Drawing Lewis Structures

A. Molecules (covalent compounds) 1. Molecules share valence electrons. The arrangement of the

atoms (structure) is important to know in order to understand their chemical and physical properties. • Boiling point

• melting point

• Polarity

• solubility

• reactivity

B. Steps for drawing Lewis structures

1. Count total # of valence electrons a. For polyatomic ions add electrons equal to

the charge if it is negatively charged b. For polyatomic ions subtract electrons equal

to the charge if it is positive Loo

k at

Gro

up

A #

B. Steps for drawing Lewis structures 2. Determine the central atom

• The atom that has the lowest electronegativity is most likely to be the central atom. However it is often easier to look for the atom that you have the least of or is written 1st.

CF4 H2O

B. Steps for drawing Lewis structures

3. Draw single bonds to outside atoms a. Each bond represents 2 electrons b. Subtract the electrons used from the total

B. Steps for drawing Lewis structures

4. Fill octet for outside atoms a. Hydrogen atoms can only accommodate a single

bond. Do not add extra electrons. b. Subtract the electrons used from the total

B. Steps for drawing Lewis structures

5. If electrons remain, add them to the central atom in pairs

a. If the central atom does not have an octet (8), try stealing electrons from the outside atoms to create double or triple bonds (no more than 3)

b. The following atoms will not have an octet if they are the central atom. These are called electron deficient exceptions (B, Be, Al)

B. Steps for drawing Lewis structures

c. If your central atom has more than 8 valence electrons from bonds and extra pairs, it is possibly an expanded octet.

Central atoms that can form these are nonmetals in the third row of the periodic table and below. These atoms have empty d orbitals. d. If double or triple bonds are present, check for

resonance.

1 e- (electron)

2 e- (bond)

4 e- (double bond)

6 e- (triple bond)

Goal: Give each atom an octet, except hydrogen

A A

A A

A A

C. Single Bonds 1. PH3

# Atoms x Valence e- P: H:

1 x 5 = 5 3 x 1 = 3

Total = 8

P H H

H

- 6 (2 for each bond)

2

*H is full with 2 e-

- 2

0

Octet Rule obeyed?

C. Single Bonds 2. CHCl3

C: H:

1 x 4 = 4 1 x 1 = 1

Total = 26

Cl: 3 x 7 = 21 C

H

Cl

Cl

Cl

- 8

18 - 18

0

Octet Rule obeyed?

C. Single Bonds 3. Br2

Br: 2 x 7 = 14 Br Br - 2

12 - 12

0 Octet Rule obeyed?

D. Multiple Bonds • 1. CO2

No Octet on C

Try double bonds C: O:

1 x 4 = 4 2 x 6 = 12

16 - 4

12 - 12

0

C O O

C O O

C O O Now carbon and oxygen

have an octet

Octet Rule obeyed?

D. Multiple Bonds 2. SiS

Si: S:

1 x 4 = 4 1 x 6 = 6

10

Si S

- 2

8 - 6

2

Si S

Si S

Octet Rule obeyed?

E. Polyatomic Ions

• When a polyatomic ion is drawn, you must add (if -) or subtract (if +) electrons from the total number of valence electrons.

• Once the structure is complete add brackets and the charge.

E. Polyatomic Ions 1. PO4

3-

P: 0:

1 x 5 = 5 4 x 6 = 24

Charge = + 3

(-3 charge = +3 e-)

= 32

P

O

O

O

O

- 8

24 - 24

0

Octet Rule obeyed?

3-

E. Polyatomic Ions 2. H3O+

H: 0:

3 x 1 = 3 1 x 6 = 6

= 8

O H H

H

- 6

2 - 2

0

Octet Rule obeyed?

Charge = -1

(+1 charge = -1 e-)

+

F. Resonance Structures • Resonance structures occur when more than one Lewis structure can

be drawn. This often occurs when the number of double bonds or triple bonds does not equal the number of outside atoms.

• When resonance structures are drawn you must draw each possibility and add arrows to show that the electrons resonate.

F. Resonance • 1. O3

O: 3 x 6 = 18 - 4

14 - 12

2

O O O

O O O

O O O

Octet Rule obeyed?

F. Resonance Structures

F. Resonance Structures 2. CO3

2-

C : 1 x 4 = 4 O : 3 x 6 = 18 Charge = +2

= 24

C O

O

O

- 6

18 - 18

0

Octet Rule obeyed?

F. Resonance Structures

G. Exceptions to the octet rule

• Expanded Octet (more than 8e- on central)

• Electron Deficient (less than 8)

G. Exceptions to Octet Rule 1. BH3

B: 1 x 3 = 3 H: 3 x 1 = 3

= 6

B H H

H

- 6

0

B, Be, Al can not obey octet rule

Electron Deficient

G. Exceptions

2. XeF4

Xe: 1 x 8 = 8 0: 4 x 7 = 28

= 36

Xe

F

F

F

F

- 8

28 - 24

4

Period 3 and below can hold

more than 8

Expanded Octet