Vocabulary: VSEPR Valence Shell Electron Pair Repulsion Theory domain = any electron pair, or any double or triple bond is considered one domain. lone pair = non-bonding pair = unshared pair = any electron pair that is not involved in bonding bonding pair = shared pair = any electron pair that is involved in bonding 2 domains on central atom 3 domains on central atom LINEAR 2 domains both are bonding They push each other to opposite sides of center (180 apart). BeCl 2 TRIGONAL PLANAR 3 domains all are bonding They push each other apart equally at 120 degrees. GaF 3 3 domains on central atom NOTE: BENT 3 domains: 2 are bonding 1 is a lone pair The 2 bonding are pushed apart by 3 rd pair (not seen) SnF 2 The geometry around the central atom is trigonal planar. The molecular shape is bent. SnF 2 1
4 domains on central atom 4 e - on central atom TETRAHEDRAL Each repels the other equally - 109.5 - not the expected 90. Think in 3D. CH 4 TRIGONAL PYRAMIDAL 3 bonding 1 lone pair The thicker, lone pair forces the others a little bit closer together (~107.3 ) NH 3 Tetrahedral vs. Trigonal pyramidal Tetrahedral vs. Trigonal pyramidal Tetrahedral geometry around the central atom Tetrahedral Molecular Shape Tetrahedral geometry around the central atom Trigonal Pyramidal Molecular Shape On the right, the 4 th lone pair, is not seen as part of the actual molecule, yet affects shape. If another one of the bonding on trigonal pyramidal were a lone pair, what is the result? 4 domains on central atom, con t Comparing the 2 bents BENT 2 bonding 2 lone The bonds are forced together still closer (104.5 ) ) by the 2 thick unshared. H 2 O Both bent molecules are affected by unshared 1 pair on the left, 2 on the right. 2
Other Molecular Geometry Just for fun, let s s look at some others that we will not study in detail in this course Note that if there are more than five domains around the central atom, it must be an exception to the octet rule! TRIGONAL BIPYRAMIDAL 5 shared Three are found in one plane ( equator( equator ) 120 apart; ; the other two are at the poles, poles, 180 apart, 90 from the equator. equator. PCl 5 SEE-SAW SAW 4 shared & 1 unshared pair One of the equator is unshared & pushes the other 2 together. The 2 poles are pushed slightly together. SF 4 T-SHAPED 3 shared & 2 unshared 2 of the 3 equator are unshared. All 3 remaining are pushed together. ClF 3 LINEAR 2 shared & 3 unshared 5 shared, 0 unshared 4 shared, 1 unshared All 3 equator are unshared. The 2 remaining are forced to the poles. XeF 2 3 shared, 2 unshared 2 shared, 3 unshared 3
OCTAHEDRAL 6 shared Each pair repels the others equally. All angles = 90 Now, if one of these was unshared SF 6 SQUARE PYRAMIDAL 5 shared & 1 unshared pair 4 shared in one plane; the 5 th pair at the pyramid s s top. If the pair at the top was unshared IF 5 SQUARE PLANAR 4 shared & 2 unshared 6 shared, 0 unshared 5 shared, 1 unshared The 4 shared are in the same plane; the 2 unshared are 90 from them. XeF 4 4 shared, 2 unshared Steps for using VSEPR: 1. Draw a Lewis Dot Structure. 2. Predict the geometry around the central atom. 3. Predict the molecular shape. also, we can try and predict the angles between atoms. All e - push each other as far apart as possible. Shared (bonding) are stretched between two atoms that want them. Longer & Thinner Unshared (non-bonding) are not stretched. stretched. Shorter & Thicker 4
Electron Pair Repulsion 2 lone require the most space & repel each other the most, resulting in the greatest distance (angle). 1 lone pair (thick) & 1 bonding pair (thin) require less space 2 bonding (both thin) require the least space & repel each other the smallest distance (angle). 5