Lewis StructuresOf Covalent Compounds

NAB

Method

Information…

• Octet rule alone does not allow us to create correct Lewis Structures

• Does not tell us where or how to place the bonded electrons

• NAB method helps

N = “NEEDED”

1. Calculate the “N” as the sum of the electrons necessary for all elements to achieve an octet

2. Important exceptions (as usual)• H = 2• Be = 4• B = 6

NEEDED

Add together the number of valence electrons for each atom in the molecule if each had a full octet.

For example, CF4

There are 5 atoms in the molecule. If each had a full octet, 5 x 8 = 40 e-s are needed

A “AVAILABLE”

1. Calculate “A” as the sum of all valence electrons available to share

AVAILABLE ELECTRONS

Add together the number of valence

electrons for each atom in the molecule.

For example, CF4

Carbon has four valence electrons and each

fluorine has seven valence electrons

= 4 + 4(7) = 32 available e-

“B” BONDS

1. This is easy– Shared is the difference between NEEDED

and AVAILABLE: N - A.– Each bond requires 2 electrons

– So B = (N – A) / 2- B is the number of bonds that will

come off the central atom

Number of Bonds

H: halogens and hydrogen can only have 1 bond

O: oxygen group can have 2 bondsN: nitrogen group can have 3 bondsC: carbon group can have 4 bondsBe and B, Al: too small to hold a full

octet; Be forms only 2 bonds and B and Al form only 3 bonds

BONDS

For example, CF4

The number of shared electrons is N – A

= 40 – 32 = 8 shared electrons

2 electrons per bond, so

8/2 = 4 bonds

Steps for Creating Lewis Structure of Molecules

1. Create a “reasonable” skeleton structure

2. The least electronegative element will be the central atom

3. Never H or any of the Halogens• WHY???

How are the atoms connected?

Skeleton structure of the molecule:

You'll generally be writing Lewis structures for molecules that have a central atom and surrounded by one or more atoms.

How do you choose the central atom?

Many times it's readily apparent:

SiBr4 CH4

H2S CO2

Molecules are many times comprised of atoms of lesser electronegativity surrounded by atoms of greater electronegativity.

SiBr4 CH4

Write out the elements of the molecule so

that the least electronegative elements is

in the center surrounded by the other

elements. For example, CF4

C FF

FF

Place a covalent bond between the central

atom and the outside atoms. Remember

each covalent bond contains two

electrons.

C F

F

F

F

After making bonds between the central atom and the surrounding atom(s), you distribute the remaining electrons in pairs first about the surrounding atom(s) and then distribute any remaining electrons about the central atom

The four covalent bonds use eight of the 32 valence electrons in CF4

There are 24 valence electrons remaining. Add electrons to the outer atoms as lone pairs to satisfy the Octet Rule.

C F

F

F

F• This uses 24

electrons. There are no electrons left, so this is The Lewis dot structure for CF4

• Rule 1: NAB

• Rule 2: Place the least electronegative element at

the center, except for H which is always an outer

atom

• Rule 3: Add covalent bonds between the center atom

and the outer atoms

• Rule 4: Add lone pairs to the outer atoms

• Rule 5: Add lone pairs to the center atom

Building Lewis Structures for Covalent Molecules

•Determine the number of electrons needed (N) by each atom to complete its octet

•Add up the total number of valence electrons available (A)

•The number of bonds (B) that come off the central atom (N-A) / 2

RESONANCE

Consider the Lewis structure of the carbonate ion, CO3

2-. The Lewis structure for this ion has a carbon-oxygen double bond, and two carbon-oxygen single bonds. But which of the three oxygens forms the double bond?  There are three possibilities:

When more than one Lewis structure can be drawn, the molecule or ion is said to have

resonance We use a double headed arrow to show that

individual structures are related by resonance.

HYPERVALENCY

Larger atoms can exhibit hypervalency, in which the octet is expanded and there are more than eight electrons in the valence orbitals.

Valence expansion (also known as hypervalency) is allowed for atoms in the 3rd period and below of the periodic table.

Hypervalency