Lec 2 - Hybridization VSEPR Theory Chem 221 Tuesday Molecular Geometry •  Hybridization– mixing of atomic orbitals(s, p, d)

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Unformatted text preview: Hybridization VSEPR Theory Chem 221 Tuesday, July 12, 2011 Molecular Geometry •  Hybridization– mixing of atomic orbitals (s, p, d...) to form hybrid (sp, sp2...) orbitals of different shapes and energies. The Three-Dimensional Symmetry of sp3 Hybrid Orbitals Hybridization of one s and three p orbitals gives four sp3 orbitals. sp3 orbitals have equivalent shapes and energies, but different orientations; each points to a vertex of an imaginary tetrahedron. •  Bond angles are 109.5°. •  •  2px orbital z y 2s orbital x+ 2py orbital 2pz orbital mixing of the 2s, 2px ,2py and 2pz orbitals yields four sp3 hybrid orbitals directed towards the corners of a tetrahedron Hybrid Orbitals and Molecular Shape CH4 sp3 hybridization The ground state electron configuration of C is 1s2 2s2 2p2 or 1s valence level 2s 2px 2py 2pz The Shape of CH4 H 1s H In-phase combination of atomic orbitals yields bonding molecular orbitals. sp3 sp3 C sp3 H 1s sp3 1s H 1s H H o 109.5 H H H C H four sigma bonds H C H a tetrahedral geometry sp2 hybrid orbitals project in a plane •  Hybridization of one s and two p orbitals give three sp2 orbitals. z z y 2px orbital x + xy plane 2s orbital 2py orbital mixing of the 2s, 2px and 2py orbitals sp2 hybridized carbon yields three hybrid sp2 orbitals Spatial arrangement of sp2 sp2 orbitals have equivalent shapes and energies, but different orientations in space. Each points to a different vertex of an imaginary triangle. •  sp2 hybridization leaves one p orbital unchanged. •  sp2 orbitals are trigonal planar; the leftover p orbital is perpendicular to the sp2’s. •  pz 2px 2py 2pz energy 2s sp2 2pz hybridization sp 2 sp 2 sp 2 valence-level orbitals 1s ground state of C 1s sp2 C sp2 bonding orbitals Three equivalent sp 2 orbitals are in the X-Y plane. The idealized inter-orbital angle is 120o . sp Hybrid Orbitals have Directionality •  Hybridization of one s and one p orbitals gives two sp orbitals. z y x 2s orbital + 2px orbital mixing of the 2s and 2px yields two 2spx orbitals Hybrid Orbitals and Molecular Shape BeCl2 sp hybridization The ground state electron configuration of Be is 1s2 2s2 or 2s 1s valence level Cl Be Cl two sigma bonds 2py 2pz Cl Cl Be a linear structure 2spx 2spx •  1s Two p orbitals remain unchanged. Hybridization •  sp orbital •  50% s character, 50% p character •  sp2 orbital •  33% s character, 66% p character •  sp3 orbital •  25% s character, 75% p character Summary of Important concepts 1)  2)  3)  4)  5)  6)  An atomic orbital (AO) corresponds to a region of space about the nucleus of a single atom where there is a high probability of Sinding an electron. s orbitals are spherical, p orbitals are like two almost ­ tangent spheres. Orbitals can hold a maximum of two electrons when their spins are paired When atomic orbitals overlap, they combine to form molecular orbitals (MOs) The number of molecular orbitals always equals the number of atomic orbitals from which they are formed Hybrid atomic orbitals are obtained by mixing (hybridizing) the wave functions for orbitals of different types (i.e., s and p orbitals) but from the same atom A sigma (σ) bond (a type of single bond) is one in which the electron density has circular symmetry when viewed along the bond axis A pi (π) bond, part of double and triple carbon–carbon bonds, is one in which the electron densities of two adjacent parallel p orbitals overlap sideways to form a bonding pi molecular orbital Valence Shell Electron Pair Repulsion Theory VSEPR theory provides a simple way to predict molecular geometries around a central atom. These predictions are consistent with hybrid molecular orbital theory. Key Features of VSEPR Theory (1) Molecules or ions may be analyzed where a central atom is covalently bonded to two or more atoms or groups. (2) All pairs of valence electrons around the central atom are counted: bonding and nonbonding. (3) Because of electron-electron repulsion, pairs of electrons tend to stay as far apart as possible. Repulsion due to nonbonding pairs of electrons is greater than repulsion due to bonding pairs. (4) The preferred geometry has all pairs of valence electrons as far apart as possible to minimize repulsion among electron pairs. Examples Methane : : : : H HCH H There are four pairs of bonding electrons in the valence level around the central carbon. Maximum separation of the four pairs is achieved with a tetrahedral geometry where each electron pair points to the corner of a tetrahedron. H H C 109.5o H H an idealized tetrahedral geometry The Importance of Nonbonding Electron Pairs : Ammonia H NH H The geometry of ammonia is often described as a trigonal pyramid. This shape is predicted by VSEPR theory. There are four pairs of electrons in the valence level of the central atom: three bonding pairs and one nonbonding pair. VSEPR theory predicts a tetrahedral geometry around nitrogen. : HN H 107o H The actual geometry is a distorted tetrahedral with a slight compression of each HN H H-N-H angle in order to H provide more space for the trigonal pyramid nonbonding electron pair. Water: Two Nonbonding Electron Pairs The water molecule is bent as predicted by VSEPR theory. H2O The are four pairs of electrons in the valence level around the central oxygen atom: two bonding and two nonbonding. : HO : H H O 105o bent H VSEPR theory predicts the four pairs will be projected to the corners of a tetrahedron for maximum separation. The actual geometry is a distorted tetrahedron with the H-O-H bond angle compressed to 105o to provide more space for the nonbonding electron pairs. Boron Trifluoride: Three Bonding Pairs BF3 F BF F Boron trifluoride has a trigonalplanar geometry as predicted by VSEPR theory. There are three pairs of bonding electrons in the valence level around the central boron. VSEPR theory predicts the three pairs of electrons will be projected to the corners of an equilateral triangle for maximum separation of the electron pairs. F B F F 120o trigonal planar Berylium Chloride: Two Bonding Pairs BeCl2 Cl Be Cl Berylium chloride has a linear geometry as predicted by VSEPR theory. There are two pairs of bonding electrons in the valence level around the berylium atom. VSEPR theory predicts the two pairs will be projected in opposite directions along an axis for maximum separation of the electron pairs. 180o Cl Be Cl linear Multiple Bonds: Sets of Electrons VSEPR theory correctly predicts the geometry around central atoms that have four and six electron bonds. The multiple bond is treated as a single set of electrons-a single unit. The preferred geometry provides maximum separation of all sets and pairs of electrons. Carbon Dioxide CO2 Two Sets Carbon dioxide has 16 valence electrons. To satisfy the octet rule , two carbon-oxygen double (four electron) bonds are required. 180o C linear :: :: O O VSEPR theory treats each four electron bond as a set. Maximum separation of the two sets around the central carbon is achieved in a linear geometry. Ethyne (Acetylene) C2H2 One Set and a Pair Ethyne has 10 electrons in the valence level. To satisfy the octet rule, a six electron (triple) bond is required between the carbon atoms. 180o 180o HC CH VSEPR theory predicts a linear geometry for maximum separation of the electron set and single pair of electrons around each carbon. Quiz 1.019 Use VSEPR theory to predict the geometry around the carbon atom in each of the following chemical species. :: H2C=O formaldehyde H3C+ methyl cation - H3C: methyl anion ...
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This note was uploaded on 08/06/2011 for the course CHEM 221x taught by Professor Broyer during the Summer '11 term at USC.

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