Lewis Structures

Electron-Dot-Diagrams For Molecules

Rules for Drawing Lewis Structures

1. Decide which atom is the central atom of the molecule

1. Hydrogen is never a central atom (1 bond only; no lone pairs)

2. Halogens usually make single bonds3. Usually the central atom is an element that

there is only one of

2. Count all valence electrons in the molecule

3. Draw a line representing a covalent bond between bonded atoms

4. Complete the “octet” of all attached atoms

Lewis Structure Rules Cont.

5. Place remaining electrons on the central atom (in pairs) even if the octet rule is exceeded

6. If the central atom lacks an octet, form multiple bonds

7. *There are exceptions to the Octet Rule*

1. An excess of electrons means that the d sub shell is involved in bonding

2. Odd number of electrons3. Not enough electrons

Vocabulary A bond is formed when a pair of electrons is

shared. (shared pairs). We indicate a bond with a line.

A double bond is formed when two pair of electrons are shared; triple when 3 pairs

A pair of electrons that is not shared is called a “lone pair”

Example (PCl3)

1. Choose the central atom

Cl P Cl

Cl

2. Count valence electronsP = 5

Cl = 7 x 3 = 21

Valence electrons = 26

3. Draw lines

4. Octet for attached atoms:

:

:

:

:

: :

::

5. Place remaining electrons

:

Example (SO3)

1. Central atom

O S O

O

2. Count valence electronsS = 6

O = 6 x 3 = 18

Valence electrons = 24

3. Draw lines

4. Octet for attached atoms

:

: :

: : :

: :

:

5. No electrons remain

6. Form multiple bonds

Example (SO42-)

1. Central atom

O

O S O

O

2. Count valence electronsS = 6

O = 6 x 4 = 24 Negative charged ion = 2

Valence electrons = 32

3. Draw lines4. Octet for attached atoms

:

: :

: : :

: :

:

5. Place remaining electrons

6. Add bracket and charge.

: :

: 2-

m.socrative.comRoom #90777 Name the following compounds:1.K2SO4

2.CuNO3

3.SeBr2

V.S.E.P.R. Theory

Valence Shell Electron Pair Repulsion

Molecular Structure

Lewis structures do not necessarily illustrate molecular geometry, but they do generate the data needed to predict molecular structure.

In the valence shell of the central atom of a molecule are either bonding electron pairs or nonbonding pairs of electrons.

These electrons repel creating bond and lone pair angles.

VSEPR Valence Shell Electron Pair Repulsion Theory

1. Shared and unshared pairs repel.2. Lone pairs repel stronger than shared pairs.3. Double and triple bonds are viewed as single;

for structure only.4. The resultant shape of the molecule is a result

of shared and lone pairs being as far apart as possible.

Reactivities The reactivity of a molecule is dependent on

its shape. The shape of a molecule is determined by its

electron configuration.

Steps1. Draw the Lewis structure of a compound.2. Count the # bonds and lone pairs.3. Apply the VSEPR rules for shape.

Table of Shapes# Pair

Groups# Lone Pairs

Molecular Shape Bond Angles

2 0 Linear 180o

3 0

1

Trigonal planar

Bent

120o

<120o

4 0

1

2

Tetrahedron

Trigonal pyramid

Bent

109.5o

<107o

<105o

“Molecular Geometry”Complete the following table for each molecule or ion

Molecule Lewis Structure Bonding Nonbonding Approx. 3-D Sketch Molecular Polar or or Ion Electron Electron Bond Shape Nonpolar

Pairs Pairs Angle Molecule

HH-C-H H

CH4 4 0 109.5

H

H

C

H

H

tetrahedral nonpolar

Multiple bonds are unidirectional and count as one area of electron density

Complete the table for each of the following molecules or ions. The central atom of each is underlined

1. CF3Cl

2. NH3

3. NH4+

4. H2O

5. SbH3

6. SF2

7. CH3-

8. PF3