Chapter 5 VSEPR, Hybridization, and MO Diagrams
What are all the molecular diagram bonds called?
sigma 1s, sigma*2s, sigma 2s, sigma*2s, pi 2p, pi*2p, sigma 2p, sigma*2p Note: Anti-bonding orbitals are marked with a *
What is a sigma bond?
A bond formed by the end to end overlap of orbitals. All bonds contain 1 sigma bond. A single bond is just a sigma bond. A sigma bond is hybridized
What is a pi bond?
A bond formed by the side to side overlap of p orbitals. They are the additional bonds in double and triple bonds. Pi bonds are unhybridized Double= 1 pi bond (1 sigma bond) Triple= 2 pi bonds (1 sigma bond)
How are molecule shape and polarity related?
A compound may be polar or nonpolar solely because of it's structure. If dipoles do not cancel out for polar bonds, then the compound is polar. Molecular shape varies so much because of lone pairs.
Ferromagnetic
A material that can be turned into a magnet. Stays magnetized after exposure to magnetic field.
What are molecular orbitals?
All the orbitals of the molecule. It is the sum of the bonding orbitals from atoms. 1s, 2s, 2px, 2py, 2pz Most of the row diatomic elements have 8 molecular orbitals.
Why do we have a bonding and anti-bonding orbital?
Atoms interact in two extreme ways to form two molecular orbitals. Think of it using Schrodinger's wave theory with bonding and anti-bonding orbital corresponding to constructive and destructive interference. Anti-bonding orbitals have less electron overlap density (higher energy), bonding have greater electron overlap density (less energy). More stability means less energy
Which atoms have their sigma 2p bond below the pi 2p?
Oxygen, Fluorine, Neon O2, F2, Ne2 Structure looks like sigma, pi, pi, sigma *
What is an electron domain?
1. A single, double, or triple bond 2. A non-bonded pair of electrons (lone pair) Note: It is NOT the central atom
How do we know if the bond angle varies?
1. Check for lone pairs! Chances are a lone pair will cause repulsion and decreases bond angles 2. Double bonds and triple bonds can cause bond angles to decrease as well 3. Check if it's a resonant structure! Resonant structures have equal bond lengths so bond angle is equal!
How do we know if a compound is polar or nonpolar?
1. Check if the outer atoms are different, that usually indicates polarity even if dipoles would cancel out 2. Check for charges, if the compound has a charge it is polar 3. If the compound has lone pairs, the structures usually transforms such that it is polar. Not always though 4. Double bonds and triple bonds are polar. This is because they cause bond angle variance which can cause dipoles to not cancel right
How do we identify molecular shape?
1. Count the number of electron domains 2. Count the number of lone pairs 3. There's a corresponding structure NOTE: DO NOT assume what a structure is based on how it looks if it is within a bigger structure
Why do we use use molecular orbital diagrams?
1. Explain bonding in terms of molecular orbital theory 2. Best suited for diatomic molecules (two atoms) 3. It can also help explain why some molecules exist and why some don't, bond strength, magnetism, and what electronic transitions can take place. *They can also show why atoms bond. If the molecular orbital diagram is lower in energy, or more stable, then that is the driving force for the formation of a diatomic molecule.
What are the shortfalls in electron configuration and orbital theory? (Hybridization)
1. How can C in CH4 make 4 bonds when it only has two free electrons for bonding? (2s and 2p) The s and p orbitals hybridize to form 4 new orbitals 2. The bond angles don't match 1s and 3p orbitals. How can CH4 have 109.5° when the 3p orbitals are at 90° angles from each other? Hybridization combines s and p to make sp3 orbitals 3. All four C-H bonds are equal in length, and it takes the same amount of energy to break all 4 bonds. How? Hybridization causes bonds to be same length and take same amount of energy to break
General point about using VSEPR to find geometry
1. If there are no lone pairs, the electron domain shape is the same as the molecular shape. 2. The electron domain shape is the same for each domain, it is the molecular shape that varys
How do we determine if a large structure is polar or nonpolar?
1. Look for polar sites. These sites include OH, O, N, P, S 2. Look at how nonpolar sites compare. These are carbon hydrogen bonds or carbon carbon bonds. 3. Then look at the structure as a whole to see if it is mostly nonpolar, if not it is polar For small structures, look at dipoles, do vector analysis, and remember central atom bonded to itself is nonpolar.
How do you draw molecular orbital diagrams?
1. Put the two atoms side by side but apart enough to draw the molecular orbital diagram in between. 2. Find the sum of electrons and orbitals in the two atoms to determine the sum for MO diagram 3. Determine the splitting order or shape of MO diagram. Be careful with the second row 4. Draw in the electrons from lowest to highest energy. For degenerate orbitals (same energy) use Hund's rule. (Degenerate orbitals filled evenly before filling higher energy levels aka same spin first). 5. Find bond order of the MO diagram.
Why hybridize?
1. To maximize bonding. More bonds=More full orbitals=More stability 2. The particular kind of hybridization that occurs is the one that yields the lowest overall energy for the molecule
How do we determine number of sigma and pi bonds with MO diagrams?
1. Use the Lewis Dot and look at bond between atoms 2. Find bond order and go from there Note: Having a bonding pi and antibonding pi does not mean 2 pi bonds. Look at 02 for example, it has 1 pi bond but bonding and antibonding pi. BOND ORDER IS ABSOLUTE
How do you find bond order?
1/2 (# electrons in bonding orbitals - # electrons in anti-bonding electrons) *This should match the bond order from lewis dot structures
What are the 5 basic types of hybridization?
2 Linear: sp 3 Trigonal Planar: sp2 4 Tetrahedral: sp3 5 Trigonal Bipyramidal: sp3d 6 Octahedral: sp3d2 (Notice these are based off electron domains)
Identify the electron domain shape for 2,3,4,5, and 6 domains.
2: Linear 3: Trigonal Planar 4: Tetrahedral 5: Trigonal Bipyramidal 6: Octahedral
Why do 5 electron domains have two bond angles?
Because they have an axial domain and equatorial domain. From the axial to equatorial it's 90 degrees, but from equatorial to equatorial it's 120 degrees. In 6 domains equatorial to equatorial becomes 90 degrees.
How do bonding and anti-bonding orbitals compare in energy?
Bonding orbitals have lower energy than anti-bonding orbitals.
Which atoms have their sigma 2p bond above the pi 2p?
Boron, Carbon, Nitrogen B2, C2, N2 Structure looks like pi, sigma, pi, sigma *
6 Electron Domains no lone pairs
Electron Domain Shape: Octahedral Molecular Shape: Octahedral Bond Angles: 90°
6 Electron Domains, 4 bonds and two lone pairs
Electron Domain Shape: Octahedral Molecular Shape: Square Planar Bond Angles: 90°
6 Electron Domains, 5 bonds and one lone pair
Electron Domain Shape: Octahedral Molecular Shape: Square Pyramidal Bond Angles: <90°
4 Electron Domains, 2 bonds 2 lone pairs
Electron Domain Shape: Tetrahedral Molecular Shape: Bent Bond Angles: <109.5° Notice this bent differs in angle to the 3 domain bent
4 Electron Domains no lone pairs
Electron Domain Shape: Tetrahedral Molecular Shape: Tetrahedral Bond Angles: 109.5°
4 Electron Domains, 3 bonds 1 lone pair
Electron Domain Shape: Tetrahedral Molecular Shape: Trigonal Pyramidal Bond Angles: <109.5°
3 Electron Domains, 2 bonds 1 lone pair
Electron Domain Shape: Trignonal Planar Molecular Shape: Bent Bond Angles: <120°
5 Electron Domains, 2 bonds and 3 lone pairs
Electron Domain Shape: Trigonal Bipyramidal Molecular Shape: Linear Bond Angles: 180°
5 Electron Domains, 4 bonds 1 lone pair
Electron Domain Shape: Trigonal Bipyramidal Molecular Shape: Seesaw Bond Angles: <90°, <120°
5 Electron Domains, 3 bonds and 2 lone pairs
Electron Domain Shape: Trigonal Bipyramidal Molecular Shape: T-shaped Bond Angles: <90°
5 Electron Domains no lone pairs
Electron Domain Shape: Trigonal Bipyramidal Molecular Shape: Trigonal Bipyramidal Bond Angles: 90°, 120°
3 Electron Domains no lone pairs
Electron Domain Shape: Trigonal Planar Molecular Shape: Trigonal Planar Bond angles: 120°
2 Electron Domains
Electron domain shape: Linear Molecular shape: Linear Bond angles are 180°
Why do paired electrons cause diamagnetism?
Electrons have spin, so a pair of electrons in the same orbital have opposite spins and cancel each other out. Unpaired electrons then align parallel to fields causing attraction.
What does paramagnetic mean and what MO diagrams are paramagnetic?
Has attraction to a magnetic field (stronger than the diamagnetism or weak repulsion) At least one unpaired electron in atom/structure More unpaired electrons means stronger attraction or paramagnetism
What is hybridization?
Hybridization is the process of combining two or more atomic orbitals to create new orbitals (hybrids) that will fulfill the geometric demands of the system.
Why do p orbitals split unequally?
It's because we have an end to end overlap then two side to side overlaps. The sigma is the end to end whereas the pi is the 2 overlapping bonds. The sigma 2p moves above the pi 2p when radius of atoms increase. There's less electron density or overlap with bigger radiuses so it is less stable and higher energy.
What does miscibility mean?
It's the property or ability of two substances to dissolve each other
How do polarity and solubility relate?
LIKE DISSOLVES LIKE Polar substances dissolve in each other (miscible) Nonpolar substances dissolve in each other (miscible) A polar and nonpolar substance don't dissolve in each other (not miscible)
What causes bond angles to differ from the norm?
Lone pairs, double bonds, and triple bonds occupy more space than a bonded electron pair. They repulse more than normal which can cause structures to have slight variance in bond angles
What does diamagnetic mean and what MO diagrams are diamagnetic
No attraction (there is a weak repulsion) to a magnetic field An atom/structure with no unpaired electrons
What is the order of the degree of repulsion between electron pairs?
Nonbonded/Nonbonded> Nonbonded/bonded> bonded/bonded This shows that lone pairs have the greatest repulsion
Which atomic orbitals interact?
Only atomic orbitals of about the same energy interact to a significant degree. When two orbitals interact, they make a bonding molecular orbital and an anti-bonding molecular orbital.
How do we know if something is planar? How do we know if it rotates?
Pi bonds cause the molecule to fix and not rotate. This usually allows for molecules to be planar. Something is planar if everything is along the same plane or in 2 dimensions. The molecular geometry using VSEPR theory, can help us identify shapes which aren't planar. Usually if a molecule rotates it isn't planar. Sigma bonds rotate without the presence of a pi bond.
What are the 2 types of bonds?
Sigma and pi bonds
What happens when two atoms move close to each other and form a bond?
The atomic orbitals overlap and make molecular orbitals.
What is the basic idea of VSEPR?
The atoms around the central atom are negatively charged species. So the compound is most stable when electron domains around central atom are as FAR APART as possible.
What are bonding orbitals?
The orbitals of the atoms that bond are called bonding orbitals. We only look at what bonding orbitals have electrons (note this is NOT molecular orbitals)
What is the VSEPR theory?
Valence shell electron pair repulsion (VSEPR) theory predicts the geometry of individual molecules from the number of electron pairs surrounding their central atoms.
Why do we skip the 1st row with 2nd row MO diagrams?
We look at the outermost row or valence electrons similar to Lewis Dot Structures with MO diagrams. That is why we skip 1s with second row MO diagrams
CO splitting order (Carbon Oxygen)
You can draw the MO diagram with both splitting orders and get that they're both diamagnetic and have a bond order of 3. (This is an example of an MO diagram where splitting order is irrelevant)
What happens if you have a molecular orbital diagram with mixed atoms? (a B/C/N mixed with O/F/NE)
You can use either molecular orbital template and get the same result.
What is the order of what "occupies" the most space?
nonbonded electron pair> triple bond> double bond> single bond Nonbonded electron pairs must occupy the most space as it is most repulsive
What are Hybrid Orbitals?
orbitals of equal energy produced by the combination of two or more orbitals on the same atom. They are mixtures of s,p,d,f and allow for bonding and shapes and properties we observe in molecules
