Structure of molecules

Objectives:•To understand molecular structure and bond angles•To learn to predict molecular geometry from the number of electron pairs•To learn to apply the VSEPR model to molecules with double bonds

3-D Molecular Structure

• Lewis structures = arrangement of valence e-

• Also want to talk about the 3-D geometric structure of molecules

• Example: H2O

• “Bent” shape with 104.5° angle between the two hydrogens

3-D Molecular Structure

• Linear structure

• All atoms in a line

• 180° angle between the oxygen atoms

3-D Molecular Structure

• Trigonal planar

• Example: BF3

• Flat (planar) with 120° angles between fluorine atoms

3-D Molecular Structure

• Tetrahedral

• Example: CH4

•Tetrahedron: A four-sided pyramid

•109.5° angles between Hs in methane

3-D VSEPR Model

• Molecular shape determines molecular properties!!!!!!!

• Valence Shell Electron Pair Repulsion (VSEPR) Model allows us to make predictions about molecular shape

• Pairs of electrons surrounding an atom repel each other

• Atoms in molecule are positioned to minimize repulsion (stable state)

• Bonding e-s and lone pairs are positioned as far apart as possible

VSEPR: 2 pairs of electrons

• BeCl2

• First, Lewis diagram:

• Be (2 valence e-) + 2 Chlorine (7 valence e-) = 16 valence e-

• Beryllium is exception to octet rule - often forms “electron deficient molecules”

• Cl - Be - Cl

• Arrange remaining e- (16-4 = 12 e-) around the Chlorines

VSEPR: 2 pairs of electrons

• BeCl2 has two pairs of electrons surrounding it

• What is best way to arrange two pairs of electrons to minimize repulsions?

• Put electron pairs 180° degrees apart

• Now we can specify 3-D molecular structure: linear

• Whenever you have 2 pairs of electrons surrounding an atom, they should always be placed at 180°

VSEPR: 3 pairs of electrons

• BF3 - Boron trifluoride

• Doesn’t follow octet rule

• Boron tends to form “electron deficient” configurations

• Boron (3 valence e-) + 3 Fluorine (7 valence e-) = 24 valence e-

• Boron has 3 e- pairs around it

• Distribute remaining e- (24-6= 18 e-) around the fluorines

VSEPR: 3 pairs of electrons

• BF3 - Boron trifluoride

• How to minimize repulsion between 3 pairs of electrons?

• 120° angles between electron pairs

• Trigonal planar structure

• Whenever 3 pairs of e-

around atom, should always be placed at corners of triangle, with 120° angle between...

VSEPR: 4 pairs of electrons

• CH4 - Methane

• Lewis Diagram

• Carbon (4 valence e-) + 4 Hydrogen (1 valence e-) = 8 valence e-

• Follows duet (hydrogen) and octet (carbon) rules

VSEPR: 4 pairs of electrons

• CH4 - Methane

• How to minimize repulsion between 4 pairs of electrons?

• 109.5° angles between electron pairs

• Tetrahedral shape (like a square pyramid)

• Whenever 4 pairs of e- around atom, should always be placed at 4 corners of tetrahedron, with 109.5° angle between...

• Whenever you have 4 pairs of e- around atom: tetrahedral geometry

Try it with Ammonia

• NH3 - Ammonia

1.Draw Lewis structure

★H - N - H | H

1.Nitrogen (5 valence e-) + 3 Hydrogen (1 valence e-) = 8 valence e-

(6 already accounted for by lines)

★Follow octet and duet rules

1. . .H - N - H | H

Try it with Ammonia

. .H - N - H | H

•How many electron pairs?

•4

•How to reduce repulsions?

•Tetrahedral geometry

•Angles?

•109.5° - but angle between hydrogens actually smaller - lone pair more repulsive than bonded pair

•We call this TRIGONAL PYRAMIDAL

Steps for predicting molecular geometry with VSEPR

1.Draw Lewis structure for molecule

2.Count electron pairs and arrange them in a way that minimizes repulsions: put them as far apart as possible

3.Determine positions of atoms from the way the electron pairs are shared

4.Determine name of molecular structure (linear, trigonal planar, tetrahedral, bent, trigonal pyramid)

VSEPR Model of Water

• First - Lewis structure

• H - O - H

• Oxygen (6 valence e-) + 2 Hydrogen (2 valence e-) = 8 valence e- (4 accounted for by lines)

• Follow octet and duet rules

• ..H - O - H ··

VSEPR Model of Water

..H - O - H ··

• How many electron pairs?

• 4

• How to reduce repulsions?

• tetrahedral geometry

• Angles?

• 109.5° angles (actually less, since lone pairs repulse more than bonded pairs)

• We call this BENT shape

VSEPR and Molecules with Double Bonds

• Carbon Dioxide

• 2 electron pairs: linear structure (180° bond angle)

• We know that there are 4 e- pairs around carbon, but not tetrahedral

• 2 pairs of electrons between Carbon and each Oxygen

• Each double bond should be treated as single bond for geometry purposes

Hybrid Orbitals

• If an s and a p orbital overlap, what happens?????

• You get a linear sp orbital - a hybrid combination of the s and p orbitals

• This is the type of hybridization you get with linear molecular geometry!

Hybrid Orbitals

• If an s and 2 p orbitals overlap, what happens?????

• You get a trigonal sp2

orbital - a hybrid combination of the s and 2 p orbitals

• This is the type of hybridization you get with trigonal molecular geometry!

Hybrid Orbitals

• If an s and 3 p orbitals overlap, what happens?????

• You get a tetrahedral sp3 orbital - a hybrid combination of the s and 3 p orbitals

• This is the type of hybridization you get with tetrahedral molecular geometry!

Guide to hybrid orbitals:

Geometry Hybridization Example

linear sp BeH2, HCN

trigonal sp2 BF3

tetrahedral sp3 CH4, H2O

Questions

• What is hybridization of CH4?

• Procedure - count regions of electron density around the atom and match this number with the corresponding hybridization