Lewis diagrams and Molecular Shape

1. Lewis diagrams and Molecular Shape There is a direct link between Lewis diagrams and molecular shape The Valence Shell Electron Pair Repulsion theory states a molecule’s shape is determined by the electron pairs that surround central atoms These electron pairs(EP) include both bond pairs (BP) and lone pairs (LP) Four electron pairs define the tetrahedral shape family, as in SiCl4 We define five shape families based on the need to accommodate mutually repelling electron pairs around a central atom The balloons illustrate these “natural” shapes

2. The 5 Shape Families The five shape families that electron pairs develop are illustrated above Linear – Trigonal-planar – Tetrahedral – Trigonal-bipyramidal – Octahedral In this case each molecule has all its electron pairs as bond pairs around the central atom Not all the shape-determining electrons pairs need be bond pairs

3. X .. .. X X X .. .. X General Shape Family Scheme (1) 0 LP 2 BP 0 LP 3 BP 1 LP 2 BP 2 LP 1 BP 2 EP’s 3 EP’s 1 LP 1 BP

4. Lone pairs and shape: 4 Electron Pairs When there are four electron pairs, the shape family is tetrahedral Four possibilities exist: with 0, 1, 2 or 3 lone pairs 0 LP – tetrahedral shape 2 LP – bent shape 1 LP – trigonal-pyramidal shape 3 LP – linear shape Linear Hydrogen Fluoride, HF 1 bond pair 3 lone pairs

5. .. : : : X X X X .. .. General Shape Family Scheme 4 BP 0 LP 3 BP 1 LP 2 BP 2 LP 1 BP 3 LP

6. .. .. .. .. .. .. .. X X X X X .. .. .. General Shape Family Scheme 5 BP 0 LP 4 BP 1 LP 3 BP 2 LP 2 BP 3 LP 1 BP 4 LP

7. Specialfeatures of 5 EP shapes In the trigonal bipyramidal geometry, alone among the 5 shape families, the sites are not identical There are three equivalent sites which are identical to those in trigonal-planar, with bond angles of 120 . These are called the equatorial sites Axial Sites - above and below the plane in a mutually linear relationship. Note - the angles are different Note - if there are LP’s, these will always first occupy equatorial sites

8. .. .. .. .. .. .. .. .. .. .. X X X X X X .. .. .. .. .. General Shape Family Scheme 6 BP 0 LP 5 BP 1 LP 4 BP 2 LP 3 BP 3 LP 2 BP 4 LP 1 BP 5 LP

9. Some special features of VSEPR multiple bonds - occupy almost the same volume of space - considered as single BP of electrons Ex) NO2 N has 4 EP’s 1 LP & 3 BP’s One double bond Bent shape LP are considered to be larger than BP because constrained by only a single positively charged nucleus as opposed to two for a BP For 6 EP case LP prefer to be opposite since otherwise at 90o they would experience strong repulsive forces For 5 EP since neighboring LP’s at120 experience much less repulsion than at 90o they tend to cluster on the equatorial sites of the trigonal-bipyramidal geometry

10. O -1 N O O -1 O O O O O ]- -1 N O S S O • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • O O O -1 O -1 Exercise Predict the shape of NO3- Triangular planar 4 EP’s 4 BP’s 3 Bonds SO42- Tetrahedral 6 EP’s 6 BP’s • • 4 Bonds

11. -1 -1 • F • P • • • P F F O F • • • • • • • • • • • • • • • O POF2- Trigonal pyramid 5 EP’s 4 BP’s 1 LP

12. Geometry of large molecules VSEPR method extends to larger molecules by treating them as a chains central atoms The geometry at the S, O and N are similar to those in water and ammonia, i.e. bent and trigonal pyramidal The geometry at the C1 and C2 atoms are tetrahedral, while at C3 it is trigonal planar The complete molecule simply moves parts around by twisting around any single bond to relieve congestion.

13. Shape and polarity Recall that most covalent bonds are polar Therefore molecules must be polar Not always!!! Symmetry in a molecule can cause bond dipole vectors to cancel each other making the molecule non-polar even though its individual bonds are very polar

14. Permanent Dipole Moments Molecules that have permanent dipole moment are called polar The unit of dipole moment is the Debye Note that for molecules like H2 and CCl4, there is no permanent dipole moment! Such molecules are non-polar.