Chapter 9Molecular Geometriesand Bonding TheoriesChemistry, The Central Science, 10th editionTheodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. BurstenJohn D. BookstaverSt. Charles Community CollegeSt. Peters, MO 2006, Prentice-Hall, Inc.
November 23 ONLINE PRACTICE QUESTIONS UNIT 8 BY NEXT SUNDAY!!!WATCH PODCASTS 8-3 is this lesson8-1 and 8-2 review of chapter 8Molecular Shapes VSEPR theory Electron Domain GeometryMolecular GeometryHw for section 9.1 and 9.2 : 1,2,3,11 to 23 odd
What Determines the Shape of a Molecule?The Lewis structure is drawn with the atoms all in the same plane. The overall shape of the molecule will be determined by its bond angles.
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.
There are five fundamental geometries for molecular shape
Electron DomainsWe can refer to the electron pairs as electron domains.Each pair of electrons count as an electron domain, whether they are in a lone pair, in a single, double or triple bond.This molecule has four electron domains.
Electron-Domain GeometriesAll one must do is count the number of electron domains in the Lewis structure.The geometry will be that which corresponds to that number of electron domains.
In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimizes this repulsion.We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.There are simple shapes for AB2 to AB6 molecules.
Electron domain geometry: When considering the geometry about the central atom, we consider all electrons (lone pairs and bonding pairs).When naming the molecular geometry, we focus only on the positions of the atoms.
To determine the shape of a molecule, we distinguish between lone pairs (or non-bonding pairs, those not in a bond) of electrons and bonding pairs (those found between two atoms).We define the electron domain geometry (or orbital geometry) by the positions in 3D space of ALL electron pairs (bonding or non-bonding).The electrons adopt an arrangement in space to minimize e--e- repulsion.VSEPR Model
Examples Draw the Lewis structures, and then determine the orbital geometry of each. Indicate the number of electron domains first:H2SCO2PCl3CH4SO2
Examples Draw the Lewis structures, and then determine the orbital geometry of each: e- domain orbital geometry or e- domain geomH2S4 tetrahedralCO22 linearPCl34 tetrahedralCH4 4 tetrahedralSO23 trigonal planar
To determine the electron pair ( electron domain) geometry:draw the Lewis structure,count the total number of electron pairs around the central atom,arrange the electron pairs in one of the above geometries to minimize e--e- repulsion, and count multiple bonds as one bonding pair.But then we have to account for the shape of the molecule
Molecular GeometriesWithin each electron domain, then, there might be more than one molecular geometry.
Linear Electron DomainIn this 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.
Nonbonding Pairs and Bond AngleNonbonding pairs are physically larger than bonding pairs.Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.
By experiment, the H-X-H bond angle decreases on moving from C to N to O:
Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs.Therefore, the bond angle decreases as the number of lone pairs increase.
Similarly, electrons in multiple bonds repel more than electrons in single bonds.
Multiple Bonds and Bond AnglesDouble and triple bonds place greater electron density on one side of the central atom than do single bonds.Therefore, they also affect bond angles.
Trigonal Planar Electron DomainThere are two molecular geometries:Trigonal planar, if all the electron domains are bondingBent, if one of the domains is a nonbonding pair.
Tetrahedral Electron DomainThere are three molecular geometries:Tetrahedral, if all are bonding pairsTrigonal pyramidal if one is a nonbonding pairBent if there are two nonbonding pairs
Examples Same examples as before, now determine the the molecular geometry of each, including shapes and bond angles:H2SCO2PCl3CH4SO2
- Examples Determine the molecular geometry of each, including shapes and bond angles: Shape AnglesH2Sbent
Molecules with Expanded Valence Shells
For elements of the 3rd shell and below, some atoms can have expanded octets. AB5 (trigonal bipyramidal) or AB6 (octahedral) electron pair geometries.
Trigonal Bipyramidal Electron Domain (5 e domains)There are two distinct positions in this geometry:AxialEquatorial
Molecules with Expanded Valence ShellsTo minimize e--e- repulsion, lone pairs are always placed in equatorial positions.
Trigonal Bipyramidal(e- domain)There are four distinct molecular geometries in this domain:
*Trigonal bipyramidal *Seesaw *T-shaped *Linear
Trigonal Bipyramidal Electron DomainLower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
Octahedral electron domain6 electron pairs
All positions are equivalent in the octahedral domain.There are three molecular geometries:*Octahedral*Square pyramidal*Square planar
Examples Determine the Shape of each, indicate the electron domain, molecular geometry and angles.PF5XeF4SF6SCl4
- Examples Determine the Shape of each:PF5trigonal bipyramid90, 120XeF4square planar 90SF6octahedral 90SCl4see-saw
November 28Large moleculesMolecular shape and PolarityHybridizationMultiple bondingHW25, 26, 35, 47, 51, 55
Larger MoleculesIn larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.
Larger MoleculesThis approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
Shapes of Larger MoleculesIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.
Examples Determine the shape and angles about each atom:
Examples Determine the shape and angles about each atom:
Trigonal planarlinearBenttrigonal pyramidTrigonal planar
When there is a difference in electronegativity between two atoms, then the bond between them is polar.It is possible for a molecule to contain polar bonds, but not be polar.For example, the bond dipoles in CO2 cancel each other because CO2 is linear.Molecular Shape and Molecular Polarity
In water, the molecule is not linear and the bond dipoles do not cancel each other.Therefore, water is a polar molecule.The overall polarity of a molecule depends on its molecular geometry.
PolarityBy adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.
To rememberTrigonal Planar molecular shape if the 3 surroundings atoms are the same the molecule is non polar molecule because the dipoles cancel each other.Tetrahedral: if 4 surrounding atoms are the same molecule non-polar. This is important for carbon compounds with 4 single bonds.
Examples Determine whether each is polar or nonpolar:CCl4PCl3BF3BrClFCHSO3
Examples Determine whether each is polar or nonpolar:CCl4nonpolarPCl3polarBF3nonpolarBrClFCHpolarSO3nonpolar
Overlap and BondingWe think of covalent bonds forming through the sharing of electrons by adjacent atoms.In such an approach this can only occur when orbitals on the two atoms overlap.
Overlap and BondingIncreased overlap brings the electrons and nuclei closer together while simultaneously decreasing electron-electron repulsion.However, if atoms get too close, the internuclear repulsion greatly raises the energy.
HYBRIDIZATIONVALENCE BOND THEORY (s and p bonds)BOND ORDER
Hybrid OrbitalsIts hard to imagine tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize.
Atomic orbitals can mix or hybridize in order to adopt an appropriate geometry for bonding.Hybridization is determined by the electron domain geometry.sp Hybrid OrbitalsConsider the BeF2 molecule (experimentally known to exist):Hybrid Orbitals
Hybrid OrbitalsConsider beryllium:In its ground electronic state, it would not be able to form bonds because it has no singly-occupied orbitals.
Hybrid OrbitalsBut if it absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital, it can form two bonds.
Hybrid OrbitalsMixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals.These sp hybrid orbitals have two lobes like a p orbital.One of the lobes is larger and more rounded as is the s orbital.
Hybrid Orbitals (sp)These two dege