Chem 1301 Ch 4
Valence Shell Electron Pair Repulsion Theory (VSEPR)
"The best arrangement of a given number of electron groups is the one that minimizes the repulsions among them" resulting geometric arrangement produces predictable shapes and bond angles
# of hybrid orbitals generated =
# of pure atomic orbitals that participate in the hybridization process
Bond Order
(# of bonding electrons - # of anti bonding electrons) / 2 difference between number of electrons in bonding and anti bonding orbitals only consider valence electrons can be a fraction higher bond order = stronger and shorter bonds if = 0, then it is unstable compared to individual atoms and no bond will form substance will be paramagnetic if its molecular orbital diagram has unpaired electrons if all electrons are paired, it is diamagnetic
Predicting Polarity of Molecules
1. Draw the Lewis structure and determine the molecular geometry 2. Determine whether the bonds in the molecule are polar - if there are not polar bonds, the molecule is nonpolar 3. determine whether the polar bonds add together to give a net dipole moment (with net dipole moment = polar)
Steps to Apply the VSEPR Model/Predicting the Shapes around Central Atoms
1. Draw the Lewis structure for the model 2. Count the electron pairs and arrange them in the way that minimizes repulsion - put the pairs as far apart as possible 3. Determine the positions of the atoms from the way the electron pairs are shared 4. Determine the name of the molecular structure from the positions of the atoms 1. Draw the Lewis structure for the model 2. Count the electron pairs and arrange them in the way that minimizes repulsion 3. Classify each electron group as a bonding or lone pair, and count each type (multiple bonds count as one group) 4. Use the next Table to determine the shape and bond angles
Predicting Hybridization and Bonding Schemes
1. Start by drawing the Lewis structure 2. Use VSEPR theory to predict the electron group geometry around each central atom 3. Use Table 10.3 to select the hybridization scheme that matches the electron group geometry 4. Sketch the atomic and hybrid orbitals on the atoms of the molecule, showing overlap of the appropriate orbitals 5. Label the bonds as σ or π
Trigonal Planar Electron Domain
2 molecular geometries: trigonal planar - if all the electron domains are bonding bent - if one of the domains is a nonbonding pair
mixing one s and one p orbital results in
2 sp hybrid orbitals
Lewis Structures
2-dimensional
Tetrahedral Electron Domain
3 molecular geometries: tetrahedral - if all are bonding pairs trigonal pyramidal - if one is a nonbonding pair bent - if there are two nonbonding pairs
mixing one s and two p orbitals results in
3 sp2 hybrid orbitals
Molecules
3-dimensional
Molecular Structures
3-dimensional arrangement of atoms in a molecule dictate how the molecule interacts determined by VSEPR
Trigonal Bipyramidal Electron Domain
4 molecular geometries: trigonal bipyramidal seesaw T-shaped linear
mixing one s and three p orbitals results in
4 sp3 hybrid orbitals
Sigma (σ) Molecular Orbitals
MO1 and MO2
Octahedral Electron Domain
all positions are equivalent 3 molecular geometries: octahedral square pyramidal square planar
Electrostatic Potential Diagram
alternative method for representing charge distribution in a molecule colors of visible light are used to show the variation in charge distribution -- red indicates the most electron-rich region; blue indicates the most electron-poor region)
Molecular Geometry vs. Electron Geometry
atoms are different sizes double vs. single bonds lone pairs -- occupy space, but "unseen" relative sizes of repulsive interactions repulsions make bond angles smaller than "perfectly predicted"
Solid Wedge
bond coming out of the page
Hatched Wedge
bond going into the page
Straight Line
bond in plane of paper
Representing 3-D Shapes on Paper
central atom placed in plane of paper put as many other atoms as possible in same plane - use straight line bond for atoms in front of plane, use solid wedge for atoms behind plane, use hatched wedge
dsp3 Hybrid Orbitals
combination of one d, one s, and three p orbitals leads to 5 degenerate dsp3 orbitals requires trigonal bipyramidal arrangement of five hybrid orbials
d2sp3 / sp3d2 Hybridization
combination of two d, one s, and three p orbitals leads to 6 degenerate d2sp3 orbitals requires an octahedral arrangement of six hybrid orbitals six electron pairs around an atom are always arranged octahedrally
Double/Triple bond
count as one electron domain or one effective pair place greater electron density on one side of the central atom than do single bonds affect bond angles
Molecular Geometry
determined by the number of bonding vs. nonbonding domains affects polarity of the molecule
Electron Geometry
determined by total number of domains
Electron Pairs
electron groups/domains/effective pairs
What determines the Shape of a Molecule?
electron pairs, whether they be bonding or nonbonding, repel each other by assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule for the central atom in the molecule, count the total number of electron domains/groups -- count the number of bonds (single, double, triple, all count as 1 bonding group); count the number of lone pairs (each pair counts as 1) the total number of domains determines electron geometry the number of bonding vs. nonbonding domains determines molecular geometry
Electrons in Molecular Orbitals
electrons are placed in molecular orbitals just as they are in atomic orbitals filled in order of increasing energy can hold a maximum of 2 electrons as long as they have opposite spins only the valence orbitals of the atoms contribute significantly to the molecular orbitals of a particular molecule molecular electron configuration shows the type of molecular orbital and the number of electrons each contains
Localized Electron Model/Valence Bond Theory
electrons in a molecule occupy atomic orbitals of individual atoms orbitals interact to make bonds -- not just electrons bonds are formed by the overlap of half-filled atomic orbitals between atoms -- the greater the orbital overlap, the stronger the bond; the extent of the orbital overlap depends on orbital shape and direction
Sigma Bonds (σ)
end-to-end overlapping of orbitals or head-to-head overlap all electron density lies between the nuclei of the bonding atoms single bonds stronger than π bonds permit rotation
Chlorprothixine
first generation antipsychotic drug first used to treat schizophrenia and other disorders in the 1950s blocks certain receptors in neurons, which requires binding of the drug to the receptor cis-chlorprothixine is biologically active trans-chlorprothixine is almost completely inactive
Antibonding Molecular Orbital
higher in energy than the atomic orbitals of which it is composed electrons will favor separated atoms
Molecular Orbital (MO) Theory
invoke the wave nature of electrons orbital wave functions combine either constrictively or destructively if waves interact constructively, the resulting orbital is lower in energy: a bonding molecular orbital if waves interact destructively, the resulting orbital is higher in energy: an anti bonding molecular orbital MO1 and MO1 are referred to as sigma (σ) molecular orbitals
Larger Molecules/Multiple Central Atoms
it makes more sense to talk about the geometry about a particular atoms rather than the geometry of the molecule as a whole
Lewis Theory Beings to Fail
lewis theory generally predicts trends in properties, but does not give good numerical predictions (ex. bond strength and bond length) lewis theory gives good first approximations of the bond angles in molecules, but usually cannot be used to get the actual angle lewis theory cannot write one correct structure for many molecules where resonance is important lewis theory often does not predict the correct magnetic behavior of molecule (ex. O2 is paramagnetic, although the lewis structure predicts it is diamagnetic)
2 Electron Groups
linear geometry bond angle: 180 degrees
Relative Sizes of Repulsive Interactions
lone pair - lone pair > lone pair - bonding pair > bonding pair - bonding pair
Bonding Molecular Orbital
lower in energy than the atomic orbitals of which it is composed electrons will favor bonding
Dipole Moment (m)
measure of bond polarity + and - end use an arrow to represent point to the negative charge center with the tail of the arrow indicating the positive center of charge by adding the individual bond dipoles, one can determine the overall dipole moment for the molecule - indicates the polarity of the molecule arrow goes toward more electronegative atom
sp Hybrid Orbitals
mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals these have two lobes like a p orbital one of the lobes is larger and more rounded as is the s orbital two degenerate orbitals would align themselves 180 degrees from each other
Valence Bond Theory / Localized Electron Model
molecules are groups of atoms connected by localized overlap of valence shell orbitals inadequately explains: magnetic property of molecules spectral properties of molecules electron delocalization conductivity of metals valence bond theory, VSEPR theory, and hybrid orbital theory work well together to explain the shapes of molecules
Energy
nrg
6 Electron Groups
octahedral geometry 8 sides all positions equivalent bond angle: 90 degrees between al electron groups
Multiple Bonds
one σ bond and one π bond do not permit rotation -- would break interaction between p orbitals
Linear Electron Domain
only 1 molecular geometry if there are only two atoms in a molecule, the molecule will be linear no matter what the electron domain is
Homonuclear Diatomic Molecules
p orbitals occur in sets of three mutually perpendicular orbitals two pairs of p orbitals can overlap in a parallel fashion, and one pair can overlap head-on σ and π boning molecular orbitals σ* and π* anti bonding molecular orbitals
Odd # of Electrons (in homonuclear diatomic molecules)
paramagnetic
Even # of Electrons (in homonuclear diatomic molecules)
paramagnetic or diamagnetic must draw diagrams
Measuring Paramagnetism
paramagnetic sample will appear heavier when the electromagnet is turned on because the sample is attracted into the inducing magnetic field
Nonbonding Pairs
physically larger than bonding pairs greater repulsions -- tends to decrease bond angles in a molecule
Polar Molecules
polar bonds - electronegativity difference (theory); bond dipole moments (measured) have an unsymmetrical shape - vector addition electron density accumulates to one side of the molecule attracted to other polar molecules ex. H-Cl - bonding electrons are pulled toward the Cl end of the molecule
Molecular Polarity Affects Solubility in Water
polar molecules are attracted to other polar molecule because water is a polar molecule, other polar molecules dissolve well in water, as well as ionic compounds some molecules have both polar and non polar parts polarity affects boiling points and solubilities -- like dissolves like
sp Hybridization
provides 2 orbitals 2 electron groups/ effective pairs around central atom
sp2 Hybridization
provides 3 orbitals 3 electron groups/ effective pairs around central atom
sp3 Hybridization
provides 4 orbitals 4 electron groups/ effective pairs around central atom
dsp3 / sp3d Hybridization
provides 5 orbitals 5 electron groups/ effective pairs around central atom
d2sp3 / sp3d2 Hybridization
provides 6 orbitals 6 electron groups/ effective pairs around central atom
Hybridization
refers to the mixing of the native atomic orbitals to form special orbitals for bonding mixing of two or more nonequivalent orbitals
Electron pairs in atoms' outer-shells
repel each other
Destructive Wave Interaction
results in higher energy orbital anti bonding molecular orbital
Constructive Wave Interaction
results in lower energy orbital bonding molecular orbital
Homonuclear
same atom
Molecular Orbitals (MO)
same characteristics as atomic orbitals can hold 2 electrons with opposite spins square or the molecular orbital wave function indicates electron probability
Cis / Trans Isomers
same molecular formula different structural formula own distinct properties
Molecular Orbital (MO) Diagram
shows the relative energy and number of electrons in each molecular orbital number of orbitals are conserved number of molecular orbitals will always be equal to the number of atomic orbitals used to construct them can be used to predict stability of species
Pi Bonds (π)
side-to-side overlap all electron density lies above and below the plane of the nuclei of the bonding atoms the interaction between parallel orbitals is not as strong as between orbitals that point at each other do not permit rotation
2 Types of Covalent Bonds
sigma bonds (σ) pi bonds (π)
Hybrid Orbitals
sp orbitals are higher in energy than the 1s orbital but lower than the 2p used only for bonding NOT in isolated atoms
What is the orbital hybridization of the central atom if it has 2 single bonds and 1 lone pair?
sp2
What is the hybridization of the central atom in XeF2
sp3d
Paramagnetism
substance is attracted into the inducting magnetic field associated with unpaired electrons
Diamagnetism
substance is repelled from the inducing magnetic field associated with paired electrons
Net Paramagnetism
substance that has both paired and unpaired electrons
4 Electron Groups
tetrahedral geometry bond angle: 109.5 degrees
Bonding in Homonuclear Diatomic Molecules
the smaller p-block elements in the second period have a sizable interaction between the s and p orbitals this flips the order of the σ and π molecular orbitals in these elements (B2, C2, N2)
Combining the Localized Electron and Molecular Orbital Models
the σ bonds in a molecule can be described as being localized the π bonds must be treated as being delocalized for molecules that require resonance: localized electron model can be used to describe the σ bonding molecular orbital model can be used to describe the π bonding
sp2 Hybrid Orbitals
three degenerate sp2 orbitals trigonal planar arrangement of atomic orbitals with bond angles of 120 degrees one p orbital is not used oriented perpendictular to the plane of the sp2 orbitals Three orbitals work by angles of 120
5 Electron Groups
trigonal bipyramidal geometry 2 tetrahedral base to base 2 bond angles: 120 degrees/90 degrees equatorial positions are 120 degrees from each other 90 degrees between axial and equatorial electron groups
3 Electron Groups
trigonal planar geometry bond angle: 120 degrees
PCl3 is an industrial chemical used in many herbicides and insecticides. What is the molecular geometry/shape of PCl3?
trigonal pyramidal
T/F: Hybridization of one s orbital and two p orbitals results in 3 hybrid orbitals
true
T/F: Just because a molecule possess polar bonds does not mean the molecule as a whole will be polar.
true
T/F: The VSEPR theory predicts that the angle between the central atom C and two O atoms in CO2 measures 180 degrees.
true
T/F: When you have 4 electron domains/effective groups, the electron geometry is tetrahedral.
true
Heteronuclear Diatomic Molecules and Ions
two different atoms may also involve, in special cases, molecules that contain atoms adjacent to each other in the periodic table combination of "identical" atomic orbitals realists in equal contribution from both atomic orbital to the molecular orbital for atomic orbitals with different energies and types, the atomic orbital closest to the the molecular orbital contributes most more electronegative atoms have lower energy orbitals lower energy atomic orbitals contribute more to bonding molecular orbitals higher energy atomic orbitals contribute more to anitbonding molecular orbitals nonbonding molecular orbitals remain on donating atom
Trigonal Bipyramidal Electron Domain
two distinct positions: axial equatorial lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial positions in this geometry
Trans Isomer
two heavy groups on opposite sides of the ring
Cis Isomer
two heavy groups on the same side of the ring
Geometries involving expanded octets on the central atom
we must use d orbitals in our hybrids
Energy-Level Diagrams for Diatomic Molecules
when the two atoms of a diatomic molecule are very different, the energy-level diagram for homonuclear molecules cannot be used
Single Bond
σ bonds permit rotation
