AP CHAPTER 9 OUTLINE
CHEMICAL BONDING AND MOLECULAR STRUCTURE
I. Some Common Molecular Strucutres
A. Linear Molecules -- atoms lie in a straight line
1. Bond angle -- angle formed by covalent bonds
a. linear = 180
B. Planar Triangular Molecules -- 3 atoms are at 3 corners of a triangle and are conected to one central atoms (2 demensional)
1. Bond angle = 120
C. Tetrahedral Molecules -- four-sided geometric shape like a pyramid with triangular faces
1. bond angle = 109.5
D. Trigonal Bipyramidal Molecules -- two trigonal pyramids that share a common base
1. Equatorial bonds -- bonds around the "equator" of the molecule
a. Bond angle = 120
2. Axial bonds -- vertical bonds along "axis" of molecule
a. Bond angle = 180
b. Bond angle between equatorial and axial bonds = 90
E. Octahedral Molecules -- eight-sided figure, two square pyramids sharing a common base
a. Bond angle = 90
II. Predicting the Shapes of Molecules: VSEPR Theory
A. Valence Shell Electron Pair Repulsion Theory
1. Valence electron pairs, being negatively charged, stay as far apart as possible so that the repulsions between them are at a minimum
B. Using Lewis Structures to Predict Molecular Shapes
1. To predict the shape of a molecule or ion, need to know how many sets of electron pairs surround the central atom
2. Refered to as "electron arrangement (geometry)"
a. 2 pairs = linear
b. 3 pairs= planar triangular
c. 4 pairs = tetrahedral
d. 5 pairs = trigonal bipyramidal
e. 6 pairs = octahedral
C. Molecular Shapes when Some Electron Pairs are not in Bonds
1. lone pairs -- unshared electron pairs (effect shpae of molecule)
a. Lone pairs have greater effective size than bonded pairs -- lone pairs repel more than bonded pairs
1. LP-LP repulsion > LP-BP repulsion > BP-BP repulsion
b. Refered to as "molecular shape (geometry, or arrangement)"
2. 2 pairs = always linear
3. 3 pairs (1 lone pair) = nonlinear, bent, or V-shaped
4. 4 pairs (tetrahedral)
a. 1 lone pair = trigonal pyramidal
b. 2 lone pairs = nonlinear or bent
5. 5 pairs (trigonal bipyramidal)
a. 1 lone pair (always on the equatorial because this gives the fewest strong neighbor repulsions) = Distorted terahedral
b. 2 lone pairs = T-shaped
c. 3 lone pairs = linear
6. 6 pairs (octahedral)
a. 1 one pair = square pyramidal
b. 2 lone pairs = square planar
D. Shapes of Molecules ond Ions with Double or Triple Bonds
1. When predicting the molecular geometry, treat double and triple bonds like single bonds
III. Molecular Shape and Molecular Polarity
A. Polarity effects many physical properties of a molecule
1. Polar molecules attract each other
a. strength of attraction depends on amount of charge on either molecule and the distance between the two molecules (dipole moment)
b. Some molecules contain polar bonds, but are are not polar molecules
1. Cancel bond dipoles by symmetry ( linear, planar triangular, tetrahedral, trigonal bipyramidal and octahedral all do this)
2. Bond dipoles -- shown by an arrow with a crossed tail
a. arrow points toward negative end of bond, tail points toward positive end of bond
B. Polar Molecules --- if all the atoms attached to the central atom are not the same, or if there are lone pairs in the valence shell, the molecule is usually polar
1. Nonpolar = (A) bonds are nonpolar or (B) no lone pairs and atoms attached to cetral atom are all the same
IV. Wave Mechanics and Covalent Bonding: Valence Bond Theory (VB Theory)
A. A bond between two atoms is formed when a pair of electrons with their spins paired is shared by two overlapping atomic orbitals, one orbital from each of the atoms joined by the bond
1. Atoms tend to position themselves so that the maximum amount of orbital overlap occurs because this yields the minimum potential energy and therefore the strongest bonds
Example:
2. a Lewis structure can be viewed, in a very qualitative sense, as a shorthand notation for the valence bond description of a molecule
a. maximum overlap occurs along bond axis
1. Accounts for geometry shape of molecules!
V. Hybrid Orbitals -- when atoms form bonds, their simple atomic orbitals often mix to form new orbitals called...
A. Formation of sp, sp2, and sp3 Hybrid Orbitals
1. Hybrid orbitals extend farther from the nucleus than do regualr orbitals
a. more overlap with ohter orbitals
b. stronger, more stable bonds
2. the number of hybrid orbitals = number of original atomic orbitals involved
3. The VB and VSEPR theories compliment each other well!
4. When one s and one p orbital are mixed, you get 2 sp hybrid orbital
Example:
B. Other Hybrids Formed from s and p orbitals
1. When one s and 2 p orbitals are mixed, you get 3 sp2 hybrid orbital
Example:
2. When one s and 3 p orbitals are mixed, you get 4 sp3 hybrid orbital
Example:
C. Hybridization when the Central Atom Has More than an Octet
1. Hybridization involves d-orbitals
2. sp3d --> point towards the corners of a trigonal bipyramid
3. sp3d2 --> point towards the corners of a octahedral
D. Using VSEPR Theory to Predict Hybridization
1. the hybridization will equal the number of bonded electron pairs on the central atom
a. 2 pairs = sp, 3 pairs = sp2, 4 pairs = sp3, 5 pairs = sp3d, 6 pairs = sp3d2
E. Hybridization in Molecules that have lone pairs of electrons
1. Lone pair of electrons can occupy hybrid orbitals
F. Coordinate Covalent Bonds and Hybrid Orbitals
1. formed by the overlapping of one filled orbital with one empty orbital
VI. Double and Triple Bonds
A. Sigma Bond -- a bond that is made along the axis between two nuclei
1. Can be made by overlapping s-orbitals, p-orbitals (end to end), or hybrid orbitals
B. Pi Bond -- a bond that is made on two opposite sides of the axis between the nuclei
1. made by p-orbitals overlapping side to side
a. allow atoms to form double and triple bonds
2. The basic framework of the molecule is determined by the formation of a sigma bond
a. Remaining unhybridized p-orbitals, one from each atom, overlap to form a pi bond
b. Rotation around the axis of a double does not happen -- misalignes the p-orbitals
c. If their are two pairs of unhybridized p-orbitals remaining, a triple bond will form
C. A Brief Summary
1. Basic molecular structure of a molecule is determined by sigma bonds
2. Hybrid orbitals are used by an atom to form sigma bonds and to hold lone pairs
3. the # of hybrid orbitals needed by an atom = the # of atoms to which it is bonded + the # of lone pairs of electrons in the valence shell
4. Double bonds consist of one sigma and one pi bond
5. Triple bonds consist of one sigma and two pi bonds
Outline based upon:
Brady, J. E., Holum, J. R., Russell, J. W. (2000). Chemistry: The Study of Matter and Its Changes. (3rd ed.). New York: John Wiley & Sons, Inc. pp. 367-403.
