Chemistry: The Central Science Fourteenth Edition Chapter 9 Molecular Geometry and Bonding Theories Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Molecular Shapes Lewis structures show bonding and lone pairs but do not denote shape. However, we use Lewis structures to help us determine shapes. Here we see some common shapes for molecules with two or three atoms connected to a central atom. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
What Determines the Shape of a Molecule? The bond angles and bond lengths determine the shape and size of molecules. Electron pairs repel each other. Electron pairs are as far apart as possible; this allows predicting the shape of the molecule. This is the valence-shell electron-pair repulsion (VSEP R) model. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Electron Domains We can refer to the directions
to which electrons point as electron domains. This is true whether there is one or more electron pairs pointing in that direction. It is also true if it is a lone pair or a bond. The central atom in this molecule, A, has four electron domains. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Valence-Shell Electron-Pair Repulsion (V S E P R) Model The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them. (The balloon analogy in the figure to
the left demonstrates the maximum distances that minimize repulsions.) Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Electron-Domain Geometries (1 of 3) The table shows the electron-domain geometries for two through six electron domains around a central atom. To determine the electron-domain geometry, count the total number of lone pairs, single, double, and triple bonds on the central atom. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Electron-Domain Geometries (2 of 3) Table 9.1 Electron-Domain Geometries as a Function of Number of Electron Domains Number of Electron Domains* Arrangement of Electron Domains Electron Domain Geometry Predicted Bond Angles
2 Two domains opposite each other, forming a straight line. Linear 180 3 Three domains forming the points of an equilateral Trigonal planar triangle. 120
4 Four domains forming the Tetrahedral points of a pyramid. 109.5 Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Electron-Domain Geometries (3 of 3) [Table 9.1 Continued] Number of Electron Domains*
Arrangement of Electron Domains Three domains forming the points of an equilateral triangle. Two additional domains are found perpendicular, above and below, the triangle. 5 Electron Domain Geometry Predicted Bond Angles Trigonal bipyramidal
120 90 Octahedral 90 Four domains forming the points of a square. Two additional domains are found perpendicular, above and below, the square 6 *The number of electron domains is sometimes called the coordination number of the atom. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Molecular Geometries 1) Draw the best Lewis structure. 2) Determine the electron-domain geometry. 3) Use the arrangement of the bonded atoms to determine the molecular geometry. Tables 9.2 and 9.3 show the potential molecular geometries. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Linear Electron Domain
In the linear domain, there is only one molecular geometry: linear. Note: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is. Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron Domains Electron-Domain Geometry Bonding Domains Nonbonding Domains
Two domains opposite each other, forming a straight line, linear. 2 Molecular Geometry Linear. 2 0 Example C O 2. A central C double bonded left
and right to O; each O has two pairs of dots. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Trigonal Planar Electron Domain (1 of 3) There are two molecular geometries: Trigonal planar, if all electron domains are bonding electrons Bent, if one of the domains is a lone pair Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron Domains
Electron-Domain Geometry Bonding Domains Nonbonding Domains Three domains forming the points of an equilateral triangle, trigonal planar. 3 Blank
3 Blank 2 Molecular Geometry Trigonal planar. B F 3. B is single left and right, angled down, and above to F; each F has three pairs of dots. bent.
N O 2. N is double bonded left, angled down, to O, which has two pairs of dots, and single bonded right, angled down, to O, which has three pairs. The entire molecule has a negative charge 0 1 Example Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Trigonal Planar Electron Domain
(2 of 3) There are three molecular geometries: Tetrahedral, if all are bonding pairs Trigonal pyramidal, if one is a lone pair Bent, if there are two lone pairs Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Trigonal Planar Electron Domain (3 of 3) Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron Domains
Electron-Domain Geometry Bonding Domains Nonbonding Domains Four domains forming the points of a pyramid, tetrahedral. 4 Blank
Blank 4 Blank Blank 3 2 Molecular Geometry Example Tetrahedral.
C H 4. C is single bonded right, into the page, and out of the page, all angled down, to H, as well as single bonded above to H. Trigonal pyramidal. N H 3. N is single bonded right, into the page, and out of the page, all angled down, to H; N has a pair of dots above. Bent H 2 O. O is single bonded left and right, angled down, to H; O
has two pairs of dots. 0 1 2 Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to compress bond angles. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Multiple Bonds and Bond Angles Double and triple bonds have larger electron domains than single bonds. They exert a greater repulsive force than single bonds, making their bond angles greater. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Expanding Beyond the Octet Rule Remember that some elements can break the octet rule and make more than four bonds (or have more than four electron domains). The result is two more possible electron domains: five = trigonal bipyramidal; six = octahedral (as was seen in the slide on electron-domain geometries). Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Trigonal Bipyramidal Electron Domain (1 of 3) There are two distinct positions in this geometry: Axial Equatorial Lone pairs occupy equatorial positions. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Trigonal Bipyramidal Electron Domain (2 of 3) There are four distinct molecular geometries in this domain: Trigonal bipyramidal Seesaw
T-shaped Linear Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Trigonal Bipyramidal Electron Domain (3 of 3) Table 9.3 Electron-Domain and Molecular Geometries for Five and Six Electron Domains around a Central Atom Number of Electron Domains Electron-Domain Geometry Bonding Domains
Nonbonding Domains Example Trigonal bi-pyramidal. Three domains forming the points of an equilateral triangle, and two more domains above and below, perpendicular to the plane of the triangle. 5 Molecular Geometry 5
P C l5 0 Seesaw. Blank Blank 4 SF4 1 T shaped. Blank
Blank 3 ClF3 2 Linear. Blank Blank 2 3
X e F2 Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Octahedral Electron Domain (1 of 2) All positions are equivalent in the octahedral domain. There are three molecular geometries: Octahedral Square pyramidal Square planar Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Octahedral Electron Domain
(2 of 2) Table 9.3 Electron-Domain and Molecular Geometries for Five and Six Electron Domains around a Central Atom Number of Electron Domains 6 Electron-Domain Geometry Four domains forming the points of a square, and two more domains above and below, perpendicular to the plane of the square.
Bonding Domains Nonbonding Domains Molecular Geometry Example Octahedral. 6 S F6 0
Square pyramidal. Blank Blank 5 BrF5 X1 Square planar. Blank Blank
4 2 XeF4 Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Shapes of Larger Molecules For larger molecules, look at the geometry about each atom rather than the molecule as a whole. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Polarity of Molecules Ask yourself: Covalent or ionic? If covalent: Are the bonds polar? (Do they have a bond dipole?)
a. No: The molecule is nonpolar! b. Yes: ContinueDo the average position of + and coincide? (Is the overall dipole moment equal to zero?) 1) Yes: The molecule is nonpolar. 2) No: The molecule is polar. Note: Different atoms attached to the central atom have different
polarity of bonds. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Comparison of the Polarity of Two Molecules A nonpolar molecule A polar molecule Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Valence-Bond Theory (1 of 2) In valence-bond theory, electrons of two atoms begin
to occupy the same space. This is called overlap of orbitals. The sharing of space between two electrons of opposite spin results in a covalent bond. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Valence-Bond Theory (2 of 2) Increased overlap brings the atoms together until a balance is reached between the like charge repulsions and the electronnucleus attraction. Atoms cant get too close because the internuclear repulsions get too great.
Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved VSEPR and Hybrid Orbitals VSEPR predicts shapes of molecules very well. How does that fit with orbitals? Lets use H2O as an example: If we draw the best Lewis structure to assign VSEPR, it becomes bent. If we look at oxygen, its electron configuration is If it shares two electrons to fill its valence shell, they should be in 2p. Wouldnt that make the angle 90? Why is it 104.5? Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Hybrid Orbitals
Hybrid orbitals form by mixing of atomic orbitals to create new orbitals of equal energy, called degenerate orbitals. This process is called hybridization. When two orbitals mix they create two orbitals; when three orbitals mix, they create three orbitals; and so on. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Besp Hybridization When we look at the orbital diagram for beryllium (B e), we see that there are only paired electrons in full sublevels. Be makes electron-deficient compounds with two bonds for Be. Why? sp hybridization (mixing of one s orbital and one p orbital). Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
sp Orbitals Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. The sp hybrid orbitals each have two lobes like a p orbital. One of the lobes is larger and more rounded, as is the s orbital. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Position of sp Orbitals These two degenerate orbitals would align themselves 180 from each other. This is consistent with the observed geometry of B e compounds (like BeF2) and VSEPR: linear. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
BoronThree Electron Domains Gives sp2 Hybridization Using a similar model for boron leads to three degenerate Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Carbon: sp3 Hybridization With carbon, we get four degenerate Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved What Happens with Water? We started this discussion with H2O and the angle question: Why is it 104.5 degrees instead of 90 degrees? Oxygen has two bonds and two lone pairsfour electron domains.
The result is hybridization! Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Hypervalent Molecules The elements that have more than an octet Valence-bond model would use d orbitals to make more than four bonds. This view works for period 3 and below. Theoretical studies suggest that the energy needed would be too great for this. A more detailed bonding view is needed than we will use in this course. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Hybrid Orbital Summary (1 of 2) 1) Draw the Lewis structure. 2) Use VSEPR to determine the electron-domain geometry. 3) Specify the hybrid orbitals needed to accommodate these electron pairs.
Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Hybrid Orbital Summary (2 of 2) Table 9.4 Geometric Arrangements Characteristic of Hybrid Orbital Sets Atomic Orbital Set Hybrid Orbital Set Geometry Examples Linear, 180 degrees. s,p BeF2, HgCl2 Two sp
Three, s p 2 Trigonal planar, 120 degrees BF3, SO3 s,p,p Four, s p 2 Tetrahedral, 109.5 degrees C H 4. N H 3. H 2 O. N H 4, plus. s,p,p,p Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Types of Bonds How does a double or triple bond form? It cant, if we use only hybridized orbitals. However, if we use the orbitals that are not hybridized, we can have a sideways overlap. Two types of bonds: Sigma () bond Pi () bond Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Sigma () and Pi () Bonds Sigma bonds are characterized by head-to-head overlap. cylindrical symmetry of electron density about the internuclear axis. Pi bonds are characterized by
sideways overlap. electron density above and below the internuclear axis. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Bonding in Molecules Single bonds are always Multiple bonds have one all other bonds are Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Localized or Delocalized Electrons Bonding electrons
that are specifically shared between two atoms are called localized electrons. In many molecules, we cant describe all electrons that way (resonance); the other electrons (shared by multiple atoms) are called delocalized electrons. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Benzene and a p The organic molecule benzene has orbital on each C atom, which form delocalized bonds using one electron from each p orbital. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
Molecular Orbital (MO) Theory (1 of 2) Wave properties are used to describe the energy of the electrons in a molecule. Molecular orbitals have many characteristics like atomic orbitals: Maximum of two electrons per orbital Electrons in the same orbital have opposite spin Definite energy of orbital Can visualize electron density by a contour diagram Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved More on MO Theory They differ from atomic orbitals because they represent the entire molecule, not a single atom.
Whenever two atomic orbitals overlap, two molecular orbitals are formed: one bonding, one antibonding. Bonding orbitals are constructive combinations of atomic orbitals. Antibonding orbitals are destructive combinations of atomic orbitals. They have a new feature unseen before: A nodal plane occurs where electron density equals zero. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Molecular Orbital (MO) Theory (2 of 2) Whenever there is direct overlap of orbitals, forming a bonding and an antibonding orbital, they are called
sigma () molecular orbitals. The antibonding orbital is distinguished with an asterisk as Here is an example for the formation of a hydrogen molecule from two atoms. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MO Diagram An energy-level diagram, or MO diagram, shows how orbitals from atoms combine to form molecular orbitals. In the two electrons go into the bonding molecular orbital (lower in energy).
Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Can He2 Form? Use MO Diagram and Bond Order to Decide! Therefore , does not exist. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Guiding Principles for the Formation of Molecular Orbitals 1) The number of MOs formed equals the number of AOs
combined. 2) AOs combine with AOs of similar energy. 3) The effectiveness with which two AOs combine is proportional to their overlap. 4) Each MO can accommodate at most two electrons with opposite spin. (They follow the Pauli exclusion principle.) 5)
When MOs of the same energy are populated, one electron enters each orbital (same spin) before pairing. (They follow Hunds rules.) Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MOs, Bonding, and Core Electrons occurs at high temperatures. Lewis structure: The MO diagram is on the right. Notice that core electrons dont play a major part in bonding, so we usually dont include them in the MO diagram.
Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MOs from p-Orbitals p-orbitals also undergo overlap. They result in either direct or sideways overlap. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MO Diagrams for the Second Period p-Block Elements There are and orbitals from s and p atomic orbitals. There are and orbitals from p atomic
orbitals. Since direct overlap is stronger, the effect of raising and lowering energy is greater for and Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved s and p Orbital Interactions In some cases, s orbitals can interact with the pz orbitals more than the px and py orbitals. It raises the energy of the pz orbital and lowers the energy of the s orbital. The px and py orbitals are degenerate orbitals. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MO Diagrams for Diatomic Molecules
of Second Period Elements Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved MO Diagrams and Magnetism Diamagnetism is the result of all electrons in every orbital being spin-paired. These substances are weakly repelled by a magnetic field. Paramagnetism is the result of the presence of one or more unpaired electrons in an orbital. Is oxygen paramagnetic or diamagnetic? Look back at the MO diagram! It is paramagnetic. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Paramagnetism of Oxygen
Lewis structures would not predict that is paramagnetic. The MO diagram clearly shows that is paramagnetic. Both show a double bond (bond order = 2). Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Heteronuclear Diatomic Molecules Diatomic molecules can consist of atoms from different elements. How does a MO diagram reflect differences? The atomic orbitals have
different energy, so the interactions change slightly. The more electronegative atom has orbitals lower in energy, so the bonding orbitals will more resemble them in energy. Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Copyright Copyright 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved
