10 February 2013

Lewis structures

Lewis structures are representations of molecules

showing all electrons, bonding and nonbonding.

Rules for writing Lewis structures

Find the sum of valence electrons of all

atoms in the molecule or polyatomic ion.

Divide that number by two to get the number

of electron pairs.

The central atom is the least electronegative

element that isnt hydrogen. Connect the

outer atoms to it by single bonds (e pairs).

Divide the remaining pairs equally over the

outer atoms.

PCl3

5 + 3(7) = 26

= 13 pairs

10 February 2013

Lewis structures

Place the remaining e-pairs on the

central atom

Check for the octet rule, and make

double or triple bonds if necessary.

More examples :NH4

+

SO3

CO32-

HCN

10 February 2013

Learning check

Draw Lewis structures for the following

CO2

NH3

N2

O3

ClO4-

SO32-

HClO2

HNO3

HClO4

H2SO3

HCNO

H3PO4

10 February 2013

Formal charges

For each atom, count the electrons in lone pairs

and half the electrons it shares with other atoms.

Subtract that from the number of valence electrons

for that atom: The difference is its formal charge.

The best Lewis structure7

7is the one with the fewest charges.

7puts a negative charge on the most electronegative atom.

10 February 2013

Resonance

What is the Lewis structure of ozone?

7but this is at odds with the observed structure of ozone, in which7

7both bonds are of the same length.

7both outer oxygens

have a charge of -1/2.

The single bond should be longer

than the double bond7

The terminal oxygens should

have formal charges of -1 and 0

10 February 2013

One Lewis structure cannot accurately

depict a molecule such as ozone.

Resonance

We use multiple structures, resonance

structures, to describe the molecule.

Just as green is a synthesis of blue and yellow7

7ozone is a synthesis of

these two resonance structures.

10 February 2013

In truth, the electrons that form the second CO bond in the

double bonds below do not always sit between that C and that O,

but rather can move among the two oxygens and the carbon.

They are not localized,

but rather are delocalized.

The organic compound

benzene, C6H6, has two

resonance structures.

It is commonly depicted

as a hexagon with a circle

inside to signify the

delocalized electrons

in the ring.

Resonance

10 February 2013

Exceptions to the octet rule

There are three types of ions or molecules

that do not follow the octet rule:

Ions or molecules with an odd number of electrons.

Ions or molecules with less than an octet.

Though relatively rare and usually quite

unstable and reactive, there are ions and

molecules with an odd number of electrons.

NO

NO2

O2-

ClO2

Also rare. Hydrogen, Helium, Boron, Beryllium

Non favorable

Structures !!

10 February 2013

Ions or molecules with more than eight

valence electrons (an expanded octet).

Exceptions to the octet rule

The only way PCl5 can exist is if

phosphorus has 10 electrons around it.

It is allowed to expand the octet of atoms

on the 3rd row or below.

Presumably d orbitals in these

atoms participate in bonding.

Another example: PO43-

10 February 2013

Learning check

Draw resonance structures where appropriate,

label any formal charges, and rank the resonance

structures in order of importance to overall structure.

SCN -

CS2

SNF3 S2CO2-

IO3-

HCO3-

PF3

SO42-

ClO4-