Chapter 10 Chemical Bonding II Chemistry II Chapter 10 Chemical Bonding II

Chemical Bonding II Molecular Shapes

Chemical Bonding II Molecular Shapes From the model of EDTA we can infer that the 3-Dimensional structure of a molecule determines its __________ the structure displays many molecular properties: Skeletal structure Bonding Shape Bond theory allows you to predict the shapes of molecules properties

Chemical Bonding II VSEPR Theory e- groups (lone pairs and bonds) are most stable when they are as far apart as possible – v________ s____ e_______ p_____ r_________ theory Maximum separation 3-D representation allows us to predict the shapes and bond angles in the molecule Valence shell electron pair repulsion

Chemical Bonding II VSEPR Theory e.g. draw the 2 possible Lewis dot structures for NO2- and discuss the behavior of the associated e- groups there are 3 e- groups on N 1 lone pair 1 single bond 1 double bond (counted as 1 group) there are _____ e- groups on N ____ lone pair ____ single bond ____ double bond (counted as 1 group)

Chemical Bonding II 2 e- Groups: Linear Geometry 5 basic shapes of molecules: linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral Also draw BeCl2, bond angles are 180 degrees

Chemical Bonding II 2 e- Groups: Linear Geometry Draw both 2-dimensional and 3-dimensional pictures of the molecules in the following slides

Chemical Bonding II 2 e- Groups: Linear Geometry occupy positions opposite, around the central atom linear geometry - bond angle is ________ e.g. CO2 Also draw BeCl2, bond angles are 180 degrees

Chemical Bonding II 3 e- Groups: Trigonal Geometry occupy triangular positions trigonal planar geometry - bond angle is __________ e.g. BF3 BF3, 120 degree bond angles

Chemical Bonding II 3 e- Groups: Trigonal Geometry e.g. Formaldehyde, CH2O 3 e– groups around central atom – why not 120° ?

Chemical Bonding II 4 e- Groups: Tetrahedral Geometry occupy tetrahedron positions around the central atom tetrahedral geometry - bond angle is ________ e.g. CH4 109.5

Chemical Bonding II 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. PCl5

Chemical Bonding II 6 e- Groups: Octahedral Geometry occupy positions in the shape of two square-base pyramids that are base-to-base octahedral geometry e.g. SF6

Chemical Bonding II The Effect of Lone Pairs lone pair groups “occupy more space” on the central atom 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

Chemical Bonding II 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. SO2 S has 6, O has 6 electrons. Double bond O to S, single bond O to S, S has lone pair…resonance hybrid

Chemical Bonding II 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. NH3

Which species has the smaller bond angle, Perchlorate (ClO4-) or Chlorate (ClO3-)?

Chemical Bonding II 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 – bond angle is 104.5 ° e.g. H2O

Chemical Bonding II Tetrahedral-Bent Shape

Chemical Bonding II 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

Chemical Bonding II Replacing Atoms with Lone Pairs in the Trigonal Bipyramid System

Chemical Bonding II See-Saw Shape

Chemical Bonding II T-Shape

Chemical Bonding II Linear Shape Tro, Chemistry: A Molecular Approach

Chemical Bonding II 6 e- Groups with Lone Pairs: Derivatives of Octahedral Geometry 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 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°

Chemical Bonding II 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

Practice – Predict the Molecular Geometry and Bond Angles in SiF5-

Practice – Predict the Molecular Geometry and Bond Angles in SiF5─ - Si Least Electronegative 5 Electron Groups on Si Si Is Central Atom 5 Bonding Groups 0 Lone Pairs Si = 4e─ F5 = 5(7e─) = 35e─ (─) = 1e─ total = 40e─ Shape = Trigonal Bipyramid Bond Angles Feq-Si-Feq = 120° Feq-Si-Fax = 90°

Practice – Predict the Molecular Geometry and Bond Angles in ClO2F (Chloryl Fluoride)

Practice – Predict the Molecular Geometry and Bond Angles in ClO2F Cl Least Electronegative 4 Electron Groups on Cl Cl Is Central Atom 3 Bonding Groups 1 Lone Pair Cl = 7e─ O2 = 2(6e─) = 12e─ F = 7e─ Total = 26e─ Shape = Trigonal Pyramidal Bond Angles O-Cl-O < 109.5° O-Cl-F < 109.5°

Chemical Bonding II 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

SF6 S F

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

Describing the Geometry of Methanol

Describing the Geometry of Glycine

Practice – Predict the Molecular Geometries in H3BO3 Tro, Chemistry: A Molecular Approach

Practice – Predict the Molecular Geometries in H3BO3 oxyacid, so H attached to O 3 Electron Groups on B 4 Electron Groups on O B Least Electronegative O has 2 Bonding Groups 2 Lone Pairs B has 3 Bonding Groups 0 Lone Pairs B Is Central Atom B = 3e─ O3 = 3(6e─) = 18e─ H3 = 3(1e─) = 3e─ Total = 24e─ Shape on B = Trigonal Planar Shape on O = Bent

Practice – Predict the Molecular Geometries in C2H4 Tro, Chemistry: A Molecular Approach

Practice – Predict the Molecular Geometries in C2H4 3 Electron Groups on C C = 2(4e─) = 8e ─ H = 4(1e─) = 4e─ Total = 12e─ 0 Lone Pairs Shape on each C = Trigonal Planar

Practice – Predict the Molecular Geometries in CH3OCH3

Practice – Predict the Molecular Geometries in Dimethyl Ether (CH3OCH3) 4 Electron Groups on C C = 2(4e─) = 8e ─ H = 6(1e─) = 6e─ O = 6(1e─) = 6e─ Total = 20e─ 2 Lone Pairs on O Shape on each C = Tetrahedral Shape on O = Bent

Reminder about Eletronegativity! Electronegativity, is a chemical property that describes the tendency of an atom to e- towards itself

Polarity of Molecules in order for a molecule to be polar it must have polar bonds electronegativity difference dipole moments (charge x distance) 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

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.

Vector Addition

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

Molecule Polarity The H-O bond is polar Both sets of bonding e- are pulled toward the O Net result is a polar molecule

Molecule Polarity

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

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

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

Molecular Polarity Affects Solubility in Water Oil and water do not mix! Mutual attraction causes polar molecules to clump together

Unique Properties 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…

Molecular Polarity Affects Solubility in Water some molecules have both polar and nonpolar parts e.g. soap

Practice - Decide Whether the Following Are Polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5 Nitrosyl chloride

Practice - Decide Whether the Following Are Polar Trigonal Bent Cl N O 3.0 3.5 Trigonal Planar O S 3.5 2.5 1) polar bonds, N-O 2) asymmetrical shape 1) polar bonds, all S-O 2) symmetrical shape polar nonpolar