Ch 13: Covalent Bonding Section 13: Valence-Shell Electron-Pair Repulsion 1. Recall the rules for drawing Lewis dot structures 2. Remember the special situations: - Resonance structures - ormal charges - Sub-octets and expanded octets 3. Use the best LDS to determine the electron pair geometry and molecular shape of the molecule/ion
Remember LDS Mechanics Three steps for basic Lewis structures: 1. Sum the valence electrons for all atoms to determine total number of electrons (add e- for any net negative charge or remove e- for any net positive charge) 2. Use pairs of electrons to form a bond between each pair of atoms (bonding pairs). 3. Arrange remaining electrons around atoms (lone pairs or multiple bonds) to satisfy the octet rule for each atom ( duet rule for hydrogen).
Remember LDS Rules of Thumb 1. In a polyatomic molecule, the atom that can make the most number of bonds typically goes in the center. This atom is also typically the least electronegative atom in the molecule. 2. H can only form one bond, so it goes on the outside of the molecule...h is a terminal atom. 3. If O and H both appear in a chemical formula, there s a good chance that they are bonded to each other. 4. When several C atoms appear in the same formula, they are probably bonded to each other in a chain. In other situations the C atoms can form a closed loop, or branching structures we ll come back to that in Ch 21! =)
Remember Beyond the Octet Rule There are numerous exceptions to the octet rule. We ll deal with two classes of violation here: Sub-octet systems (less than 8 electrons) Valence shell expansion (more than 8 electrons)
Remember Sub-Octet Systems Some atoms (for example: Be, B, and Al) can form stable molecules that do not fulfill the octet rule. B B Experiments demonstrate that the B- bond strength is consistent with single bonds only.
Remember Valence Shell Expansion or third-row elements ( Period 3 ), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding. When this occurs, more than 8 electrons can surround a thirdrow element. Example: Cl 3 (a 28 e - system) obeys octet rule Cl Cl is breakin the law! It has 10 electrons.
Remember Valence Shell Expansion (cont.) Typical atoms that demonstrate valence-shell expansion are P, S, and larger halogens (Cl, Br, and I). Example: PCl 5 Cl Cl 40 e - Cl P Cl Cl
Remember Valence Shell Expansion (cont.) Lewis-dot structure & valence shell expansion: As before, assign electrons to bonds and lone pairs to give each atom an octet. Assign any remaining electrons to elements with accessible d orbitals. When it is necessary to exceed the octet for one of several third-row or higher) atoms, put extra electrons on the central atom. ormal charge is used to discriminate between multiple LDSs. DO NOT expand an octet unless you HAVE to!! Example: I 3-22 e - I I I I I I I I I C = 0-1 0-2 +1 0
What Do Molecules Look Like? The Lewis Dot Structure approach provides some insight into molecular structure in terms of bonding, but what about 3D geometry? Recall that we have two types of electron pairs: bonding and lone. Valence-Shell Electron-Pair Repulsion (VSEPR). 3D structure is determined by minimizing repulsion of electron pairs.
Electron pairs (both bonding and lone) are distributed around a central atom such that electron-electron repulsions are minimized.
Electron pairs (both bonding and lone) are distributed around a central atom such that electron-electron repulsions are minimized. 2 electron pairs 3 electron pairs 4 electron pairs Period 1, 2 5 electron pairs 6 electron pairs Period 3 & beyond
Arranging Electron Pairs Must consider both bonding and lone pairs when minimizing repulsion. Example: CH 4 (bonding pairs only) H H C H H Lewis Structure VSEPR Structure
Arranging Electron Pairs (cont.) Example: NH 3 (both bonding and lone pairs). H H N H Lewis Structure VSEPR Structure Note: electron pair geometry vs. molecular shape
VSEPR Structure Guidelines The previous examples illustrate the strategy for applying VSEPR to predict molecular structure: 1. Construct the Lewis Dot Structure 2. Arrange bonding/lone electron pairs in space such that repulsions are minimized (electron pair geometry). 3. Name the molecular shape from the position of the atoms. VSEPR Shorthand: 1. Refer to central atom as A 2. Attached atoms are referred to as X 3. Lone pair are referred to as E Examples: CH 4 : AX 4 NH 3 : AX 3 E H 2 O: AX 2 E 2 B 3 : AX 3
VSEPR: 2 electron pairs Linear (AX 2 ): angle between bonds is 180 Experiments show that molecules with multiple bonds can also be linear. Example: Be 2 Be Multiple bonds are treated as a single effective electron group. Be Be 180 More than one central atom? Determine shape around each.
VSEPR: 3 electron pairs Trigonal Planar (AX 3 ): angle between bonds is 120 Example: B 3 Multiple bond is treated as a single effective electron group. B B 120
VSEPR: 4 electron pairs (cont.) Tetrahedral (AX 4 ): angle between bonds is ~109.5 Example: CH 4 H 109.5 H C H H tetrahedral e- pair geometry AND tetrahedral molecular shape
Bonding vs. Lone pairs Bond angle in a tetrahedral arrangement of electron pairs may vary from 109.5 due to size differences between bonding and lone pair electron densities. bonding pair is constrained by two nuclear potentials; more localized in space. lone pair is constrained by only one nuclear potential; less localized (needs more room).
VSEPR: 4 electron pairs Trigonal pyramidal (AX 3 E): Bond angles are <109.5, and structure is nonplanar due to repulsion of lone pair. Example: NH 3 H H N H 107 tetrahedral e- pair geometry; trigonal pyramidal molecular shape
VSEPR: 4 electron pairs (cont.) Classic example of tetrahedral angle shift from 109.5 is water (AX 2 E 2 ): 104.5 o bent tetrahedral e- pair geometry; bent molecular shape
VSEPR: 4 electron pairs (cont.) Comparison of CH 4 (AX 4 ), NH 3 (AX 3 E), and H 2 O (AX 2 E 2 ):
AX 2 E AX 3 E AX 2 E 2 1. Refer to central atom as A 2. Attached atoms are referred to as X 3. Lone pair are referred to as E
Molecular vs. Electron-Pair Geometry H O C N H H H Central Atom Compound Electron-Pair Geometry Molecular Shape Carbon, C CH 4 Nitrogen, N NH 3 Oxygen, O luorine, H 2 O H
What is the electron-pair geometry and the molecular shape for HCS? a) trigonal planar, bent b) trigonal planar, trigonal planar c) tetrahedral, trigonal planar d) tetrahedral, tetrahedral
VSEPR: Beyond the Octet Systems with expanded valence shells will have five or six electron pairs around a central atom. Cl Cl P Cl Cl Cl 90 S 90 120 S 90
VSEPR: 5 electron pairs Consider the structure of S 4 (34 e -, AX 4 E) What is the optimum arrangement of electron pairs around S? S?? S Compare e pair angles lone-pair / bond-pair: two at 90 o, two at 120 o bond-pair / bond-pair: four at 90 o, one at 120 o S three at 90 o three at 90 o, three at 120 o Repulsive forces (strongest to weakest): lone-pair/lone-pair > lone-pair/bond-pair > bond-pair/bond-pair
VSEPR: 5 electron pairs Driving force for last structure is to maximize the angular separation of the lone pairs. I 3 - (AX 2 E 3 ):
5-electron-pair geometries AX 4 E our previous example AX 3 E 2 AX 2 E 3
VSEPR: 6 electron pairs Which of these is the more likely structure? See-saw Square Planar
6-electron-pair geometries AX 5 E AX 4 E 2 our previous example
Molecular Dipole Moments We can use VSEPR to determine the polarity of a whole molecule. 1. Draw Lewis structures to determine 3D arrangement of atoms. 2. If one side of the molecule has more EN atoms than the other, the molecule has a net dipole. Shortcut: completely symmetric molecules will not have a dipole regardless of the polarity of the bonds.
Molecular Dipoles The C=O bonds have dipoles of equal magnitude but opposite direction, so there is no net dipole moment. The O-H bonds have dipoles of equal magnitude that do not cancel each other, so water has a net dipole moment.
Molecular Dipoles (cont.) symmetric asymmetric symmetric
Molecular Dipole Example Write the Lewis dot and VESPR structures for C 2 Cl 2. Does it have a dipole moment?
Advanced VSEPR Application Molecules with more than one central atom methanol (CH 3 OH) H H C O H H tetrahedral e- pairs tetrahedral shape tetrahedral e- pairs bent shape
The VSEPR Table # e - pairs e - Geom. Molec. Geom. 2 AX 2 Be 2 linear linear 3 AX 3 B 3 trigonal planar trigonal planar AX 2 E O 3 trigonal planar bent 4 AX 4 CH 4 tetrahedral tetrahedral AX 3 E NH 3 tetrahedral pyramidal AX 2 E 2 H 2 O tetrahedral bent
The VSEPR Table # e - pairs e - Geom. Molec. Geom. 5 AX 5 P 5 trigonal bipyramidal AX 4 E S 4 trigonal bipyramidal AX 3 E 2 Cl 3 trigonal bipyramidal AX 2 E 3 I - 3 trigonal bipyramidal trigonal bipyramidal see saw T-shaped linear 6 AX 6 S 6 octahedral octahedral AX 4 E 2 Xe 4 octahedral square planar
What is the expected shape of ICl 2+? A. linear C. tetrahedral B. bent D. square planar