Notes

Outline

Molecular Geometry Bonding Theories

Bonding Theories Valence Shell Electron Pair Repulsion (VSEPR) Valence Bond Approach (Atomic Orbital Approach) Molecular Orbital Approach

VSEPR Approach Molecular Geometries are determined by minimizing the repulsion that takes place between valence  electron pairs

Valence Electron Pair Repulsion Bonding pair repulsion Bonding pair-lone pair repulsion Lone pair-lone pair repulsion

Two Kinds Of Electron Pairs Bonding Pairs Lone Pairs

Two Geometries Molecular Geometry-the spacial arrangement of atoms relative to one another Geometry seen in X-Ray Diffraction Only Bonding Pairs considered Electron Pair(domain) Geometry-the spacial arrangement of all electron pairs attached to Central atom All Electron Pairs Considered (Bonding and Lone Pair)

Molecules Containing Two Electron Pairs AX2

Two Bonding Pairs + zero lone pairs On Central Atom AX2 molecular profile Two bonding pairs can separate with 180 degree separation for minimum repulsion Molecular Geometry = Linear molecular geometry = electron domain geometry Examples: CO2, BeH2,C2H2

Molecules Containing Three Electron Pairs On Central Atom AX3 ( 3 bonding pairs + 0 lone pairs) :AX2 ( 2 bonding pairs + 1 lone pair)

Molecules With Three bonding pairs + zero lone pairs on Central Atom AX3 molecular profile Three bonding pairs can separate 120 degrees apart in the same plane for minimum repulsion Molecular Geometry = Trigonal Planar = Electron Domain Geometry Examples: BF3, AlH3, C2H4,H2CO

Molecules With Two Bonding Pairs + One Lone Pair on Central Atom :AX2 molecular profile Molecular Geometry = Angular Electron Domain Geometry = Trigonal Planar Examples:   :SO2

Molecules With Four Electron Pairs On Central Atom Molecular Profiles AX4 ( 4 bonding pairs + 0 lone pairs) :AX3 ( 3 bonding pairs + 1 lone pair) ::AX2 ( 2 bonding pairs + 2 lone pairs) :::AX ( 1 bonding pair + 3 lone pairs)

Four Bonding Pairs + Zero Lone Pairs on Central Atom AX4 molecular profile Four bonding pairs can separate 109.5 degrees apart for minimum repulsion Molecular Geometry= Tetrahedral = Electron Domain Geometry Examples: CH4, NH4+, BH4-, C2H6, CHCl3, CH2Br2

Three Bonding Pairs + One Lone Pair on Central Atom :AX3 molecular profile Lone Pair repels bonding pairs out of plane and moves them closer to each other Molecular Geometry = Trigonal Pyramidal Electron Domain Geometry = Tetrahedral Examples: NH3, RNH2, PH3, H3O+

Two Bonding Pairs + Two Lone Pairs On Central Atom ::AX2 molecular profile Two Lone Pairs push bonding pairs closer to one another Molecular Geometry = angular Electron  Domain Geometry = Tetrahedral Examples: H2O, H2S, R-O-H, R-O-R

One Bonding Pair + Three Lone Pairs On Central Atom :::AX Molecular Geometry = Linear Electron  Domain Geometry = Tetrahedral Examples: HCl, HF, HBr

T-10.4 Geometries of Molecules With 2-4 Electron Pairs

Molecules With Five Electron Pairs On Central Atom Exceptions to Octet Rule Molecular profiles AX5 (5 bonding pairs + 0 lone pairs) :AX4 (4 bonding pairs + 1 lone pair) ::AX3 (3 bonding pairs + 2 lone pairs) :::AX2 (2 bonding pairs + 3 lone pairs)

Five Bonding Pairs + Zero Lone Pairs On Central Atom AX5 molecular profile Five bonding pairs can separate with three pairs 120 degrees apart in a trigonal planar array and the fourth and fifth pair above and below the trigonal planar array Molecular Geometry = Trigonal Bipyramidal = Electron Domain Geometry Examples: PCl5, Fe(CO)5

T-10.10 Electron Pair Geometry Of Five Electron Pairs

Four Bonding Pairs + One Lone Pair On Central Atom :AX4 molecular profile Molecular Geometry=Seesaw or distorted tetrahedron Electron Domain Geometry = Trigonal Bipyramidal Examples:  :SF4

Three Bonding Pairs + Two Lone Pairs on Central Atom ::AX3 molecular profile Molecular Geometry = T-shaped Electron  Domain Geometry = Trigonal Pyramidal Examples: BrCl3, FBr3, ClF3

Two Bonding Pairs + Three lone pairs On Central Atom :::AX2 molecular profile Molecular Geometry = Linear Electron  Domain Geometry = Trigonal Bipyramidal Example: XeF2

T-10.9 Part 1 Geometries of Molecules With Five Electron Pairs On Central Atom

Molecules Containing Six Electron Pairs On Central Atom Exceptions To Octet Rule Molecular Profiles AX6 ( 6 bonding pairs + 0 lone pairs) :AX5 ( 5 bonding pairs + 1 lone pair) ::AX4 ( 4 bonding pairs + 2 lone pairs)

Six Bonding Pairs + Zero Lone Pairs On Central Atom AX6 molecular profile Four bonding pairs in same plane 90 degrees apart pointing to the corners of a square with fifth and sixth pairs 90 degrees above and below the square Molecular Geometry = Octahedral = Electron Domain Geometry Examples:SF6, CoCl6 –4

Molecules With Five Bonding Pairs + One Lone Pair On Central Atom :AX5 molecular profile Molecular Geometry = Square Pyramidal Electron  Domain Geometry = Octahedral Examples: IF5

Molecules With Four Bonding Pairs + Two Lone Pairs On Central Atom ::AX4 molecular profile Molecular Geometry = Square Planar Electron  Domain Geometry = Octahedral Examples: XeF4

T-10.9 Part 2 Geometries of Molecules With 6 Electron Pairs on Central Atom

Molecular Polarity Polar Molecule-molecule whose bond dipoles do not cancel. Molecular Dipole ¹ 0 Non-Polar Molecule-Molecule where all Bond Dipoles cancel.  Molecular Dipole = 0

T-10.18 Effect of Electric Field On Polar Molecules

Predicting Molecular Polarity From Molecular Profile Six Electron Pairs AX6 and ::AX4 If X’s all the same = non-polar If one X is different = polar :AX5 = polar Five Electron Pairs AX5, ::AX3, :::AX2 If X’s are the same = non-polar If one X is different = polar :AX4 = polar

Predicting Molecular Polarity (cont) Four Electron pairs AX4 If all X’s the same = non-polar If one X is different = polar :AX3, ::AX2, :::AX All are polar Three Electron Pairs AX3 If all X’s the same=non-polar If one X is different = polar :AX2 = polar Two Electron Pairs AX2 If X’s same = nonpolar If different = polar

Valence Bond(Atomic Orbital) Approach The atomic orbitals of atoms can share a common volume(orbital overlap) Overlapping of atomic orbitals between two atoms lowers energy state of molecule Greater the overlap more stable the co-valent bond Electron density greatest in overlap region

T-10.21 Atomic Orbital Overlap of Hydrogen Molecule

Hybridization Theory of Atomic Orbitals Proposed by Linus Pauling to account for the tetravalency of Carbon and the Trivalency of Boron and Aluminum Atomic orbitals can combine (mix) to form hybrid atomic orbitals These hybrid atomic orbitals can overlap with other hybrid orbitals or non-hybrid atomic orbitals to form co-valent bonding

T-? SP3 Hybridization of Carbon Atom in Methane

T-10.27 sp2 Hybridization of Carbon Atoms in Ethylene

T10.28 SP Hybridization of Carbon Atoms in Acetylene

T-10.24 Five Hybridization of Atomic Orbitals Using s,p,d atomic orbitals

Geometrical Isomerism Pi overlap between two parallel “p” orbitals overlapping creates a Pi bond Ethylene has one Pi Bond to go with its sigma bond Restricted Rotation occurs with one Pi Bond Attempting to rotate either atom Pi Bonded severely reduces the amount of overlap destabilizing the bond Can result in isomerism due to steric arrangement of similar groups attached to Pi bonded atoms Similar groups on same side of molecule = Cis isomer Similar groups on opposite sides of molecule = Trans isomer Geometrical Isomerism is a form of stereoisomerism

Delocalization of Pi electrons Pi electrons can go from one “p” orbital to another “p” orbital parallel to and adjacent This process of bonding electrons moving outside its original locality is called “delocalization” Results in stabilizing further the molecular system called resonance stabilization Examples of resonance stabilization Carbonate ion (CO3 –2 ) Benzene molecule Ozone(O3) SO2 molecule

Molecular Orbital Approach When atoms get close enough to bond the atomic orbitals combine or repel one another to produce a set of orbitals that are the domain of entire molecule Different than atomic orbitals that atoms retain control over even in a hybridized state Advantage in explaining resonance and absorption spectroscopy

Types of Molecular Orbitals Bonding Molecular Orbitals (BMO)-Molecular Orbitals that are lower in energy than the original atomic orbitals due to combining orbital space Anti-Bonding Molecular Orbital (ABMO)- Molecular Orbitals that are higher in energy state than original atomic orbitals due to repulsion of orbital space Non-Bonding Molecular Orbital-Molecular Orbitals that are the same in energy as the original atomic orbitals reserved for lone pairs or unpaired electrons

T-10.31 BMO and ABMO in Hydrogen Molecule

T-10.38 Pi Electron Delocalization of Ozone

T-10.35 Molecular Orbital energy Diagram for Period 2 atom bonding

Bonding Order Bonding Order- The number of bonding pairs between two atoms Bonding Order = (# BMO electrons - #ABMO electrons) / 2 BO = 0 means no bond such as Ne2 He2,etc BO = 1 means a single co-valent bond BO = 2 means a double covalent bond BO = 3 means a triple co-valent bond