Section 3.3 – Part A Pg. 91-96 Objective: 1) Apply VSEPR theory to predict molecular shapes.

VSEPR TheorySection 3.3 – Part A

Pg. 91-96

Objective:

1) Apply VSEPR theory to predict molecular shapes

Stereochemistry – is the study of the 3-D spatial configuration of molecules and how this affects their reactions.

The shape of molecules is determined by the repulsion that happens between electron pairs

The theory behind molecular shapes is called VSEPR Theory (Valence Shell Electron Pair Repulsion)

Molecular Shapes

Solid = in plane of page Dashed = behind (away) Wedge = ahead (toward)

General Rule: ◦ Pairs of electrons in the valence shell of an atom

stay as far apart as possible because of the repulsion of their negative charges

◦ The type, number and direction of bonds to the central atom of a molecule determine the shape of the resulting molecule.

So how do we predict these molecular shapes?

VSEPR

We will be using the following compounds to analyze the 6 shapes possible

◦ BeH2(s), BH3(g), CH4(g), NH3(g), H2O(l), HF(g)

To start, draw a Lewis formula for each of the molecules and then consider the arrangement of all pairs of valence electrons.

◦ (Remember – all pairs of valence e-’s repel each other and want to get as far apart as possible)

Using VSEPR to Predict Molecular Shapes

Shape #1 = LinearLewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

2 0 2 AX2 linear X – A – Xlinear

Be

• This Lewis formula indicates that BeH2(s) has two bonds and no lone pairs on the central atom.

• VSPER theory suggests that the two bond pairs will be farthest apart by moving to opposite sides to a bond angle of 180°

• This gives the molecule a linear orientation

* A is the central atom; X is another atom

*Exception* Beryllium does not follow OCTET RULE

Shape #2 = Trigonal PlanarLewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

3 0 3 AX3trigonal planar

• This Lewis formula indicates that BH3(g) has three bonds and no lone pairs on the central atom.

• VSPER theory suggests that the three bond pairs will be farthest apart by moving to a bond angle of 120° to each other.

• This gives the molecule a trigonal planar orientation.

* A is the central atom; X is another atom

B

*Exception* - Boron Does not follow OCTET RULE

Draw the Lewis Formula for BF3

Practice

Does not obey the octet rule

Trigonal Planar

F

F

F

Shape #3 =TetrahedralLewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

4 0 4 AX4 tetrahedral

• This Lewis formula indicates that CH4(g) has four bonds and no lone pairs on the central atom.

• VSPER theory suggests that the four bond pairs will be farthest apart by arranging in three dimensions so that every bond makes an angle of 109.5° with each other.

• This gives the molecule a tetrahedral orientation.

* A is the central atom; X is another atom

Draw the Lewis Formula for SiH4

Practice

H

H

H

H

Tetrahedral

Shape #4 =Trigonal PyramidalLewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

3 1 4 AX3E tetrahedral Trigonal

pyramidal

• This Lewis formula indicates that NH3(g) has three bonds and one lone pair on the central atom.

• VSPER theory suggests that the four groups of e-’s should repel each other to form a tetrahedral shape (bond angle = 109.5°)

• But the lone pair is very repulsive, thus pushes the atoms more to a 107.3° bond angle

• This gives the molecule a trigonal pyramidal orientation.

* A is the central atom; X is another atom, E is a lone pair of electrons

Draw the Lewis Formula for PCl3

Practice

Cl

Cl

Cl

Trigonal pyramidal

Shape #5 =Angular (Bent)Lewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

2 2 4AX2E

2

tetrahedral Angular

(Bent)

• This Lewis formula indicates that H2O(l) has two bonds and two lone pairs on the central atom.

• VSPER theory suggests that the four groups of e-’s should repel each other to form a tetrahedral shape (bond angle = 109.5°)

• But the TWO lone pairs are very repulsive, thus pushes the atoms more to a 105° bond angle

• This gives the molecule an angular (bent) orientation.

* A is the central atom; X is another atom, E is a lone pair of electrons

Draw the Lewis Formula for OCl2

Practice

Angular (bent)

Shape #6 =Linear (Tetrahedral)Lewis

FormulaBond Pairs

Lone

Pairs

Total

Pairs

General

Formula

Electron Pair

Arrangement

Stereochemical Formula

1 3 4 AXE3

Linear(Tetrahedra

l)

• This Lewis formula indicates that H2O(l) has two bonds and two lone pairs on the central atom.

• VSPER theory suggests that the four groups of e-’s should repel each other to form a tetrahedral shape (bond angle = 109.5°)

• But since there are only two atoms with one covalent bond holding them together, by definition, the shape is linear, as is the shape of every other diatomic molecule.

* A is the central atom; X is another atom, E is a lone pair of electrons

FH

Draw the Lewis Formula for HCl

Practice

VSEPR theory describes, explains, and predicts the geometry of molecules by counting pairs of electrons that repel each other to minimize repulsion. The process for predicting the shape of a molecule is summarized below:

Step 1: Draw the Lewis formula for the molecule, including the electron pairs around the central atom.

Step 2: Count the total number of bonding pairs (bonded atoms) and lone pairs of electrons around the central atom.

Step 3: Refer to Table 7, and use the number of pairs of electrons to predict the shape of the molecule.

Summary

Pg. 95

Draw the Lewis and stereochemical formulas for a sulfate ion, SO4

2- and predict the shape◦ See pg. 95

Draw the Lewis and stereochemical formulas for a chlorate ion, ClO3

- and predict the shape◦ See pg. 96

On your own: Pg. 96 #3

Practice

It is important to remember that a double or triple bond is one bond, and to treat it as such, when predicting the VSEPR shapes of molecules.

Example: Predict the shape of C2H4(g)

◦ Draw the Lewis formula for the molecule

◦ Count the # of pairs of e-’s around the central carbon atoms. The carbon atoms have 3 bonds (2 single, 1 double) and no

lone pairs. This is the same as a trigonal planar configuration.

Practice: Predict the shape for C2H2(g).

Multiple Bonds in VSEPR Models

HH

HH

Answer: See pg. 97

1) Finish pg. 96 #1-3

2) Pg. 98 #6-7 (Multiple Bond Practice)◦ For 7 c, d, e - If there is more than one central atom involved,

tell me the shape around each of the central atoms ◦ Example:

3) Pg. 104 #1, 2, 3 #2: If there is more than one central atom involved, tell

me the shape around each of the central atoms

Homework

trigonal planar—first two carbonstetrahedral—third carbon

Draw the Lewis Formula for PCl3

Practice