Chapter 10 Chemical bonding: Molecular shapes, valence bond theory, and molecular orbital theory

Valence Shell Electron Pair Repulsion Theory VSEPR theory: Electrons repel each other Electrons groups in a molecule arrange themselves so as to be as far apart as possible Minimize repulsion Determines molecular geometry Electron groups are atoms and lone pairs

Defining Molecular Shape Electron pair geometry: the geometrical arrangement of electron groups around a central atom Atoms and lone pairs count as electron groups Molecular Geometry: the geometrical arrangement of atoms around a central atom Ignore lone pair electrons

2 e- groups surrounding the central atom e- pair geometry: linear MG: linear AXE designation: AX2E0 A: Central Atom X: Bonding pairs E: Non-bonding pairs Example: BeCl2, CO2

3 e- groups 3 Bonds, 0 Lone Pairs 2 Bonds, 1 Lone Pair e- PG: Trigonal Planar (Triangular planar) MG: Trigonal Planar AX3E0 BF3 2 Bonds, 1 Lone Pair e- PG: Trigonal Planar (Triangular planar) MG: Bent/angular AX2E1 GeCl2

4 e- groups 4 bonds, 0 Lone Pairs 3 bonds, 1 Lone Pair e- PG: Tetrahedral MG: Tetrahedral AX4E0 CH4 3 bonds, 1 Lone Pair e- PG: Tetrahedral MG: Triangular Pyramidal AX3E1 NH3

2 bonds, 2 Lone Pairs e- PG: Tetrahedral MG: Bent/Angular AX2E2 H2O

Molecular Polarity

Determining molecular polarity Draw Lewis dot structure taking into account 3-D molecular geometry Indicate the dipole moment for each bond using EN values for each atom Sum up the dipole moments as vectors and determine if there is a net dipole moment

Valence bond theory Electrons are more appropriately represented in a quantum mechanical manner Electrons exist in atomic orbitals that can interact with other atomic orbitals Interactions between two atoms can be analyzed through their potential energy

Valence bond theory summary When bringing atoms close together, when orbitals with unpaired electrons interact this results in a stabilizing (more negative) energy, promoting a chemical bond Geometry and shape of orbitals interacting will affect the overall molecular geometry

Valence bond theory: Hybridization Orbitals in a molecule are not the same as in an atom Orbitals can mix together to form hybrid orbitals This occurs mostly with atoms that tendto make more bonds, like carbon Hybrid orbitals will have altered energy and shape, depending on the orbitals that are mixing Reasoning for hybridization….ENERGY!! Results in overall lower potential energy for the molecule

sp3 hybridization Mixing one s orbital and three p orbitals will give you four sp3 orbitals Can explain 4 EPG geometries

sp2 hybridization and double bonds Mixing one s orbital and two p orbitals makes three sp2 orbitals Explains 3 EPG geometries with double bonds

Sigma (σ) and pi (π) bonds

Bond rotation

sp hybridization Mixing one s orbital and one p orbital makes 2 sp orbitals Explains triple bonds

sp3d hybridization Mixing one s, three p, and one d orbital produce five sp3d orbitals This can explain 5 EPG geometries

sp3d2 hybridization Mixing one s, three p, and two d orbitals produce five sp3d2 orbitals This can explain 6 EPG geometries

Molecular orbital (MO) theory!!! Solving the Schrodinger equation allowed for the calculation of the atomic orbitals (1s, 2p, 3d, etc.) A similar calculation can be applied for molecular orbitals, this results in a process similar to hybridization Linear combination of atomic orbitals (LCAOs) – a weighted linear sum of the valence atomic orbitals a mixing of the atomic orbitals to produce molecular orbitals

Making molecular orbitals from atomic orbitals

Molecular orbital energy diagram Similar to orbital diagrams for atoms, but now for molecules Molecular orbitals are labeled based on the interaction between the orbitals (σ in the case below) The type of bonding orbital (bonding or antibonding) The orbitals involved (1s orbitals)

Bond order Bond order – relates to the strength of a bond and is dependant on the number of electrons in bonding and antibonding orbitals

Summary of LCAO-MO Theory We can represent molecular orbitals (MOs) as a linear combination of atomic orbitals (AOs), where the total number of atomic orbitals will equal the total number of produced MOs When two AOs mix, a lower energy bonding MO is produced, and a higher energy antibonding MO is produced Filling in the molecular orbital energy diagram with the total number of electrons from the atoms, following general electron configuration rules Bond order can be determined from the bond order formula

Period two homonuclear diatomic molecules With period two diatomic molecules 2s and 2p orbitals must be considered (the valence orbitals in these atoms)

p orbital based MOs For B2, C2, and so on, we need more orbitals to store electrons Mixing of the positive and the negative to produce the bonding and antibonding MOs

Explanation of change in orbital ordering Orbitals with similar phases and similar energies can mix The more 2s and 2px mixing lowers the σ2s energy and raises the σ2p energy

Summary

Power of the MO theory Lewis dot structure shows oxygen with no unpaired electrons From the MO diagram you can see that it does have 2 unpaired electrons Real life liquid O2 displays paramagnetism, as expected from a compound with unpaired electrons

Second period heteronuclear diatomic molecules Different atoms have different energies for their atomic orbitals Mixing is not as strong in heteronuclear diatomic molecules

Polyatomic molecules

No MO Chapter 10