Chemistry, The Central Science, 10th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Chapter 9Molecular Geometriesand Bonding Theories John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice-Hall, Inc.
November 23 ONLINE PRACTICE QUESTIONS UNIT 8 – BY NEXT SUNDAY!!! WATCH PODCASTS 8-3 is this lesson 8-1 and 8-2 review of chapter 8 • Molecular Shapes • VSEPR theory • Electron Domain Geometry • Molecular Geometry • Hw 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.
Electron Domains • We can refer to the electron pairs as electrondomains. • 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 Geometries • All 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 ValenceShellElectronPair 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.
VSEPR Model • 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.
Examples – Draw the Lewis structures, and then determine the orbital geometry of each. Indicate the number of electron domains first: • H2S • CO2 • PCl3 • CH4 • SO2
Examples – Draw the Lewis structures, and then determine the orbital geometry of each: • e- domain orbital geometry • or e- domain geom • H2S 4 tetrahedral • CO2 2 linear • PCl3 4 tetrahedral • CH4 4 tetrahedral • SO2 3 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 Geometries Within each electron domain, then, there might be more than one molecular geometry.
Linear Electron Domain • In 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 Angle • Nonbonding 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 Angles • Double 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 Domain • There are two molecular geometries: • Trigonal planar, if all the electron domains are bonding • Bent, if one of the domains is a nonbonding pair.
Tetrahedral Electron Domain • There are three molecular geometries: • Tetrahedral, if all are bonding pairs • Trigonal pyramidal if one is a nonbonding pair • Bent if there are two nonbonding pairs
Examples – Same examples as before, now determine the the molecular geometry of each, including shapes and bond angles: • H2S • CO2 • PCl3 • CH4 • SO2
Examples – Determine the molecular geometry of each, including shapes and bond angles: • Shape Angles • H2S bent <109° • CO2linear 180° • PCl3trigonal pyramid <109° • CH4tetrahedral 109.5° • SO2bent <120°
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: • Axial • Equatorial
Molecules with Expanded Valence Shells • To 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 Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.
Octahedral electron domain • 6 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. • PF5 • XeF4 • SF6 • SCl4
Examples – Determine the Shape of each: • PF5trigonal bipyramid 90°, 120° • XeF4square planar 90° • SF6octahedral 90° • SCl4see-saw <90°,< 120° See moving chart
November 28 • Large molecules • Molecular shape and Polarity • Hybridization • Multiple bonding • HW • 25, 26, 35, 47, 51, 55
Larger Molecules In 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 Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.
Shapes of Larger Molecules • In acetic acid, CH3COOH, there are three central atoms. • We assign the geometry about each central atom separately.
Trigonal planar linear • Examples – Determine the shape and angles about each atom: • 1 • 2 Bent trigonal pyramid Trigonal planar
Molecular Shape and Molecular Polarity • 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.
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.
Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.