Ch 9: Polarity of Molecules Polarity of polyatomic molecules polarity of a polyatomic molecule depends on: individual bond polarity molecular geometry e.g., CO2, H2O, NH3: are these molecules polar? for each polar bond in a molecule, we must look at the individual bond dipole then, for all of the bond dipoles, we must ask what overall effect results from adding up the individual bond dipoles bond dipoles are vector quantities - they possess both direction and magnitude 194 The overall dipole moment of a polyatomic molecule is the sum of its bond dipoles e.g., CO2: e.g., H2O: e.g., NH3 In general: for ABn type molecules in which all B atoms are the same, the following symmetrical geometries lead to nonpolar molecules regardless of how polar the individual bonds are: linear (AB2) trigonal planar (AB3) tetrahedral and square planar (AB4) trigonal bipyramid (AB5) octahedral (AB6) 195 E.g., are the following molecules polar? CCl4 CS2 SO3 SF4 PF5 196 Now: we’ve learned to predict the geometries and polarities of molecules – these will have a major influence on physical properties of substances…… We now want to examine more closely the nature of the chemical bond and relate what we know about electronic structure of atoms to the electronic structure and geometries of molecules Two models: Valence Bond (VB) Model Molecular Orbital (MO) Model Valence bond (VB) model Combines the notion of electron-pair bonding (Lewis valence) with the idea of atomic orbitals Review atomic orbitals from Ch 6! 197 Buildup of e- density between nuclei occurs when valence AOs overlap, or share a region of space orbital overlap allows two unpaired e- of opposite spin to share space between nuclei, forming a covalent bond e.g., 1s + 1s case for H2 The equilibrium H-H bond length is a compromise between: AO overlap (attraction) nuclear-nuclear repulsion this can be shown with a potential energy curve 198 Can s & p -type atomic orbitals overlap? can atomic p orbitals overlap with other p-orbitals? e.g., HF, N2, O2, F2 VB model successful in explaining bonding in diatomics in terms of atomic orbitals……. How to reconcile observed and VSEPR geometries of polyatomic molecules using VB model? 199 Hybrid orbitals VB model helps explain why covalent bonds form Overlap of AOs on atoms permits sharing of electrons How to reconcile observed molecular geometries with formation of bonds from atomic orbitals? e.g., bonding in BeF2: How does Be bond to 2 F atoms? Be: 1s22s2 F: 1s22s22p5 Each F can accommodate an additional electron in its 2p subshell, but how can Be bind 2 fluorine atoms when it has no unpaired electrons? According to the VB model a bond results from sharing of unpaired electrons via overlap of AOs… What if we promote a 2s electron of Be to the 2p level.... 200 Now, according to VB theory, Be can bind 2 F atoms.... Would the 2 Be-F bonds be identical? Experimentally, both Be - F bonds in BeF2 are equivalent (same length) Can we propose another model? allow the occupied 2s and one 2p orbital of Be to mix and form two hybrid orbitals: 201 What are the characteristics of these sp "hybrid orbitals" ? like p-orbitals: have two ‘lobes’ unlike p-orbitals, one ‘lobe’ is larger than the other large lobes of the sp hybrid orbitals point in opposite directions Why should orbital hybridization occur? Isn't energy required to promote a 2s e- of Be into a 2p orbital? The increased overlap between the sp hybrid orbitals of Be and the 2p orbitals of F makes up for energy required to promote the 2s e- The formation of BeF2 from overlap of 2sp and 2p orbitals is more exothermic than promotion of a 2s e- is endothermic 202 Similarly, how does C [1s22s22p2] combine with 4 hydrogens to form CH4 ? how to get 4 unpaired e- on C to share with 4 H atoms? promote a 2s e- to the 2p subshell; hybridize the 2s and all 3 2p orbitals: gives 4 sp3 hybrid orbitals sp3 hybridization of C in CH4 results in 4 equivalent hybrid atomic orbitals which point to the vertices of a tetrahedron So the bonding in CH4 is described (in the VB theory) as the overlap between 4 equivalent sp3 hybrid AOs of C and 1s AOs of H 203 What about the bonding in BF3? sp2 hybridization of B: Mixing a 2s AO with 2-2p AOs produces 3 equivalent sp2 hybrid orbitals each sp2 orbital has a large lobe and a small lobe the large lobe of each sp2 hybrid AO points to the corner of an equilateral triangle 204 Based upon our knowledge of geometries (the VSEPR model), can we predict which set of hybrid orbitals an atom uses in bond formation? note the following: sp hybridization 2 equivalent sp hybrid orbitals; point in opposite directions (i.e., linear) sp2 hybridization 3 equivalent sp2 hybrid orbitals; point to corners of an equilateral triangle (i.e., trigonal planar) sp3 hybridization 4 equivalent sp3 AOs; point to the vertices of a tetrahedron To determine which hybrid orbital set the center atom uses, count the total number of e- pairs on the atom; then…. Total # e- pairs hybrid orbitals 2 sp 3 sp2 4 sp3 5 ____ 205 Problems du Jour What set of hybrid orbitals is used by the central atom in each of the following? CS2 XeF4 PF6 H3O+ NH3 BH4206