Chapter 11: Chemical Bonding II - Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory

Valence shell electron pair repulsion (VSEPR) theory

A theory that allows prediction of the shapes of molecules based on the idea that electrons—either as lone pairs or as bonding pairs—repel one another.

electron groups

A general term for lone pairs, single bonds, multiple bonds, or lone electrons in a molecule.

Linear Geometry

The molecular geometry of two electron groups with a 180° bond angle due to the repulsion of two electron groups.

How many electron groups are in a double bond?

One

Trigonal Planar Geometry

The molecular geometry of three electron groups with 120° bond angles in a plane.

How many electron groups are in a triple bond?

One

Tetrahedral Geometry

The molecular geometry of four electron groups with 109.5° bond angles.

What is the molecular geometry of the HCN molecule?

Linear

Trigonal Bipyramidal Geometry

The molecular geometry of five electron groups, with 120° bond angles between the three equatorial electron groups and 90° bond angles between the two axial electron groups and the trigonal plane.

Octahedral Geometry

The molecular geometry of six electron groups with 90° bond angles.

electron geometry

The geometrical arrangement of electron groups in a molecule.

molecular geometry

The geometrical arrangement of atoms in a molecule.

A X2 E0

EG: Linear
MG: Linear
BA: 180°

A X3 E0

EG: Trigonal Planar
MG: Trigonal Planar
BA: 120°

A X2 E1

EG: Trigonal Planar
MG: Bent (Bent-Trigonal Planar)
BA: < 120°

A X4 E0

EG: Tetrahedral
MG: Tetrahedral
BA: 109.5°

A X3 E1

EG: Tetrahedral
MG: Trigonal Pyramidal
BA: < 109.5°

A X2 E2

EG: Tetrahedral
MG: Bent (Bent-Tetrahedral)
BA: < 109.5°

A X5 E0

EG: Trigonal Bipyramidal
MG: Trigonal Bipyramidal
BA: 120° equatorial, 90° axial

A X4 E1

EG: Trigonal Bipyramidal
MG: Seesaw
BA: 120° equatorial, 90° axial

A X3 E2

EG: Trigonal Bipyramidal
MG: T-Shaped
BA: <90°

A X2 E3

EG: Trigonal Bipyramidal
MG: Linear
BA: 180°

A X6 E0

EG: Octahedral
MG: Octahedral
BA: 90°

A X5 E1

EG: Octahedral
MG: Square Pyramidal
BA: <90°

A X4 E2

EG: Octahedral
MG: Square Planar
BA: 90°

The geometry of a molecule is determined by the number of electrons on which atom(s)?

The central atom. Or on all interior atoms, if there is more than one.

How do you determine the number of electron groups?

From the Lewis Structure

Which of the following counts as a single electron group: lone pair, single bond, double bond, triple bond, free radical single electron?

Each of those counts as only one.

Which electron group interaction is most repulsive?

lone pair-lone pair

When drawing 3D structures, what does a hatched wedge indicate?

A bond going through the back of the page.

When drawing 3D structures, what does a solid wedge indicate?

A bond coming out of the front of the page.

valence bond theory

An advanced model of chemical bonding in which electrons reside in quantum-mechanical orbitals localized on individual atoms that are a hybridized blend of standard atomic orbitals; chemical bonds result from an overlap of these orbitals.

hybridized atomic orbitals

a kind of blend or combination of two or more standard atomic orbitals

hybrid orbitals

Orbitals formed from the combination of standard atomic orbitals that correspond more closely to the actual distribution of electrons in a chemically bonded atom.

hybridization

A mathematical procedure in which standard atomic orbitals are combined to form new, hybrid orbitals.

What is the hybridization of four electron groups?

sp3 hybridization

What is the hybridization of three electron groups?

sp2 hybridization - has one leftover unhybridized p orbital

What is the hybridization of two electron groups?

sp hybridization - has two leftover unhybridized p orbitals

What is the hybridization of five electron groups?

sp3d hybridization

What is the hybridization of six electron groups?

sp3d2 hybridization

pi (π) bond

The bond that forms between two p orbitals that overlap side to side.

sigma (σ) bond

The resulting bond that forms between a combination of any two s, p, or hybridized orbitals that overlap end to end.

One (and only one) ___________________ bond forms between any two atoms. Additional bonds must be ________________ bonds.

σ ; π

rotation about a _____________ bond is highly restricted

double bond, or π bond

rotation about a ____________ bond is relatively unrestricted

sigma bond

isomers

compounds with the same molecular formula but different structures or spatial arrangement of atoms

molecular orbital theory (MO)

An advanced model of chemical bonding in which electrons reside in molecular orbitals delocalized over the entire molecule. In the simplest version, the molecular orbitals are simply linear combinations of atomic orbitals.

bonding orbital

A molecular orbital that is lower in energy than any of the atomic orbitals from which it was formed.

antibonding orbital

A molecular orbital that is higher in energy than any of the atomic orbitals from which it was formed.

bond order

For a molecule, the number of electrons in bonding orbitals minus the number of electrons in nonbonding orbitals divided by two; a positive bond order implies that the molecule is stable. The higher the bond order, the more stable.

When assigning the electrons of a molecule to MOs of the same energy, follow Hund's rule, which requires

filling the orbitals singly first, with parallel spins, before pairing.

Molecular orbital energy diagram for B2, C2, and N2:

σ2s < σ2s < π2p < σ2p < π2p < σ*2p

Molecular orbital energy diagram for F2, O2, Ne2:

σ2s < σ2s < σ2p < π2p < π2p < σ*2p

nonbonding orbital

an orbital whose electrons remain localized on an atom