AP. Chemistry Chapter 9 Reading Notes

Return to AP. Chemistry Homepage and Contents

You must relate the concepts of molecular shapes (Chapter 9), to the concepts of Lewis dot structures and electronic configurations.

Molecular Shape, Valence Bond Theory, and Valence Shell Electron Pair Repulsion Theory

These three theories are related and result in determining the shape of a molecule. The shape of the molecule is a contributing factor to molecular polarity. Molecular polarity determines many of the physical and chemical properties of the substance. Read the chemistry of vision given on p. 372. Biological enzymes are another application of molecular shape.

Valence Shell Electron Pair Repulsion Theory - VSEPR Theory- The basis is that bonding pairs of electrons and lone pairs of electrons in the valence shell repel each other. The electron pairs align themselves around the central atom to minimize this repulsive force. This author describes electron pairs as electron domains. I will use the terms electron pair and electron domain interchangeably. A bonding pair of electrons, a lone pair of electrons, or the electrons in a multiple bond are counted as a single electron domain the VSEPR theory.

Define and differentiate electron pair geometry and molecular shape. Know the term ligand. (A ligand is an atom or group of atoms bonded to the central atom.) In the substance water the H atoms are the ligands.

Electron pair geometry deals with the alignment of the bonding and lone pairs of electrons around the central atom. This in turn determines the positions of the ligands around the central atom. If there are no lone pairs of electrons around the central atom there is no difference between the electron pair geometry and the molecular shape.

Be able to predict the electron pair geometry and molecular shape of a compound. Drawing the Lewis Structure and counting the electron pairs around the central atom most easily accomplish this. Remember when two or three pairs of electrons are shared in the process of creating a double or triple bond they count as a single group in the VSEPR theory.

FIVE ELECTRON DOMAIN GEOMETRIES OR FIVE ELECTRON PAIR GEOMETRIES

p. 349 Table 9.1 Brown/LeMay “Chemistry The Central Science”

Notice the Molecular Shapes are derived from the electron domain geometry. The electron domain geometry and the molecular shape are different when there are lone pairs of electrons around the central atom. You need to memorize the names of all five electron domains and all the molecular geometries. Be able to visualize each shape in 3-D space.

Examples	Electron Pair Geometry Or Electron Domain Geometry	Molecular Shape	Bonding Pairs	Lone Pairs
BeI2	Linear	Linear	2	0
BH3	Trigonal Planar	Trigonal Planar	3	0
SO2	Trigonal Planar	Bent	2	1
CF4	Tetrahedral	Tetrahedral	4	0
NH3	Tetrahedral	Trigonal Pyramidal	3	1
H2O	Tetrahedral	Bent	2	2
PCl5	Trigonal Bipyramidal	Trigonal Bipyramidal	5	0
SF4	Trigonal Bipyramidal	Seesaw	4	1
ClF3	Trigonal Bipyramidal	T shaped	3	2
XeF2	Trigonal Bipyramidal	Linear	2	3
SF6	Octahedral	Octahedral	6	0
BrF5	Octahedral	Square Pyramidal	5	1
XeF4	Octahedral	Square Planar	4	2

More Applications of the VSEPR Theory

• Distortions of bond angles by lone pairs of electrons around the central atom
• Distortions of bond angles by multiple bonds
• Differentiate between molecular polarity and bond polarity.
• Note that bond polarity describes electron pair density within a bond.
• Molecular polarity measures the electron pair density throughout the entire molecule.
• Molecular Polarity- arrange the bond dipoles and look at the symmetry of the molecular shape.
• Polar molecules- lopsided with charge creating a slightly positive and slightly negative ends of the molecule
• Nonpolar molecules- symmetrical- charge is evenly distributed over the molecule.
• There is no relationship between bond polarity and molecular polarity. Note the substance Carbon Dioxide is composed of polar covalent bonds but the molecule is nonpolar.
• Guidelines for the Recognition of Molecular Polarity- (There are exceptions to these guidelines.
• Polar Molecules
• Different ligands and/or
• Lone pairs of electrons around the central atom
• Nonpolar Molecules
• Identical ligands and
• No lone pairs of electrons around the central atom

Valence Bond Theory – Pure atomic orbitals mix to form hybrid orbitals. The hybrid orbitals align to minimize electron-electron repulsion giving the electron domain geometries predicted by the VSEPR Theory.

• Understand the relationship between Electron Domain Geometries and the type of hybrid orbital used.

Electron Domain Geometry	Linear	Trigonal Planar	Tetrahedral	Trigonal Bipyramidal	Octahedral
Type of Hybrid	sp	sp^2	sp^3	sp^3d	sp^3d^2

• Show how electrons in the electronic configuration of an atom can be promoted to higher energy orbitals which then hybridize to form the hybrid orbitals. (See p. 363)
• Be able to illustrate molecules showing the hybrid orbitals overlapping and pure p orbitals overlapping to show sigma and pi bonds.
• Relate localized and delocalized pi bonds to resonance structures.

Molecular Orbital Theory - related to the pure atomic orbitals of the atoms in a molecule combining to form molecular orbitals.

This theory tends to be a bit more complicated than the valence bond theory. It is more general and explains many of the anomalies that do not align with the valence bond theory. A case in point is the paramagnetism of elemental diatomic oxygen. The valence bond theory predicts that oxygen should be diamagnetic yet experiments clearly show oxygen is paramagnetic.

Compare and contrast bonding versus antibonding orbitals. Study figures 9.34 , and 9.38 on pages 373 and 377. These figures should give you a geometric frame of reference for the analysis of basic sigma s, sigma p, and pi p bonding and antibonding orbitals.

You should be able to write the electronic configurations of diatomic homonuclear and heteronuclear elements of the first and second periods using bonding and antibonding molecular orbitals. You should then be able to find the bond order based on these configurations. You will be given the order of filling for the bonding and antibonding orbitals. Look at the orbital box diagram given at the bottom of p. 381.

Magnetism - related to the electronic configuration of an element.

Substances with one or more unpaired electrons are affected by a magnetic field and are said to be paramagnetic. Paramagnetic substances are attracted to a magnetic field. Substances in which all of the electrons are paired are not affected appreciably by a magnetic field, in fact they very slightly repelled by a magnetic field.

CHAPTER 8 & 9 SUMMARY

To check your understanding of the relationship between the Valence Bond Theory and the VSEPR Theory take each example in the left hand column, draw the Lewis Structure, and then predict the electron pair geometry, molecular shape, # of bonding pairs, # of lone pairs, type of hybrid orbital used by the central atom, and the resulting bond angle between the central atom and the ligands. Finally write the electronic configuration of the central atom using a spectroscopic notation and show how the pure atomic orbitals can be mixed to create the hybrid orbitals.

Example	Electron Pair Geometry	Molecular Shape	# Bonding Pairs of e^-	# Lone Pairs of e^-	Type of Hybrid Orbital	Bond Angle between ligands & central atom
BeCl2	Linear	Linear	2	0	sp	180
SO3	Trigonal Planar	Trigonal Planar	3	0	sp^2	120
CH4	Tetrahedral	Tetrahedral	4	0	sp^3	109.5
PH3	Tetrahedral	Pyramidal	3	1	sp^3	<109.5
water	Tetrahedral	Bent	2	2	sp^3	<109.5
PCl5	Trigonal Bipyramidal	Trigonal Bipyramidal	5	0	sp^3d	120, 180
SCl4	Trigonal Bipyramidal	SeeSaw	4	1	sp^3d	120,180
ICl3	Trigonal Bipyramidal	T-Shaped	3	2	sp^3d	90,180
XeCl2	Trigonal Bipyramidal	Linear	2	3	sp^3d	180
SCl6	Octahedral	Octahedral	6	0	sp^3d^2	90
ICl5	Octahedral	Square Pyramidal	5	1	sp^3d^2	90
XeCl4	Octahedral	Square Planar	4	2	sp^3d^2	90