Chapter 10 Chemical Bonding II Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro

Tro, Chemistry: A Molecular Approach2 Structure Determines Properties! properties of molecular substances depend on structures the structure includes many factors, including: skeletal structure Bonding - ionic, polar covalent, or covalent Shape bonding theory should allow you to predict the shapes of molecules

Tro, Chemistry: A Molecular Approach3 Molecular Geometry Molecules are 3-D Describe molecular shape using geometric terms Geometry has characteristic angles that we call bond angles

Tro, Chemistry: A Molecular Approach4 Using Lewis Theory to Predict Molecular Shapes Lewis theory - regions of e - in an atom based on placing shared pairs and unshared pairs of valence e - predicts the shapes of molecules based on negatively charged regions which repel

Tro, Chemistry: A Molecular Approach5 VSEPR Theory e - groups (lone pairs and bonds) are most stable when they are as far apart as possible – valence shell electron pair repulsion theory Maximum separation the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule 3-D representation

Tro, Chemistry: A Molecular Approach6 Electron Groups the Lewis structure predicts the arrangement of valence e - around the central atom(s) each lone pair of e - constitutes one e - group on a central atom each type of bond constitutes one electron group on a central atom e.g. NO 2 there are 3 e - groups on N 1 lone pair 1 single bond 1 double bond (counted as 1 group)

Tro, Chemistry: A Molecular Approach7 5 Basic Molecular Geometries 5 arrangements of e - groups for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same O S O

Tro, Chemistry: A Molecular Approach8 2 e - Groups: Linear Geometry occupy positions opposite, around the central atom linear geometry - bond angle is 180° e.g. CO 2

Tro, Chemistry: A Molecular Approach9 3 e - Groups: Trigonal Geometry occupy triangular positions trigonal planar geometry - bond angle is 120° e.g. BF 3

Tro, Chemistry: A Molecular Approach10 Not Quite Perfect Geometry 3 e – groups around central atom – why not 120° ? Because the bonds are not identical, the observed angles are slightly different from ideal.

Tro, Chemistry: A Molecular Approach11 4 e - Groups: Tetrahedral Geometry occupy tetrahedron positions around the central atom tetrahedral geometry - bond angle is 109.5° e.g. CH 4

Tro, Chemistry: A Molecular Approach12 5 e - Groups: Trigonal Bipyramidal Geometry occupy positions in the shape of a two tetrahedra that are base-to-base trigonal bipyramidal geometry e.g. PCl 5

Tro, Chemistry: A Molecular Approach13 6 e - Groups: Octahedral Geometry occupy positions in the shape of two square-base pyramids that are base-to- base octahedral geometry e.g. SF 6

Tro, Chemistry: A Molecular Approach14 The Effect of Lone Pairs lone pair groups “occupy more space” on the central atom because their e - density is exclusively on the central atom rather than shared like bonding electron groups relative sizes of repulsive force interactions is: Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair – Bonding Pair this effects the bond angles, making them smaller than expected

Tro, Chemistry: A Molecular Approach15 Effect of Lone Pairs The bonding electrons are shared by two atoms, so some of the negative charge is removed from the central atom. The nonbonding electrons are localized on the central atom, so area of negative charge takes more space.

Tro, Chemistry: A Molecular Approach16 Effect of Lone Pairs: Derivative Shapes the molecule’s shape will be one of basic molecular geometries if all the e - groups are bonds and all the bonds are equivalent molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes

Tro, Chemistry: A Molecular Approach17 3 e - Groups with Lone Pairs Derivative of Trigonal Geometry when there are 3 e- groups around central atom, and 1 of them is a lone pair trigonal planar - bent shape - bond angle < 120° e.g. SO 2

Tro, Chemistry: A Molecular Approach18 4 e - Groups with Lone Pairs Derivatives of Tetrahedral Geometry when there are 4 e - groups around the central atom, and 1 is a lone pair trigonal pyramidal shape – bond angle is 107 ° e.g. NH 3

Tro, Chemistry: A Molecular Approach19 Bond Angle Distortion from Lone Pairs

Tro, Chemistry: A Molecular Approach20 HW: Which species has the smaller bond angle, Perchlorate (ClO 4 - ) or Chlorate (ClO 3 - )?

Tro, Chemistry: A Molecular Approach21 4 e - Groups with Lone Pairs Derivatives of Tetrahedral Geometry when there are 4 e - groups around the central atom, and 2 are lone pairs tetrahedral-bent shape e.g. H 2 O it looks similar to the trigonal planar-bent shape, except the angles are smaller 104.5°

Tro, Chemistry: A Molecular Approach22 Tetrahedral-Bent Shape

Tro, Chemistry: A Molecular Approach23 Bond Angle Distortion from Lone Pairs

Tro, Chemistry: A Molecular Approach24

Tro, Chemistry: A Molecular Approach25 5 e - Groups with Lone Pairs Derivatives of Trigonal Bipyramidal Geometry when there are 5 e - groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room when there are 5 e - groups around the central atom, and 1 is a lone pair, the result is called see-saw shape aka distorted tetrahedron when there are 5 e - groups around the central atom, and 2 are lone pairs, the result is called T-shaped when there are 5 e - groups around the central atom, and 3 are lone pairs, the result is called a linear shape the bond angles between equatorial positions is < 120° the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial

Tro, Chemistry: A Molecular Approach26 Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System

Tro, Chemistry: A Molecular Approach27 See-Saw Shape

Tro, Chemistry: A Molecular Approach28 T-Shape

Tro, Chemistry: A Molecular Approach29 Linear Shape

Tro, Chemistry: A Molecular Approach30 when there are 6 e - groups around the central atom, and 1 is a lone pair, the result is called a square pyramid shape the bond angles between axial and equatorial positions is < 90° 6 e - Groups with Lone Pairs Derivatives of Octahedral Geometry

Tro, Chemistry: A Molecular Approach31 when there are 6 e - groups around the central atom, and 2 are lone pairs, the result is called a square planar shape the bond angles between equatorial positions is 90° 6 e - Groups with Lone Pairs Derivatives of Octahedral Geometry

Tro, Chemistry: A Molecular Approach32

Tro, Chemistry: A Molecular Approach33 Predicting the Shapes Around Central Atoms 1) Draw the Lewis Structure 2) Determine the Number of Electron Groups around the Central Atom 3) Classify Each Electron Group as Bonding or Lone pair, and Count each type remember, multiple bonds count as 1 group 4) Use Table 10.1 to Determine the Shape and Bond Angles

Tro, Chemistry: A Molecular Approach34 Practice – Predict the Molecular Geometry and Bond Angles in SiF 5 -

Tro, Chemistry: A Molecular Approach35 Practice – Predict the Molecular Geometry and Bond Angles in SiF 5 ─ Si = 4e ─ F 5 = 5(7e ─ ) = 35e ─ (─) = 1e ─ total = 40e ─ 5 Electron Groups on Si 5 Bonding Groups 0 Lone Pairs Shape = Trigonal Bipyramid - Bond Angles F eq -Si-F eq = 120° F eq -Si-F ax = 90° Si Least Electronegative Si Is Central Atom

Tro, Chemistry: A Molecular Approach36 Practice – Predict the Molecular Geometry and Bond Angles in ClO 2 F (Chloryl Fluoride)

Tro, Chemistry: A Molecular Approach37 Practice – Predict the Molecular Geometry and Bond Angles in ClO 2 F Cl = 7e ─ O 2 = 2(6e ─ ) = 12e ─ F = 7e ─ Total = 26e ─ 4 Electron Groups on Cl 3 Bonding Groups 1 Lone Pair Shape = Trigonal Pyramidal Bond Angles O-Cl-O < 109.5° O-Cl-F < 109.5° Cl Least Electronegative Cl Is Central Atom

Tro, Chemistry: A Molecular Approach38 Representing 3-Dimensional Shapes on a 2- Dimensional Surface one of the problems with drawing molecules is trying to show their dimensionality by convention, the central atom is put in the plane of the paper put as many other atoms as possible in the same plane and indicate with a straight line for atoms in front of the plane, use a solid wedge for atoms behind the plane, use a hashed wedge

Tro, Chemistry: A Molecular Approach39

Tro, Chemistry: A Molecular Approach40 SF 6 S F F F FF F

Tro, Chemistry: A Molecular Approach41

Tro, Chemistry: A Molecular Approach42 Multiple Central Atoms many molecules have larger structures with many interior atoms we can think of them as having multiple central atoms when this occurs, we describe the shape around each central atom in sequence e.g. acetic acid shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent

Tro, Chemistry: A Molecular Approach43 Describing the Geometry of Methanol

Tro, Chemistry: A Molecular Approach44 Describing the Geometry of Glycine

Tro, Chemistry: A Molecular Approach45 Practice – Predict the Molecular Geometries in H 3 BO 3

46 Practice – Predict the Molecular Geometries in H 3 BO 3 B = 3e ─ O 3 = 3(6e ─ ) = 18e ─ H 3 = 3(1e ─ ) = 3e ─ Total = 24e ─ 3 Electron Groups on B B has 3 Bonding Groups 0 Lone Pairs Shape on B = Trigonal Planar B Least Electronegative B Is Central Atom oxyacid, so H attached to O 4 Electron Groups on O O has 2 Bonding Groups 2 Lone Pairs Shape on O = Bent

Tro, Chemistry: A Molecular Approach47 Practice – Predict the Molecular Geometries in C 2 H 4

48 Practice – Predict the Molecular Geometries in C 2 H 4 C = 2(4e ─ ) = 8e ─ H = 4(1e ─ ) = 4e ─ Total = 12e ─ 3 Electron Groups on C Shape on each C = Trigonal Planar 0 Lone Pairs

Tro, Chemistry: A Molecular Approach49 Practice – Predict the Molecular Geometries in CH 3 OCH 3

50 Practice – Predict the Molecular Geometries in Dimethyl Ether (CH 3 OCH 3 ) C = 2(4e ─ ) = 8e ─ H = 6(1e ─ ) = 6e ─ O = 6(1e ─ ) = 6e ─ Total = 20e ─ 4 Electron Groups on C Shape on each C = Tetrahedral 2 Lone Pairs on O Shape on O = Bent

Tro, Chemistry: A Molecular Approach51 Reminder about Eletronegativity! Electronegativity, is a chemical property that describes the tendency of an atom to e - towards itself

Tro, Chemistry: A Molecular Approach52 Polarity of Molecules in order for a molecule to be polar it must 1)have polar bonds  electronegativity difference  dipole moments (charge x distance) 2)have an unsymmetrical shape  vector addition polarity affects the intermolecular forces of attraction therefore boiling points and solubilities  like dissolves like nonbonding pairs strongly affect molecular polarity

Tro, Chemistry: A Molecular Approach53 Molecule Polarity The H-Cl bond is polar Bonding e - are pulled toward the Cl end of the molecule Net result is a polar molecule.

Tro, Chemistry: A Molecular Approach54 Vector Addition

Tro, Chemistry: A Molecular Approach55

Tro, Chemistry: A Molecular Approach56 Molecule Polarity The O-C bond is polar The bonding e - are pulled equally toward both O’s Symmetrical molecule Net result is a nonpolar molecule

Tro, Chemistry: A Molecular Approach57 Molecule Polarity The H-O bond is polar Both sets of bonding e - are pulled toward the O Net result is a polar molecule

Tro, Chemistry: A Molecular Approach58 Molecule Polarity

Tro, Chemistry: A Molecular Approach59 Molecule Polarity The H-N bond is polar All the sets of bonding electrons are pulled toward the N Not symmetrical Net result is a polar molecule

Tro, Chemistry: A Molecular Approach60 Molecule Polarity The C-H bond is polar Four equal dipoles cancel each other out due to symmetry Net result is a non-polar molecule

Tro, Chemistry: A Molecular Approach61 Molecular Polarity Affects Solubility in Water polar molecules are attracted to other polar molecules since water is a polar molecule, other polar molecules dissolve well in water and ionic compounds as well

Tro, Chemistry: A Molecular Approach62 Molecular Polarity Affects Solubility in Water Oil and water do not mix! Mutual attraction causes polar molecules to clump together

Water shrinks on melting (ice floats on water) Unusually high melting point Unusually high boiling point Unusually high surface tension Unusually high viscosity Unusually high heat of vaporization Unusually high specific heat capacity And more… Unique Properties

Tro, Chemistry: A Molecular Approach64 Molecular Polarity Affects Solubility in Water some molecules have both polar and nonpolar parts e.g. soap

Tro, Chemistry: A Molecular Approach65 Practice - Decide Whether the Following Are Polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5

Tro, Chemistry: A Molecular Approach66 Practice - Decide Whether the Following Are Polar polar nonpolar 1) polar bonds, N-O 2) asymmetrical shape 1) polar bonds, all S-O 2) symmetrical shape Trigonal Bent Trigonal Planar Cl N O O O O S 2.5