VSEPR Theory (notes)

VSEPR TheoryChemistry Warm-up:

1. Pick up a set of the skeleton notes from the first lab table.

2. Complete the words of the week assignment.

You need to have answers for Tuesday, Thursday and today.

Today’s :Draw Lewis Dot Structures for boron

triiodide and oxygen difluoride.

VSEPR Theory

Valence

VSEPR Theory

ValenceShell

VSEPR Theory

ValenceShell

Electron

VSEPR Theory

ValenceShell

ElectronPair

VSEPR Theory

ValenceShell

ElectronPair

Repulsion

VSEPR Theory

ValenceShell

ElectronPair

RepulsionTheory

VSEPR Theory

Valence Shell

VSEPR Theory

Valence ShellRemember that the outermost energy

level of an atom is referred to as the valence shell.

VSEPR Theory

Valence ShellRemember that the outermost energy

level of an atom is referred to as the valence shell.

The electrons within this energy level, valence shell, are the only ones that are used in forming chemical bonds.

VSEPR Theory

Electron PairA “lone pair” of electrons is described as

two electrons, on a central atom, that are not used within a chemical bond.

VSEPR Theory

Electron PairA “lone pair” of electrons is described as

two electrons, on a central atom, that are not used within a chemical bond.

NH

HH

Nitrogen has one LONE PAIR of electrons on the central atom.

VSEPR Theory

RepulsionAs you learned long ago, the two north

ends of two different magnets will repel each other.

VSEPR Theory

RepulsionAs you learned long ago, the two north

ends of two different magnets will repel each other.

The same holds true for like charges within the atom. Lone pairs of electrons, on a central atom, will repel each other as well as the electrons within a bond to make the most stable arrangement for that molecule.

VSEPR Theory

RepulsionAs you learned long ago, the two north ends of two

different magnets will repel each other.The same holds true for like charges within the

atom. Lone pairs of electrons, on a central atom, will repel each other as well as the electrons within a bond to make the most stable arrangement for that molecule.

NH

HH

That lone pair of electrons within the NH3 molecule

occupy space and repel the electrons within the bonds causing the molecule to be shaped sort of like a tripod.

VSEPR TheoryVSEPR Theory is based on the idea that the geometry of a molecule is determined primarily by repulsions among the pairs of electrons associated with a central atom. Only valence electrons of the central atom influence the molecular shape in a meaningful way.

VSEPR TheoryThere are some basic assumptions that must be considered:

1. Pairs of electrons in the valence shell of a central atom repel each other.

2. These pairs of electrons tend to occupy positions in space that minimize repulsion and maximize the distance of separation between them.

3. The valence shell is taken as a sphere with electron pairs localizing on the spherical surface at maximum distance from one another.

4. A multiple bond is treated as if it is a single electron pair and the two or three electron pairs of a multiple bond are treated as a single super pair.

VSEPR TheoryThere are five basic shapes of molecules that can be predicted using the above assumptions. They are:

1. Linear2. Angular / V-shaped / Bent3. Tetrahedral4. Trigonal Planarand5. Trigonal Pyramidal.

For each of these we will follow the same format of shape, number of atoms bonded to the central atom,

number of lone pairs of electrons on the central atom, and an example with a Lewis Dot Structure.

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Linear

HBr(hydrogen

monobromide)

H Br

None(there is no central

atom)

None(there is no central

atom)

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Linear

BeH2(beryllium hydride)

Two None

H Be H

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Angular(V-shaped, Bent)

H2O(dihydrogen monoxide)

Two Two

H O

H

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Tetrahedral

CH4(methane

or carbon tetrahydride)

Four None

CH

HH

H

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Trigonal planar

BF3(boron trifluoride)

Three None

BF F

F

VSEPR TheoryExample Lewis Dot Structure

# of atoms bonded to the central atom

# of lone pairs of electrons on the

central atom

Trigonal pyramidal

NH3(ammonia or nitrogen

trihydride)

Three One

NH HH

VSEPR Theory

Name the shape for each of the following molecules.

A: B:

C: D: