4 electron domains: 3 bonding and 1 non-bonding. 2 electron domains: 2 bonding and 0 non-bonding. 3 electron domains: 2 bonding and 1 non-bonding

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1 [4.3D VSEPR] pg. 1 f 7 Curriculum The use f VSEPR thery t predict the electrn dmain gemetry and the mlecular gemetry fr species with tw, three and fur electrn dmains. Shapes f species are determined by the repulsin f electrn pairs accrding t VSEPR thery. Predictin f bnd angles frm mlecular gemetry and presence f nn- bnding pairs f electrns. Predictin f mlecular plarity frm bnd plarity and mlecular gemetry. A. Overview VSEPR: Valence Shell Electrn Pair Repulsin Thery Wrks fr mlecules where a central atm is surrunded by uter atms AX n, where A = central atm X = uter atm n = # f B atms Ex. CO 2, H 2O, BF 3, NH 3, CCl 4, CHCl 3 Accrding t VSEPR, in the valence shell f an atm, electrn pairs repel each ther, and want t get as far away frm each ther as pssible These electrn pairs can be bnding pairs r lne pairs B. Electrn Dmain Electrn lcatin in the valence shell Tw types: Bnding pair (can be single, duble r triple bnd, cunted as ne unit) Lne pair ie. electrn grups regins f high electrn density, s repel eachther Ex. Determine the number f electrn dmains: 4 electrn dmains: 3 bnding and 1 nn-bnding 2 electrn dmains: 2 bnding and 0 nn-bnding 3 electrn dmains: 2 bnding and 1 nn-bnding

2 [4.3D VSEPR] pg. 2 f 7 C. Gemetry Electrn dmain gemetry: arrangement f electrn dmains arund central atm Best arrangement has electrn dmains as far away frm each ther as pssible Minimizes repulsins Fund by cunting number f electrn dmains and cnsidering the arrangement that minimizes repulsins Mlecular gemetry: The arrangement in space f the atms in a mlecule r plyatmic in cnsequence f electrn dmain gemetry. Atms in the mlecules ccupy psitins arund the central atm that minimizes repulsin between all electrn dmains. Electrn dmain gemetry = mlecular gemetry if all electrn dmains are bnding dmains. D. Electrn pair repulsin Nn-bnding pairs (lne pairs): higher charge cncentratin than a bnding pair e- nt shared between tw atms cause slightly mre repulsin than bnding pairs. Repulsin decreases in the fllwing rder: lne pair lne pair > lne pair bnding pair > bnding pair bnding pair E. Species with tw electrn dmains Electrn dmain gemetry: Linear Electrn dmains psitined 180 t each ther Mlecular gemetry: Linear bnding dmains: 2 nn-bnding dmains: 0 CO 2 BeCl 2 C 2H 2

3 [4.3D VSEPR] pg. 3 f 7 F. Species with 3 electrn dmains Electrn dmain gemetry: triangular/trignal planar Electrn dmains 120 away frm eachther 1) Mlecular Gemetry: triangular/trignal planar (AX 3) bnding dmains: 3 nn-bnding dmains: 0 BF 3 HCHO 2) Mlecular Gemetry: bent (AX 2E) bnding dmains: 2 nn-bnding dmains: 1 ie. ne electrn dmain is a lne pair mlecular determined by psitin f bnding pairs lne pairs cause slightly mre repulsin than bnding pairs Ex. Ozne 3 electrn dmains Lne pair f electrns distrts, making angle 117 instead f 120 O 3 G. Species with 4 electrn dmains Electrn dmain gemetry: tetrahedral Electrn dmains away frm eachther 1) Mlecular Gemetry: tetrahedral (AX 4) bnding dmains: 4 nn-bnding dmains: 0 + Ex. CH 4 Ex. NH 4 2) Mlecular Gemetry: trignal pyramidal (AX 3E) bnding dmains: 3 nn-bnding dmains: 1

4 [4.3D VSEPR] pg. 4 f 7 bnd angles 107 Ex. NH 3 3) Mlecular Gemetry: Bent r V-d (AX 2E 2) bnding dmains: 2 nn-binding dmains: 2 bnd angles 105 Ex. H 2O Here is a summary f the steps used in determining the f a mlecule. 1) Draw the Lewis structure 2) Cunt the ttal number f electrn dmains n the central atm. 3) Determine the electrn dmain gemetry as fllws: a. 2 electrn dmains linear b. 3 electrn dmains triangular planar c. 4 electrn dmains tetrahedral 4) Always draw the Lewis structure befre 5) Determine the mlecular gemetry frm the number f bnding electrn dmains. 6) Cnsider the extra repulsin caused by the lne pairs and adjust the bnd angles accrdingly.

5 [4.3D VSEPR] pg. 5 f 7 H. VSEPR and plarity 1 Bnd plarity is nt as mlecular plarity Many mlecules are nnplar, but have plar bnds. A bnd is plar if the tw atms n either end are different. Exceptin #1 C-H behave as thugh nn-plar Exceptin #2 N-Cl, N & Cl have almst same electrnegativities, s N-Cl is essentially nn-plar Bnd plarity is based n charge separatin between tw bnded atms Mlecular plarity depends n: the plar bnds that it cntains; rientatin f the plar bnds Diples in a bnd 2 hypthetical mlecule AB. B is mre electrnegative and has the greater share f the electrns in the AB bnd. partial negative charge (-δ) n B and a partial psitive charge f equal magnitude (+δ) n A. Diple mment: μ (AB) = δ x d δ = charge magnitude d = bnd length μ measures degree f plarity μ = 1.03D measured in Debyes (D) value can give an idea f the plar character f a mlecule. vectr quantity, has directin and magnitude. Directin shwn by arrw pinting frm negative t psitive end Ex. HF = 1.91 D, HCl is 1.03 D (F is mre electrnegative than Cl). 3 Diples in a mlecule Diatmic Mlecules 2 atms jined by cvalent bnd Diple mment f bnd gives diple mment f mlecule Greater electrnegativity difference between atms = greater diple mment f mlecule Plyatmic Mlecules Mre than tw atms bnded by cvalent bnds. Idea f a diple can be applied t individual bnds within Bnd diple: diple mment f individual bnd in a plyatmic mlecule Diple mment f mlecule depends n rientatins f varius bnd diples. Diple mment f mlecule = vectr sum bnd diples If the vectr sum is zer, the mlecule is nn-plar Fr a mlecule t be plar, it must have plar bnds Cl 2 is nn-plar Hydrcarbns are nn-plar, ex. CH

6 [4.3D VSEPR] pg. 6 f 7 Plar Mlecules Bnds f different plarity Bnds are nt symmetrical, diples dn t cancel Fr a mlecule t be plar, it must have a psitive and negative end, like a magnet H 2O Side atms are Line test cannt SOCl 2 Side atms are Line test cannt Plar / Nn-Plar Plar / Nn-Plar Nn plar mlecules Bnds are f equal plarity Arranged symmetrically with respect t each ther, diples cancel What des it mean t be symmetric? There are tw cmpnents: 4 Mlecule must have symmetrical : linear, trignal planar, and tetrahedral AND All f the atms attached t the central atm must be When bth cnditins are present, electrns have unifrm distributin Lack f net diple weak intermlecular frces CO 2, C-O bnds identical in magnitude, ppsite in directin, vectr sum f zer. Side atms are Line test cannt Plar / Nn-Plar BF 3 resultant mment f any tw B- F diples is equal in magnitude but ppsite in directin t the mment f the third ne Side atms are Line test cannt Plar / Nn-Plar SO 3 Side atms are Line test cannt CH 4 Side atms are Line test cannt Plar / Nn-Plar Plar / Nn-Plar 4

7 [4.3D VSEPR] pg. 7 f 7 Steps fr finding mlecular diple mment: 1. Is the mlecule a hydrcarbn? a. Yes the mlecule is nn-plar b. Determine the mlecule s Lewis structure 2. Use the Lewis structure t determine the mlecule s a. Symmetric s: linear, trignal planar, and tetrahedral i. assciated with mlecules whse central atms have n lne pairs b. Unsymmetric s: bent and trignal pyramidal i. assciated with mlecules whse central atms have ne r mre lne pairs. ii. Will be plar even if all uter atms are, due t lne pairs giving uneven electrn distributin. 3. Is the symmetric? a. N: The mlecule is plar b. Yes: Examine the utside atms 4. If the is symmetric, are all the utside atms? a. N: The mlecule is plar b. Yes: the mlecule is nn-plar Mlecular Plarity Practice: a) draw the VSEPR structure fr each mlecule b) Determine if the mlecule is plar c) If pssible, draw the diples fr each bnd, and the resulting diple IBr CCl 4 PCl 3 H 2S C 2H 2 AsH 3 SO SO 3 NH 2Cl C 3H 8 NH 3 CH 3Cl BeF 2 HCN SiCl 4 I 2 N 2H 2 HNO SO 2 CH 2O