Chemistry: The Central Science Fourteenth Edition Chapter 9 Molecular Geometry and Bonding Theories Copyright © 2018, 2015, 2012 Pearson Education, Inc. All Rights Reserved Molecular Shapes • Lewis structures show bonding and lone pairs but do not denote shape. • Here we see some common shapes for molecules with two or three atoms connected to a central atom. Valence-Shell Electron-Pair Repulsion (VSEPR) Model - need Lewis Dot structure to get shape of molecule Best shape: Electrons stay as far apart as possible. (like tied balloons) (The balloon analogy in the figure to the left demonstrates the maximum distances that minimize repulsions.) Atoms are visible. Lone pair electrons are invisible but occupy an electron domain. Electron-Domain Geometries Table 9.1 Electron-Domain Geometries as a Function of Number of Electron Domains - Number of Arrangement of Electron Domain Predicted Electron Domains* Electron Domains Geometry Bond Angles 2 Two domains opposite each other, forming a straight line. Linear 180 degrees 3 Three domains forming the points of an equilateral triangle. Trigonal planar 120 degrees 4 Four domains forming the points of a pyramid. Tetrahedral 109.5 degrees Electron-Domain Geometries Table 9.1 [Continued] Number of Arrangement of Electron Domain Predicted Electron Domains* Electron Domains Geometry Bond Angles Trigonal 120 5 Three domains forming the points of an equilateral triangle. Two additional domains are found perpendicular, above and below, the triangle. 120 degrees, 90 degrees bipyramidal 90 6 Four domains forming the points of a square. Two additional domains are found perpendicular, above and below, the square Octahedral 90 degrees *The number of electron domains is sometimes called the coordination number of the atom. Molecular Geometries 1) Draw the best Lewis structure. 2) Determine the electron-domain geometry. 3) Use the arrangement of the bonded atoms to determine the molecular geometry. (only atoms are visible – electrons are invisible) – Tables 9.2 and 9.3 show the potential molecular geometries. Tetrahedral trigonal pyramid Linear Electron Domain • In the linear 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. Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron Electron-Domain Bonding Nonbonding Molecular Domains Geometry Domains Domains Geometry Example 2 Two domains opposite each other, forming a straight line, linear. 2 0 Linear. C O 2. A central C double bonded left and right to O; each O has two pairs of dots. Trigonal Planar Electron Domain (1 of 3) • There are two molecular geometries: – Trigonal planar, if all electron domains are bonding electrons – Bent, if one of the domains is a lone pair Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron Electron-Domain Bonding Nonbonding Molecular Domains Geometry Domains Domains Geometry Example 3 Three domains forming the points of an equilateral triangle, trigonal planar. 3 0 Trigonal planar. B F 3. B is single left and right, angled down, and above to F; each F has three pairs of dots. Blank Blank 2 1 bent. N O 2. N is double bonded left, angled down, to O, which has two pairs of dots, and single bonded right, angled down, to O, which has three pairs. The entire molecule has a negative charge Trigonal Planar Electron Domain (3 of 3) Table 9.2 Electron-Domain and Molecular Geometries for Two, Three, and Four Electron Domains around a Central Atom Number of Electron- Electron Domain Bonding Nonbonding Molecular Domains Geometry Domains Domains Geometry Example 4 Four domains forming the points of a pyramid, tetrahedral. 4 0 Tetrahedral. C H 4. C is single bonded right, into the page, and out of the page, all angled down, to H, as well as single bonded above to H. Blank Blank 3 1 Trigonal pyramidal. N H 3. N is single bonded right, into the page, and out of the page, all angled down, to H; N has a pair of dots above. Blank Blank 2 2 Bent H 2 O. O is single bonded left and right, angled down, to H; O has two pairs of dots. Trigonal Bipyramidal Electron Domain (3 of 3) Table 9.3 Electron-Domain and Molecular Geometries for Five and Six Electron Domains around a Central Atom Number of Electron-Domain Bonding Nonbonding Molecular Example Electron Domains Geometry Domains Domains Geometry Three domains forming the points of an equilateral triangle, and two more domains above and below, perpendicular to the plane of the triangle. Trigonal bi-pyramidal. P C l sub 5 5 5 0 PCl5 Blank Blank Seesaw. S F sub 4 4 1 SF4 Blank Blank T shaped. C l F sub 3 3 2 CIF3 Blank Blank Linear. X e F sub 2 2 3 XeF2 Octahedral Electron Domain (2 of 2) Table 9.3 Electron-Domain and Molecular Geometries for Five and Six Electron Domains around a Central Atom Number of Electron-Domain Bonding Nonbonding Molecular Electron Domains Geometry Domains Domains Geometry Example Four domains forming the points of a square, and two more domains above and below, perpendicular to the plane of the square. Octahedral. SFS F sub 6 6 6 0 6 Blank Blank 5 1 Square pyramidal. B r F sub 5 BrF5 Blank Blank 4 2 Square planar. X e F sub 4 XeF4 Nonbonding Pairs and Bond Angle • Nonbonding pairs (lone pairs) are physically larger than bonding pairs. • Lone pairs take up more space. Multiple Bonds and Bond Angles • Double and triple bonds have larger electron domains than single bonds. • Double and triple bonds take up more space. Shapes of Larger Molecules For larger molecules, look at the geometry about each atom rather than the molecule as a whole. To do VSEPRT (using chart next slide) 1. Draw Lewis Dot Structure. 2. Electron Domains = # bonds + # lone pairs # of bonds to central atom (single, double and triple bonds count as one bond for VSEPRT) ( # of lone pairs on the central atom - not # of electrons in lone pairs but # of pairs) 3. Use geometry e pairs & geometry of molecule for VSEPRT (# lone pairs/nonbonding e – these e are invisible for molecular shape) 4. Use hybridization for Valence Bond VSEPR & valence bond hybrization chart (e pair/domains ) To do VSEPRT examples on document camera 1. Draw Lewis Dot Structure. 2. Electron Domains = # bonds + # lone pairs # of bonds to central atom (single, double and triple bonds count as one bond for VSEPRT) ( # of lone pairs on the central atom - not # of electrons in lone pairs but # of pairs) 3. Use geometry e pairs & geometry of molecule for VSEPRT (# lone pairs/nonbonding e – these e are invisible for molecular shape) 4. Use hybridization for Valence Bond NH3 CCl3F SF6 H2CO HW 9-1: Do VSEPRT (on central atom) 1. Draw Lewis Dot Structure. 2. # e domains = # bonds (single, double, triple bonds all count as one bond for charge cloud counting) + # lone pairs (not # e in lone pairs) & use chart . End class A. : Cl. .— N — .Cl . : 4/8/21 Thursday : .Cl . : O B. H—C—Cl C. PF5 Comparison of the Polarity of Two Molecules • A nonpolar molecule • A polar molecule Polarity of Molecules: examples on document camera (a) Is bond polar ? Is there a difference in electronegativity ? (larger difference- larger polarity) (b) What is the VSEPRT shape? (c) What is vector sum? (“little horses”) vector sum = zero, nonpolar molecule vector sum = polar molecule HW 8-2: Is the molecule polar or nonpolar? . A. : ..F — N — .F . : : F : . O B. H—C—H C. PF5 Valence-Bond Theory • In valence-bond theory, electrons of two atoms begin to occupy the same space. • This is called “overlap” of orbitals. • The sharing of space between two electrons of opposite spin results in a covalent bond. • Have hybrid orbitals for covalent bonds. Hybrid Orbitals • Hybrid orbitals form by “mixing” of atomic orbitals to create new orbitals of equal energy, called degenerate orbitals. • This process is called hybridization. • When two orbitals “mix” they create two orbitals; when three orbitals mix, they create three orbitals; and so on. Be—sp Hybridization • When we look at the orbital diagram for beryllium (Be), we see that there are only paired electrons in full sublevels. • Be makes electron-deficient compounds with two bonds for Be. Why? sp hybridization (mixing of one s orbital and one p orbital). BeF2 Be = can’t bond to 2 F because paired e in 2s subshells Be hybridized sp sp Orbitals • Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals. – The sp hybrid orbitals each have two lobes like a p orbital. – One of the lobes is larger and more rounded, as is the s orbital. Position of sp Orbitals • These two degenerate orbitals would align themselves 180 from each other. • This is consistent with the observed geometry of Be compounds (like BeF2 ) and VSEPR: linear. 2 Boron—Three Electron Domains Gives sps p super 2 Hybridization Using a similar model for boron leads to three degenerate sp2 orbitals. 3 Carbon: sps p super 3 Hybridization With carbon, we get four degenerate sp3 orbitals. Hybrid Orbital Summary (1 of 2) 1) Draw the Lewis structure. 2) Use VSEPR to determine the electron-domain geometry. 3) Specify the hybrid orbitals needed to accommodate these electron pairs. VSEPR & valence bond hybrization chart (e pair/domains ) HW 8-3: what is the hybridization of the central atom ? . A. : Cl. .— N — .Cl . : : Cl : . O B. H—C—H C. PF5 Types of Bonds • How does a double or triple bond form? • It can’t, if we use only hybridized orbitals.