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: AX3E1 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: AX3E0 S O

AX2E0 AX3E0 AX2E1 AX4E0 AX3E1 AX2E2 AX5E0 AX4E1 AX3E2 AX2E3 AX6E0 06/10/99 Electron pair geometry AXE Molecule geometry linear AX2E0 trigonal planar AX3E0 AX2E1 bent tetrahedral AX4E0 AX3E1 Trigonal pyrimidal AX2E2 trigonal bipyramidal AX5E0 AX4E1 See-saw AX3E2 T-shape AX2E3 octahedral AX6E0 AX5E1 Square pyramidal AX4E2 AX3E3 AX2E4

Molecular Geometries S O AX3E0 Geometry: Trigonal Planar Bond Angle: 120º Example: SO3 S O

Molecular Geometries S O VSEPR Formula: AX2E1 Geometry: Bent (Angular) Bond Angle: Less than 120º Example: SO2 S O

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 C Cl AX4E0 Geometry: Tetrahedral Bond Angle: 109.5º Example: CCl4 C Cl

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 N H VSEPR Formula: AX3E1 Geometry: Trigonal Pyramidal Bond Angle: Less than 109.5º Example: NH3 N 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

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

Five Electron Groups AX5E0 In a molecule of PCl5 There are five electron groups around P. Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement. The shape with five bonded atoms is trigonal bipyramidal AX5E0

Five Electron Groups AX4E1 In a molecule of SF4 There are five electron groups around S - 4 bonding pairs and 1 non-bonding pair. Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement. The non-bonding pair is on the trigonal plane The shape with four bonded atoms is see-saw AX4E1

Five Electron Groups AX3E2 In a molecule of ClF3 There are five electron groups around Cl - 3 bonding pairs and 2 non-bonding pair. Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement. The two non-bonding pairs are on the trigonal plane The shape with four bonded atoms is T-shape AX3E2

Five Electron Groups AX2E3 In a molecule of XeF2 There are five electron groups around Xe - 2 bonding pairs and 3 non-bonding pair. Repulsion is minimized by placing five electron groups at angles of 120°, and 90o which is a trigonal bipyramidal arrangement. The three non-bonding pairs are on the trigonal plane The shape with four bonded atoms is linear AX2E3

Six Electron Groups AX6E0 In a molecule of SF6 There are six electron groups around S - 6 bonding pairs and 0 non-bonding pair. Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o The shape with six bonded atoms is octahedral AX6E0

Six Electron Groups AX5E1 In a molecule of BrF5 There are six electron groups around Br - 5 bonding pairs and 1 non-bonding pair. Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o The lone pair does not go in the plane but above or below it The shape with six bonded atoms is square pyramidal AX5E1

Six Electron Groups AX4E2 In a molecule of XeF4 There are six electron groups around Xe - 4 bonding pairs and 2 non-bonding pairs. Repulsion is minimized by placing four electron groups at angles of 90o on a plane and 2 perpendicular to the plane at 90o The lone pairs go above or below the plane The shape with six bonded atoms is square planar

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