7.10 – NOTES Shapes for Covalent Structures

IV. Molecular shape A. VSEPR Model – Valence Shell Electron Pair Repulsion e- pairs get as far apart as possible because of similar charges  

B. To determine shapes of molecules: 1. Draw the Lewis diagram for the compound. 2. Count the number of electron clouds around the central atom. Multiple bonds and single bonds are treated alike. Nonbonded e- pairs (lone pairs) are counted, too. 3. Shapes are determined by the number of e- clouds:

2 clouds: CO2 2 shared pairs (bonds), 0 unshared Linear; 180° largest distance possible;

3 clouds: BH3, SO3 3 shared pairs, 0 unshared 120° angle b/t electron clouds; called equilateral triangle or trigonal planar;

4 clouds: CH4, SiF4 4 shared pairs, 0 unshared 109.5° angle, tetrahedron, angles do NOT have to = 360 unless flat circle

*5 clouds: PF5 5 shared, 0 unshared 90° angles and 120° angles; trigonal bipyramid

*6 clouds: SF6 6 shared, 0 unshared All angles are 90°; octahedron

4. “Shape” or geometry refers to the positions of the atoms only 4. “Shape” or geometry refers to the positions of the atoms only. If one or more electron clouds are non bonded, you must look at the atom positions alone. Examples: H2O, NH3, SO2 electron clouds determine angle; atoms determine shape; once know the shape, draw it as such; Unshared pairs repel bonding pairs! **Angular = bent

C. Hybridization In order to make bond angles larger (less repulsion), atoms blend their orbitals into hybrids.

Determining the hybridization: 1. The orbitals available for bonding are s, p, and d. Listing them one orbital at a time: s, p, p, p, d, d, d, d, d. 2. For a particular compound, draw the Lewis structure, then count the number of e- cloud around an atom, including lone pairs. Count double bonds as 1 cloud. The atom needs 1 orbital for each cloud. 3. For 2 clouds, 2 orbitals are needed, so the hybrids are sp. For 3 clouds, 3 orbitals are needed, so the hybrids are sp2. For 4 clouds, the hybridization is sp3; for 5 sp3d, and for 6 sp3d2.  

CO2 CH4 NH3