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 three atoms with a 180° bond angle due to the repulsion of two electron groups.

trigonal planar geometry

The molecular geometry of four atoms with 120° bond angles in a plane.

tetrahedral geometry

The molecular geometry of five atoms with 109.5° bond angles.

trigonal bipyramidal geometry

The molecular geometry of six atoms 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 seven atoms 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.

trigonal pyramidal geometry

The molecular geometry of a molecule with tetrahedral electron geometry and one lone pair.

bent geometry

A molecular geometry in which the angle between three bonded atoms is approximately 109 degrees (but could be less).

seesaw geometry

The molecular geometry of a molecule with trigonal bipyramidal electron geometry and one lone pair in an equatorial position.

T-shaped geometry

The molecular geometry of a molecule with trigonal bipyramidal electron geometry and two lone pairs in equatorial positions.

square pyramidal geometry

The molecular geometry of a molecule with octahedral electron geometry and one lone pair.

square planar geometry

The molecular geometry of a molecule with octahedral electron geometry and two lone pairs.

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.

hybridization

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

pi bond (π)

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

sigma bond (σ)

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

molecular orbital (MO) theory

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.

nonbonding orbitals

An orbital whose electrons remain localized on an atom.

What determines the geometry of a molecule according to VSEPR theory?

Repulsion between electron groups around a central atom

Which molecule has a trigonal planar electron geometry?
a) CH₄
b) CO₂
c) BF₃
d) NH₃

c

What is the bond angle for a tetrahedral geometry?

b

A molecule with five electron groups and one lone pair has which molecular geometry?

b

Which type of orbital overlap forms a pi bond?

b

T/F: Lone pairs exert less repulsion than bonding pairs.

False

T/F: In an sp³d² hybridized atom, all positions are equivalent and the bond angles are 90°.

True

T/F: Double and triple bonds count as one electron group in VSEPR theory.

True

T/F: A molecule is nonpolar if its bond dipoles cancel out.

True

T/F: Valence bond theory assumes that electrons are delocalized throughout the molecule.

False

Describe how lone pairs affect bond angles in molecular geometry.

Lone pairs occupy more space than bonding pairs and push bonded atoms closer together, decreasing bond angles.

Predict the hybridization of the central atom in NH₃ and describe the shape.

NH₃ has 4 electron groups → sp³ hybridized → molecular geometry is trigonal pyramidal.

Define molecular orbital theory and explain how bonding and antibonding orbitals form.

MO theory combines atomic orbitals into molecular orbitals over the whole molecule. Bonding MOs form from constructive interference; antibonding from destructive.

How do you determine if a molecule is polar?

Draw Lewis structure, determine geometry and bond polarity, use vector addition to see if dipoles cancel or not.

What is the bond order of O₂ according to molecular orbital theory, and is it paramagnetic or diamagnetic?

O₂ has bond order of 2 and has unpaired electrons in its MO diagram → paramagnetic.