VSEPR theory Molecular Polarity

Taste The taste of a food: Interaction between food molecules and taste cells Factors: Shape of the molecule and charge distribution within the molecule Food or “spicy” molecule fit snugly into the active site of specialized proteins on the surface of taste cells When this happens, changes in the protein structure cause a nerve signal Tro: Chemistry: A Molecular Approach, 2/e

Sweet Taste: Key/Lock model Sugar molecules (“Key”) fit into the active site of taste cell receptors (“Lock”): When the sugar molecule enters the active site, parts of the taste cell receptor split apart ion channels in the cell membrane to open resulting in nerve signal transmission Artificial sweeteners also fit into the same receptor, sometimes binding even stronger than sugar (making them “sweeter” than sugar) Tro: Chemistry: A Molecular Approach, 2/e

Structure Determines Properties! Properties of molecular substances depend on the structure of the molecule The structure includes many factors, such as: the skeletal arrangement of the atoms the kind of bonding between the atoms ionic, polar covalent, or covalent the shape of the molecule Bonding theory should allow you to predict the shapes of molecules Tro: Chemistry: A Molecular Approach, 2/e

Molecular Geometry Molecules: 3-dimensional objects Molecular Geometry: Shape of a molecule Two important factors in Molecular Geometry: Bond Angle Bond Length Tro: Chemistry: A Molecular Approach, 2/e

Lewis Theory Predicts Electron Groups Lewis theory predicts there are regions of electrons in an atom Electron groups (= VSEPR group): Bonding pairs and Lone pairs Bonding pairs: Shared pairs of valence electrons between bonding nuclei Nonbonding pairs (= Lone Pairs): Unshared valence electrons on a single nuclei Tro: Chemistry: A Molecular Approach, 2/e

O N • VSEPR Theory Electron groups around the central atom repel each other (same “-” charge) Electron groups are spaced away to minimize the repulsion –valence shell electron pair repulsion theory (VSEPR theory) VSEPR theory can predict the shapes and bond angles in the molecule Tro: Chemistry: A Molecular Approach, 2/e

Identify Electron Groups VSEPR groups: Regions of Valence Electrons Each lone pair of electrons constitutes one electron group on a central atom Each bonding region constitutes one electron group on a central atom regardless of whether it is single, double, or triple O N • ____ electron groups on N Tro: Chemistry: A Molecular Approach, 2/e

Electron Group Geometry #Electron groups on central atom = 2, 3, 4, 5, 6 5 basic arrangements of electron groups around a central atom Rarely >6, like IF7 e-group arrangements leads to different geometries Lone pair causes slight different geometry than bonding pair Resonance does NOT affect the electron geometry Tro: Chemistry: A Molecular Approach, 2/e

Linear Electron Geometry Two electron groups around the central atom: electron groups occupy positions on opposite sides of the central atom Linear geometry Bond angle = 180° Tro: Chemistry: A Molecular Approach, 2/e

Trigonal Planar Electron Geometry Three electron groups around the central atom: electron groups taking a trigonal planar geometry bond angle = 120° Examples: COCl2, CH2O, SO3, BF3 Tro: Chemistry: A Molecular Approach, 2/e

Trigonal Geometry Tro: Chemistry: A Molecular Approach, 2/e

3D Animation: Three Electron Groups on Central Atom (bond angle  _________) Three terminal atoms (no lone pair electrons) Two terminal atoms (ONE lone pair electrons)

Tetrahedral Electron Geometry Four electron groups around the central atom: Tetrahedral geometry bond angle = 109.5° Examples: CF4, SiCl4, CH2Cl2, CHCl3 Tro: Chemistry: A Molecular Approach, 2/e

Tetrahedral Geometry Tro: Chemistry: A Molecular Approach, 2/e

Non-Octet: Trigonal Bipyramidal Electron Geometry 5 electron groups around the central atom: Trigonal Bipyramidal geometry Axial positions are above and below the central atom Equatorial positions are in the same base plane as the central atom <Eq-Central-Eq = 120° <Ax-Central-Eq = 90° Examples: SF6, PF6-, SiF62- Tro: Chemistry: A Molecular Approach, 2/e

Trigonal Bipyramid Tro: Chemistry: A Molecular Approach, 2/e

Non-Octet: Octahedral Electron Geometry 6 electron groups around the central atom: Two square-base pyramids that are base-to-base with the central atom in the center of the shared bases: Octahedral geometry eight sides All positions are equivalent The bond angle is 90° Tro: Chemistry: A Molecular Approach, 2/e

Octahedral Geometry Tro: Chemistry: A Molecular Approach, 2/e

Molecular Geometry vs. Electron Geometry The actual geometry of the molecule may be different from the electron geometry when: The electron groups are attached to atoms of different size, or when the bonding to one atom is different than the bonding to another Lone pairs occupy more space on the central atom than bonding pairs Tro: Chemistry: A Molecular Approach, 2/e

Not Quite Perfect Geometry Because the bonds and atom sizes are not identical in formaldehyde, the angles are slightly off Tro: Chemistry: A Molecular Approach, 2/e

The Effect of Lone Pairs <H-O-H in water = 104.5° < 109.5°, why? Lone pair (LP) groups “occupy more space” bonding pair (BP). electron density in LP is on the central atom only, not shared like bonding electron groups Ranking the repulsive force interactions: LP-LP > LP-BP > BP-BP Stronger repulsive force from LP  ____ angles from LP  _____ BP-BP angle Tro: Chemistry: A Molecular Approach, 2/e

Repulsive force: BP vs. LP BP electrons are shared by two atoms Negative charge in BP partially “neutralized” by nuclear charge less repulsive LP electrons are localized on the central atom  negative charge from LP takes more space Tro: Chemistry: A Molecular Approach, 2/e

Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

Bond Angle Distortion from Lone Pairs Tro: Chemistry: A Molecular Approach, 2/e

Bent Molecular Geometry: Derivative of Trigonal Planar 3 electron groups around the central atom (2 BP + 1 LP): trigonal planar — bent shape The bond angle is less than 120° because the lone pair takes up more space, “pushing” bonding electrons closer to each other. Tro: Chemistry: A Molecular Approach, 2/e

Molecular Geometry: Pyramidal & Bent Tetrahedral Electron Geometry (1 LP + 3 BPs): pyramidal shape Larger LP-BP repulsion  Bond angle ___109.5° Example: Ammonia, PCl3 Tetrahedral Electron Geometry (2 BPs + 2 LPs): tetrahedral—bent shape it is planar Bond angle ____ 109.5° Example: SCl2 , H2O Tro: Chemistry: A Molecular Approach, 2/e

Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

Pyramidal Shape Tro: Chemistry: A Molecular Approach, 2/e

Tetrahedral–Bent Shape Tro: Chemistry: A Molecular Approach, 2/e

Tetrahedral–Bent Shape Tro: Chemistry: A Molecular Approach, 2/e

3D Animation: Four Electron Groups on Central Atom (bond angle  _________) Three Terminal Atoms (One lone pair electrons) Four Terminal Atoms (no lone pair electrons) Two Terminal Atoms (Two lone pair electrons)

Non-Octet: Trigonal Bipyramidal Tro: Chemistry: A Molecular Approach, 2/e 33 33

Lone Pair in Trigonal Bipyramidal Electron Geometry Lone Pair(s) has priority to occupy the equatorial positions Because larger angle (<eq-Cen-eq =120°) gives less repulsion than LP at axial position (90°) Tro: Chemistry: A Molecular Approach, 2/e

Derivatives of the Trigonal Bipyramidal Electron Geometry Since Lone Pair(s) occupy equatorial positions 1 LP: seesaw shape aka distorted tetrahedron 2 LP: T-shaped 3 LP: linear shape Note: Distortion due to LP <Eq-Central-Eq < 120° <Axial-Central-Eq < 90° Tro: Chemistry: A Molecular Approach, 2/e

1x LP in Trigonal Bipyrimidal e-Geometry: Seesaw Shape Tro: Chemistry: A Molecular Approach, 2/e

2x LPs in Trigonal Bipyrimidal e-Geometry: T–Shape Tro: Chemistry: A Molecular Approach, 2/e

3x LPs in Trigonal Bipyrimidal e-Geometry: Linear Shape Tro: Chemistry: A Molecular Approach, 2/e

Non-Octet: Octahedral S F Tro: Chemistry: A Molecular Approach, 2/e

1x LP Derivatives of the Octahedral Geometry: square pyramid shape the bond angles between axial and equatorial positions is less than 90°

2x LP Derivatives of the Octahedral Geometry: square pyramid shape When more than ONE LP in Octahedral electron groups geometry, LPs are positioned opposite to each other (LP-LP repulsion is the strongest) the bond angles between equatorial positions is 90°

Tro: Chemistry: A Molecular Approach, 2/e

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 one group 4. Determine the shape and bond angles Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict the geometry and bond angles of PCl3 26 valence electrons Four electron groups on P atom: 1 LP, 3 BP Electron group geometry = __________ Bond angle = Molecular geometry ________ Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict the geometry and bond angles of SiF5− 40 valence electrons ____ electron groups on P atom: ___ LP, ___ BP Electron group geometry = __________ Bond angle = Molecular geometry ________ Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict the geometry and bond angles of ClO2F 26 valence electrons ____ electron groups on P atom: ___ LP, ___ BP Electron group geometry = __________ Bond angle = Molecular geometry ________ Tro: Chemistry: A Molecular Approach, 2/e

Representing 3-Dimensional Shapes on a 2-Dimensional Surface General guideline: 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 Atoms in front of the plane: solid wedge Atoms behind the plane: hashed wedge Tro: Chemistry: A Molecular Approach, 2/e

Tro: Chemistry: A Molecular Approach, 2/e

SF6 S F Tro: Chemistry: A Molecular Approach, 2/e

Multiple Central Atoms Many molecules have larger structures with many interior atoms They have multiple central atoms Consider each central atom in sequence shape around left C is tetrahedral shape around center C is trigonal planar shape around right O is tetrahedral-bent Tro: Chemistry: A Molecular Approach, 2/e

Geometry of Methanol: Smallest Alcohol molecule Tro: Chemistry: A Molecular Approach, 2/e

Geometry of Glycine: Smallest Amino Acid (units of Protein) Tro: Chemistry: A Molecular Approach, 2/e

Practice – Predict the molecular geometries in H3BO3 Tro: Chemistry: A Molecular Approach, 2/e

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 pone pairs B Is Central Atom Total = 24e─ Shape on B = trigonal planar Shape on O = tetrahedral bent Tro: Chemistry: A Molecular Approach, 2/e

Polarity of Molecules For a molecule to be polar it must have polar bonds electronegativity difference - theory bond dipole moments - measured have an unsymmetrical shape vector addition Polarity affects the intermolecular forces of attraction therefore boiling points and solubilities like dissolves like Nonbonding pairs affect molecular polarity, strong pull in its direction Tro: Chemistry: A Molecular Approach, 2/e

Molecule Polarity The H─Cl bond is polar. The bonding electrons are pulled toward the Cl end of the molecule. The net result is a polar molecule. Tro: Chemistry: A Molecular Approach, 2/e

Vector Addition: A. Simple Addition/Cancellation Tro: Chemistry: A Molecular Approach, 2/e

Vector Addition: B. Addition using Parallelogram Tro: Chemistry: A Molecular Approach, 2/e 58 58

Vector Addition C. Addition of Multiple vectors Tro: Chemistry: A Molecular Approach, 2/e 59 59

Tro: Chemistry: A Molecular Approach, 2/e

Molecule Polarity The O─C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule. Tro: Chemistry: A Molecular Approach, 2/e

Molecule Polarity The H─O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. Tro: Chemistry: A Molecular Approach, 2/e

Predicting Polarity of Molecules 1. Draw the Lewis structure and determine the molecular geometry. Bent, Trig. Pyr., Seesaw, T-shape: always polar!!! 2. Determine if bonds are polar if there are no polar bonds (DEN = 0), the molecule is nonpolar 3. Determine whether the polar bonds add together to give a net dipole moment Tro: Chemistry: A Molecular Approach, 2/e

Practice – Decide whether the following molecules are polar EN O = 3.5 N = 3.0 Cl = 3.0 S = 2.5 H = 2.1 Tro: Chemistry: A Molecular Approach, 2/e

Molecular Polarity Affects Solubility in Water Polar molecules are attracted to other polar molecules Because water is a polar molecule, other polar molecules dissolve well in water and ionic compounds as well Some molecules have both polar and nonpolar parts Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict the geometry and bond angles of PCl3 Tro: Chemistry: A Molecular Approach, 2/e

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° Tro: Chemistry: A Molecular Approach, 2/e

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°

Example: Predict whether NH3 is a polar molecule 1. Draw the Lewis structure and determine the molecular geometry a) eight valence electrons b) three bonding + one lone pair = trigonal pyramidal molecular geometry Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict whether NH3 is a polar molecule 2. Determine if the bonds are polar a) electronegativity difference b) if the bonds are not polar, we can stop here and declare the molecule will be nonpolar ENN = 3.0 ENH = 2.1 3.0 − 2.1 = 0.9 therefore the bonds are polar covalent Tro: Chemistry: A Molecular Approach, 2/e

Example: Predict whether NH3 is a polar molecule 3) Determine whether the polar bonds add together to give a net dipole moment a) vector addition b) generally, asymmetric shapes result in uncompensated polarities and a net dipole moment The H─N bond is polar. All the sets of bonding electrons are pulled toward the N end of the molecule. The net result is a polar molecule. Tro: Chemistry: A Molecular Approach, 2/e

Practice – Decide whether the following molecules Are polar Trigonal Bent Trigonal Planar 2.5 1. polar bonds, N-O 2. asymmetrical shape 1. polar bonds, all S-O 2. symmetrical shape polar nonpolar Tro: Chemistry: A Molecular Approach, 2/e