Molecular Geometry 1 Molecular Structure Molecular geometry is

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Molecular Geometry 1

Molecular Geometry 1

Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement

Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space. Physical and chemical properties depend on the geometry of a molecule. 2

3 -D Molecular Structures 3 -D Model 3 -D Drawing 3

3 -D Molecular Structures 3 -D Model 3 -D Drawing 3

Does it matter? The Thalidomide Story The chemical structure of thalidomide. models – enantiomers

Does it matter? The Thalidomide Story The chemical structure of thalidomide. models – enantiomers (mirror image) “The Same and Not the Same”, by Roald Hoffmann 1995, Columbia University Press 4

Does it matter? Fatty Acids ‘trans’ fatty acid ‘cis’ fatty acid 5

Does it matter? Fatty Acids ‘trans’ fatty acid ‘cis’ fatty acid 5

VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules

VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them. 6

VSEPR Notation AX 3 AXE shorthand notation: • A - central atom • X

VSEPR Notation AX 3 AXE shorthand notation: • A - central atom • X - substituent atoms • E - lone pair electrons 7

Predicting Molecular Geometry: VSEPR n n n Only five basic shapes. When a lone

Predicting Molecular Geometry: VSEPR n n n Only five basic shapes. When a lone pair replaces an atom, the molecular geometry changes as well as the angles. Double and triple bonds are treated as single bonds. 8

Linear lone pairs 2 0 1 1 -3 Molecular Geometry linear . . bond

Linear lone pairs 2 0 1 1 -3 Molecular Geometry linear . . bond pairs linear 9

Predicting Molecular Geometry Example 1: Be. Cl 2 1. Draw the Lewis structure Cl

Predicting Molecular Geometry Example 1: Be. Cl 2 1. Draw the Lewis structure Cl Be AX 2 Cl 2. Two electron pairs around the central atom. Two substituents Zero lone pairs. Geometry is Linear. Bond angle is 180 o. 10

Predicting Molecular Geometry Example 2: HF 1. Draw the Lewis structure AX 2. One

Predicting Molecular Geometry Example 2: HF 1. Draw the Lewis structure AX 2. One electron pair between the two atoms. Three lone pairs on fluorine. Geometry is Linear. 11

Trigonal Planar bond pairs 3 lone pairs 0 Molecular Geometry trigonal planar Three e-

Trigonal Planar bond pairs 3 lone pairs 0 Molecular Geometry trigonal planar Three e- pairs about central atom 12

Predicting Molecular Geometry Example 3: BF 3 . . : F: . . :

Predicting Molecular Geometry Example 3: BF 3 . . : F: . . : F. . B AX 3 . . F: . . Three electron pairs around the central atom. Three substituents and Zero lone pairs. trigonal planar 13

Tetrahedral bond pairs 4 lone pairs 0 Molecular Geometry tetrahedral Four e- pairs about

Tetrahedral bond pairs 4 lone pairs 0 Molecular Geometry tetrahedral Four e- pairs about central atom 14

Predicting Molecular Geometry Example 4: CH 4 AX 4 H H C H H

Predicting Molecular Geometry Example 4: CH 4 AX 4 H H C H H Four electron pairs around the central atom. Four substituents and Zero lone pairs. tetrahedral 15

Trigonal Pyramidal 3 lone pairs Molecular Geometry 1 . . bond pairs trigonal pyramidal

Trigonal Pyramidal 3 lone pairs Molecular Geometry 1 . . bond pairs trigonal pyramidal Four e- pairs about central atom 16

Predicting Molecular Geometry Example 5: NH 3 H N H AX 4 : AX

Predicting Molecular Geometry Example 5: NH 3 H N H AX 4 : AX 3 H Four electron pairs around the central atom. Three substituents and One lone pair. N H H molecular geometry = triangular pyramidal H 17

Bent 2 Molecular Geometry bent . . 2 lone pairs . . bond pairs

Bent 2 Molecular Geometry bent . . 2 lone pairs . . bond pairs Four e- pairs about central atom 18

Predicting Molecular Geometry Example 6: H 2 O AX 4 H O : AX

Predicting Molecular Geometry Example 6: H 2 O AX 4 H O : AX 2 H Four electron pairs around the central atom. Two substituents and Two lone pairs. O molecular geometry = angular or bent H H 19

Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion: bonding pair-bonding pair

Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion: bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair 20