MOLECULAR GEOMETRYChapter 6.5

Objectives1. Explain VSEPR theory2. Predict the shapes of molecules or

polyatomic ions using VSEPR theory3. Explain how the shapes of

molecules are accounted for by the hybridization theory

4. Describe dipole-dipole forces, hydrogen bonding, induced dipoles, and London dispersion forces

5. Explain what determines molecular polarity

Molecular Polarity Uneven distribution of molecular charge

Determined by:1. Polarity of each bond2. As well as, molecular geometry

Review: Polarity of each bond1. Find the difference in

electronegativity2. Use 0 – 0.3 nonpolar-covalent

0.3 – 1.7 polar-covalent 1.7 – 3.3 Ionic3. H – O 2.1 3.54. Difference is 1.4, so its polar-

covalent

H – O

H

_

+

VSEPR Theory Valence-Shell Electron-Pair Repulsion

States that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible

Molecules with no unshared pairs

• Only two atoms, always linear shape

• BeF2 , Beryllium does NOT follow octet rule

Hydrogen, H2

Hydrogen Chloride, HCl

F Be F

• No lone pairs• No repulsion• Linear

180 AB

2

BF3, Boron also does NOT follow the octet rule

F F B F

• No lone pairs• No repulsion• Trigonal Planar

CF4, Each fluorine is exactly the same distance from each other

F F C F F

• No lone pairs• Equal distance• Tetrahedral

120

109.5 AB

4

AB3

Molecules with unshared pairs of electrons NH3, Unshared pair pushes down on the

three Hydrogens

H N H H

• ONE lone pairs• Repulsion• Trigonal Pyramidal

H2O, the unshared pairs push down on the two Hydrogens

O HH

• TWO lone pairs• Repulsion• Bent 0r angular

107

105 AB2E2

AB3E

Others: SF6, Octahedral AB6

PCl5, Trigonal bipyramidal AB5

Hybridization The mixing of two or more atomic

orbitals of similar energies on the same atom to produce new orbitals of equal energies. VSEPR doesn’t show bonding orbitalsC __ __ __ __ __

1s 2s 2p

C __ __ __ __ __

1s 2s 2p

Form hybrid orbitals Hybrid orbitals

Orbitals of equal energy produced by the combination of two or more orbitals on the same atom.

sp3

Other hybrids: sp2 : trigonal planar sp : linear

Sp3 hybrid

sp3 movie

C __ __ __ __ __

1s 2s 2p

N __ __ __ __ __

1s 2s 2p

O __ __ __ __ __

1s 2s 2p

OR

sp2 Hybrid

B __ __ __ __ __

1s 2s 2p

sp Hybrid

Be __ __ __ __ __

1s 2s 2p

Intermolecular Forces Forces of attraction between

molecules Boiling point (BP) is a good measure of

IMF (intermolecular forces) Higher the BP , the stronger the IMF Boiling points and bond types

Non polar-covalent H2 -253⁰C

O2 -183 ⁰C

Cl2 -34 ⁰C

CH4 -164 ⁰C

Polar-covalent NH3 -88⁰C

H2O 100⁰C

HCl -85 ⁰C

Ionic NaCl 1413⁰CMgF2 2239⁰C

Metallic Cu 2567⁰CFe 2750⁰C

Molecular Polarity and Dipole-Dipole Forces

Dipole Created by equal but opposite charges

that are separated by a short distance

H – Cl

_

+

Dipole-dipole forces Forces of attraction between polar

molecules

Polar Water, H2O

Ammonia, NH3

Molecular Polarity and Dipole-Dipole Forces

H 2.1O 3.5

_

+

H 2.1N 3.0

_

+ Nonpolar Carbon tetrachloride, CCl4

Carbon Dioxide, CO2

Hydrogen bonding Intermolecular force in which a hydrogen

atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.

Creates an unusually higher boiling point.

London Dispersion forces Intermolecular attractions resulting from the

constant motion of electrons and the creation of instantaneous dipoles. Very weak intermolecular forces Only intermolecular forces for noble gas atoms and

nonpolar molecules

He

Br2