. Electrons are found in various domains. Lone pairs Single bonds Double bonds Triple bonds 1 single bond 1 double bond 1 lone pair 3 single bonds 1 lone pair 2 double bonds 2 electron domains (on central atom) 3 electron domains (on central atom) 4 electron domains (on central atom)

Electron-Domain Geometry and Molecular Geometry The basis of the VSEPR model is that electrons repel each other. Electrons will arrange themselves to be as far apart as possible. Arrangements minimize repulsive interactions. 2 electron domains Linear 3 electron domains Trigonal planar

Electron-Domain Geometry and Molecular Geometry 4 electron domains Tetrahedral 5 electron domains Trigonal bipyramidal 6 electron domains Octahedral

Electron-Domain Geometry and Molecular Geometry The electron domain geometry is the arrangement of electron domains around the central atom. The molecular geometry is the arrangement of bonded atoms. In an ABx molecule, a bond angle is the angle between two adjacent A-B bonds. Linear Trigonal planar 180° 120° Trigonal bipyramidal Octahedral Tetrahedral 90° 109.5° 120° 90°

Electron-Domain Geometry and Molecular Geometry AB5 molecules contain two different bond angles between adjacent bonds. Axial positions; perpendicular to the trigonal plane 90° Equatorial positions; three bonds arranged in a trigonal plane. 120° Trigonal bipyramidal

Electron-Domain Geometry and Molecular Geometry When the central atom in an ABx molecule bears one or more lone pairs, the electron-domain geometry and the molecular geometry are no longer the same. O •• = −

Electron-Domain Geometry and Molecular Geometry ~ ~ ~

Electron-Domain Geometry and Molecular Geometry The steps to determine the electron-domain and molecular geometries are as follows: Step 1: Draw the Lewis structure of the molecule or polyatomic ion. Step 2: Count the number of electron domains on the central atom. Step 3: Determine the electron-domain geometry by applying the VSEPR model. Step 4: Determine the molecular geometry by considering the positions of the atoms only. https://phet.colorado.edu/en/simulation/molecule-shapes

Worked Example 7.1 Determine the shapes of SO3. Strategy Use Lewis structures and the VSEPR model to determine first the electron-domain geometry and then the molecular geometry (shape).

Deviation from Ideal Bond Angles Some electron domains are better than others at repelling neighboring domains. Lone pairs take up more space than bonded pairs of electrons. Multiple bonds repel more strongly than single bonds.

Geometry of Molecules with More Than One Central Atom For more complex molecules each central atom can have its own geometry label. Central O atom No. of electron domains: 4 Electron-domain geometry: tetrahedral Molecular geometry: bent Central C atom No. of electron domains: 4 Electron-domain geometry: tetrahedral Molecular geometry: tetrahedral

Worked Example 7.2 Acetic acid, the substance that gives vinegar its characteristic smell and sour taste, is sometimes used in combination with corticosteroids to treat certain types of ear infections. Its Lewis structure is

6.2 Bond Polarity There are two extremes in the spectrum of bonding: covalent bonds occur between atoms that share electrons ionic bonds occur between a metal and a nonmetal and involve ions Bonds that fall between these extremes are polar. Text Practice: 6.10 In polar covalent bonds, electrons are shared but not shared equally. M:X Pure covalent bond Neutral atoms held together by equally shared electrons Mδ+Xδ− Polar covalent bond Partially charged atoms held together by unequally shared electrons M+X− Ionic bond Oppositely charged ions held together by electrostatic attraction

Electronegativity and Polarity Electron density maps show the distributions of charge. Electrons spend a lot of time in red and very little time in blue. Electrons are shared equally nonpolar covalent Electrons are not shared equally and are more likely to be associated with F polar covalent Electrons are not shared but rather transferred from Na to F ionic

Electronegativity There is no sharp distinction between nonpolar covalent and polar covalent or between polar covalent and ionic. The following rules help distinguish among them: If both atoms need the electrons to complete their octets, it will be MOSTLY covalent EXAMPLES: If only one atom of the pair needs the electrons to complete its octet and the other makes an octet by avoiding sharing, it will be MOSTLY ionic

Worked Example 6.1 Classify the following bonds as nonpolar, polar, or ionic and rank them in order of increasing bond polarity: (a) the bond in ClF, (b) the bond in CsBr, and (c) the carbon-carbon double bond in C2H4. Strategy Electronegativity values are: Cl (3.0), F (4.0), Cs (0.7), Br (2.8), C (2.5). Use this information to determine which bonds have identical, similar, and widely different electronegativities. Text Practice: 6.18

Dipole Moment, Partial Charges, and Percent Ionic Character Regions where electrons spend little time Regions where electrons spend a lot of time H− F •• An arrow is used to indicate the direction of electron shift in polar covalent molecules. The consequent charge separation can be represented as: Deltas (δ) denote a partial positive or negative charge. H− F •• δ+ δ−

Dipole Moment, Partial Charges, and Percent Ionic Character A quantitative measure of the polarity of a bond is its dipole moment (μ). Q is the quantity of charge. r is the distance between the charges. μ is always positive and expressed in debye units (D). μ = Q x r

Dipole Moment, Partial Charges, and Percent Ionic Character A quantitative measure of the polarity of a bond is its dipole moment (μ). μ = Q x r

Dipole Moment, Partial Charges, and Percent Ionic Character Although the designations “covalent,” “polar covalent,” and “ionic” can be useful, sometimes chemists wish to describe and compare chemical bonds with more precision. Comparing the calculated dipole moment with the measured values gives us a quantitative way to describe the nature of a bond using the term percent ionic character. percent ionic character = ×100% μ (observed) μ (calculated assuming discrete charges) μ = Q x r

Dipole Moment, Partial Charges, and Percent Ionic Character The figure below demonstrates the relationship between percent ionic character and the electronegativity difference in a heteronuclear diatomic molecule.

Study Guide for Sections 7.1, 6.2 DAY 13, Terms to know: Sections 7.1, 6.2: VSEPR, linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, trigonal bipyramidal, octahedral, electron domain geometry, molecular geometry, bond angle, polar covalent bonds DAY 13, Specific outcomes and skills that may be tested on exam 2: Sections 7.1, 6.2 Be able to determine the electron domain geometry for a central atom Be able to determine the molecular geometry for a central atom Be able to use electronegativities to quantify and rank bond polarity and classify as either nonpolar, polar covalent, or ionic Be able to use polarity arrows or partial charge symbols to represent bond polarity

Extra Practice Problems for Sections 7.1, 6.2 Complete these problems outside of class until you are confident you have learned the SKILLS in this section outlined on the study guide and we will review some of them next class period. 7.7 7.9 7.13 7.99 (just geometry) 7.101 a) c) 7.103 (a-d, f, g) 7.105 (a, b, e) 7.143 (a) 6.17 6.19

Prep for Day 14 Must Watch videos: Other helpful videos: https://www.youtube.com/watch?v=q3g3jsmCOEQ (Dipole moments, Khan Academy) http://www.youtube.com/watch?v=P79WCZIZXwk (Dipole moments) https://www.youtube.com/watch?v=1iYKajMsYPY (London forces, Bozeman) http://www.youtube.com/watch?v=PyC5r2mB4d4 (H-bonding, Tyler DeWitt) Other helpful videos: http://ps.uci.edu/content/chem-1a-general-chemistry (UC-Irvine lecture 11) https://www.youtube.com/watch?v=PVL24HAesnc (polar vs. nonpolar, crash course chemistry) http://ocw.mit.edu/courses/chemistry/5-111-principles-of-chemical-science-fall-2008/video-lectures/ (MIT lecture 13) http://echem1a.cchem.berkeley.edu/modules/module-4/ (UC-Berkeley lesson 12) hhttp://www.youtube.com/watch?v=PwveQxLLqD0 (intermolecular forces, Isaacs) ttps://www.youtube.com/watch?v=V_Fm25prTvg&list=PLqOZ6FD_RQ7mco4Yb_aYDD8wHMPVYhQrU (UV-Irvine) Read Section 7.2-7.3, 5.7