Drawing Lewis Structures, Lecture notes of Chemistry

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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. 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. CF 4 H 2 O

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

1. 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)

 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. PH 3

Atoms x Valence e-

P: H:

1 x 5 = (^5) 3 x 1 = (^3) Total = 8

H P H

H

• 6 (2 for each bond) 2

*H is full with 2 e-

Octet Rule obeyed?

C. Single Bonds

1. Br 2 Br: 2 x 7 = (^14)

• 2 Br Br 12
• 12 0

Octet Rule obeyed?

D. Multiple Bonds

• 1. CO 2

No Octet on C

C: Try double bonds O:

1 x 4 = (^4) 2 x 6 = (^12) 16

• 4 12
• 12 0

O C O

O C O

O C O^

Now carbon and oxygen have an octet

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. PO 4 3-

P: 0:

1 x 5 = 5 4 x 6 = 24 Charge (^) = + 3

(-3 charge = +3 e-)

P

O

O

O

O

Octet Rule obeyed?

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. O 3

O: (^) 3 x 6 = 18

• 4 14
• 12 2

O O O

O O O

O O O

Octet Rule obeyed?