Basic Chemistry Copyright © 2011 Pearson Education, Inc. 1 Chapter 10 Structures of Solids and Liquids 10.2 Shapes of Molecules and Ions (VSEPR Theory)

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 2 VSEPR Theory 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 the molecular shape

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 3 Shapes of Molecules The three-dimensional shape of a molecule is the result of bonded groups and lone pairs of electrons around the central atom is predicted using the VSEPR (valence-shell electron-pair repulsion) theory

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 4 Two Electron Groups In BeCl 2 two electron groups are bonded to the central atom Be (exception to the octet rule) repulsion is minimized when the two electron groups are opposite each other at 180° the shape of the BeCl 2 molecule is linear

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 5 Two Electron Groups with Double Bonds In CO 2 two electron groups are bonded to C (electrons in a double bond count as one group) minimal repulsion occurs when the two electron groups are opposite each other (180°) the shape of the CO 2 molecule is linear

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 6 Three Electron Groups In BF 3 three electron groups surround the central atom B (B is an exception to the octet rule) minimal repulsion occurs when the three electron groups are at angles of 120° the shape of the BF 3 molecule is trigonal planar

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 7 Two Electron Groups and a Lone Pair In SO 2 three electron groups, two bonded groups and one lone pair, surround the S atom repulsion is minimized with three electron groups at angles of 120°, a trigonal planar arrangement. with two O atoms bonded to S and one lone pair of electrons, the shape of the SO 2 molecule is bent (120  )

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 8 Four Electron Groups In a molecule of CH 4 four electron groups are bonded to a C atom repulsion is minimized by placing the four electron groups at angles of 109°, a tetrahedral arrangement the shape with four bonded atoms is tetrahedral

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 9 Three Bonding Atoms and One Lone Pair In a molecule of NH 3 four electron groups, three bonding groups, and one lone pair, surround a N atom repulsion is minimized with four electron groups at angles of 109°, which is a tetrahedral arrangement of electron groups with three bonded atoms and one lone pair of electrons, the shape is trigonal pyramidal

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 10 Two Bonding Atoms and Two Lone Pairs In a molecule of H 2 O, four electron groups, two groups bonded to H atoms and two lone pairs, surround the O atom four electron groups minimize repulsion in a tetrahedral arrangement the shape of the H 2 O molecule with two bonded atoms is bent(109  )

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 11 Shapes with Two or Three Electron Groups

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 12 Shapes with Four Electron Groups

Basic Chemistry Copyright © 2011 Pearson Education, Inc. Predicting Molecular Shape (VSEPR Theory) 13

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 14 Learning Check The shape of a molecule of N 2 O (N N O) is 1) linear 2) trigonal planar 3) bent (120°)

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 15 Solution STEP 1 Draw the electron-dot formula. In the electron-dot structure with 16 e , octets are acquired using two double bonds to the central N atom. : N : : N : : O : STEP 2 Arrange the electron groups around the central atom to minimize repulsion. Repulsion is minimized with two electron groups at 180°, a linear arrangement.

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 16 Solution (continued) STEP 3 Use the atoms bonded to the central atom to determine the molecular shape. The shape of a N 2 O molecule with two bonded atoms and no lone pairs on the central N is linear. : N=N=O : linear, 180° (1)

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 17 Learning Check State the number of electron groups, lone pairs, bonded atoms, 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 D. PO 4 3-

Basic Chemistry Copyright © 2011 Pearson Education, Inc. 18 Solution A. PF 3 4 electron groups, 3 bonded atoms, 1 lone pair, pyramidal (2) B.H 2 S 4 electron groups, 2 bonded atoms, 2 lone pairs, bent (3) C. CCl 4 4 electron groups, 4 bonded atoms, 0 lone pairs, tetrahedral (1) D. PO electron groups, 4 bonded atoms, 0 lone pairs, tetrahedral (1)