Shapes and Polarity of Molecules VSPER Shapes and Polarity of Molecules Adapted from: Pearson Education, Inc. Publishing as Benjamin Cummings

Shapes of Molecules Molecules (covalent chemicals) form certain shapes depending on how many lone and bonding pairs of electrons it has. Because the electron pairs repel each other we get certain shapes being formed. These are due to a certain rule called VSEPR (Valence Shell Electron Pair Repulsion) Bonding pair – these can also be drawn as straight lines Lone pair

VSEPR Theory 06/10/99 Based on Lewis structures we can know the shape or “geometry” of molecules VSEPR, as the name suggests, predicts geometry based on the repulsion of electron pairs (bonding pairs and lone pairs) Electrons around the central nucleus repel each other. Thus, resulting structures have atoms maximally spread out.

The 6 Basic Shapes for electron pair repulsion: 2 Electron Repulsion Zones 3 4 5 6 Shape: Linear Trigonal Planar (Flat) Tetrahedral Trigonal Bipyramid Octahedral

Planar triangular Tetrahedral Octahedral Trigonal bipyramidal

AXE Lewis structures do not show geometry, only electron pair placement. However, the 3-D shape (geometry) of a molecule can be determined from a properly-drawn Lewis structure. All monocentric molecules can be represented by an AXE formula: A = central atom X = outer atoms (doesn’t matter what they actually are or how many bonds they are held by) E = lone pairs of electrons on the central atom only.

Cl O What AXE formula corresponds to the chlorate ion, ClO3-1? First draw a proper Lewis structure: One central atom, three outer atoms, one lone pair: AX3E Cl O -1

S O What AXE formula corresponds to sulfur trioxide, SO3? Draw a Lewis structure. 1 central atom, 3 outer atoms, no lone pairs: AX3 S O

AX2 AX3 AX2E AX4 AX3E AX2E2 AX5 AX4E AX3E2 AX2E3 AX6 AX5E AX4E2 AX3E3 06/10/99 Electron pair geometry AXE Molecule geometry linear AX2 trigonal planar AX3 AX2E bent tetrahedral AX4 AX3E Trigonal pyrimidal AX2E2 trigonal bipyramidal AX5 AX4E See-saw AX3E2 T-shape AX2E3 octahedral AX6 AX5E Square pyramidal AX4E2 AX3E3 AX2E4

Molecular Geometries S O AX3 Geometry: Trigonal Planar Bond Angle: 120º Example: SO3 S O

Molecular Geometries S O VSEPR Formula: AX2E Geometry: Bent (Angular) Bond Angle: Less than 120º Example: SO2 S O

Molecular Geometries C Cl AX4 Geometry: Tetrahedral Bond Angle: 109.5º Example: CCl4 C Cl

Four Electron Groups In a molecule of CH4 There are four electron groups around C. Repulsion is minimized by placing four electron groups at angles of 109°, which is a tetrahedral arrangement. The shape with four bonded atoms is tetrahedral. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Molecular Geometries N H VSEPR Formula: AX3E Geometry: Trigonal Pyramidal Bond Angle: Less than 109.5º Example: NH3 N H

Three Bonding Atoms and One Lone Pair In a molecule of NH3 Three 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 three bonded atoms, the shape is pyramidal. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Molecular Geometries O H VSEPR Formula: AX2E2 Geometry: Bent (Angular) Bond Angle: Less than 109.5º Example: H2O O H

Two Bonding Atoms and Two Lone Pairs In a molecule of H2O Two electron groups are bonded to H atoms and two are lone pairs (4 electron groups). Four electron groups minimize repulsion in a tetrahedral arrangement. The shape with two bonded atoms is bent(~109). Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check The shape of a molecule of N2O (N N O) is 1) linear 2) trigonal planar 3) bent (120°)

Solution The shape of a molecule of N2O (N N O) is 1) linear In the electron-dot structure with 16 e-, octets are acquired using two double bonds to the central N atom. The shape of a molecule with two electron groups and two bonded atoms (no lone pairs on N) is linear. two electron groups • • • • : N :: N :: O : • • • • : N = N=O : linear, 180°

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) pyramidal 3) bent A. PF3 B. H2S C. CCl4

Solution A. PF3 4 electron groups, 1 lone pair, (2) pyramidal B. H2S 4 electron groups, 2 lone pairs, (3) bent C. CCl4 4 electron groups, 0 lone pairs, (1) tetrahedral

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

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

Determining Molecular Polarity STEP 1 Write the electron-dot formula. STEP 2 Determine the polarity of the bonds. STEP 3 Determine if dipoles cancel. Example: H2O . . H─O: H2O is polar │ H dipoles do not cancel

Learning Check Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr3 B. HBr C. Br2 D. SiBr4

Solution Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr3 1) pyramidal; dipoles don’t cancel; polar B. HBr 1) linear; one polar bond (dipole); polar C. Br2 2) linear; nonpolar bond; nonpolar D. SiBr4 2) tetrahedral; dipoles cancel; nonpolar