2 VSEPR In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible. have the least amount of repulsion of the negatively charged electrons. have a geometry around the central atom that determines molecular shape.
5 Three Bonding Atoms and One Lone Pair In a molecule of NH 3, 3 electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair. repulsion is minimized with 4 electron groups in a tetrahedral arrangement. with 3 bonded atoms, the shape is pyramidal.
6 Two Bonding Atoms and Two Lone Pairs In a molecule of H 2 O, 2 electron groups are bonded to H atoms and 2 are lone pairs (4 electron groups). 4 electron groups minimize repulsion in a tetrahedral arrangement. the shape with 2 bonded atoms is bent.
8 Learning Check State the number of electron groups, lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions. 1) tetrahedral 2) pyramidal3) bent A. PF 3 B. H 2 S C. CCl 4
9 Study Tip To determine shape, 1. draw the electron-dot structure. 2. count the electron pairs around the central atom. 3. count the bonded atoms to determine shape. 4 electron pairs and 4 bonded atoms = tetrahedral 4 electrons pairs and 3 bonded atoms = pyramidal 4 electron pairs and 2 bonded atoms = bent
10 Solution A. PF 3 4 electron groups, 1 lone pair, (2) pyramidal B.H 2 S 4 electron groups, 2 lone pairs, (3) bent C. CCl 4 4 electron groups, 0 lone pairs, (1) tetrahedral
11 Polar Molecules A polar molecule contains polar bonds. has a separation of positive and negative charge called a dipole, indicated with + and -. has dipoles that do not cancel. + - H–Cl H — N — H dipole H dipoles do not cancel
12 Nonpolar Molecules A nonpolar molecule contains nonpolar bonds. Cl–Cl H–H or has a symmetrical arrangement of polar bonds. O=C=O Cl Cl–C–Cl Cl dipoles cancel
13 Determining Molecular Polarity STEP 1: Write the electron-dot formula. STEP 2: Determine the shape. STEP 3: Determine if dipoles cancel or not. Example: H 2 O.. H─O : H 2 O is polar │ H dipoles do not cancel
14 Learning Check Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr 3 B. HBr C. Br 2 D. SiBr 4
15 Solution Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr 3 1) pyramidal; dipoles do not cancel; polar B. HBr1) linear; one polar bond (dipole); polar C. Br 2 2) linear; nonpolar bond; nonpolar D. SiBr 4 2) tetrahedral; dipoles cancel; nonpolar