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Lewis Structures of Molecules

Lewis Structure Assumptions

Only valance electrons are involved in bonding.

Atoms react to form molecules, so to achieve stable noble gas electron configurations.

Atoms in molecules want eight valance electrons (octet rule) except for hydrogen which wants two electrons (duet rule).

In covalent compounds atoms share electrons to form bonds in order to achieve stable noble gas electron configurations. In ionic compounds electrons are transferred from one atom to another to achieve stable noble gas electron configurations.

Covalent Bonds

Lewis Structures are only for covalently bonded molecules.

Covalent bonds mean that electrons are shared between atoms.

Single bond = 2 electrons to each atom

Double bond = 4 electrons to each atom

Triple Bond = 6 electrons to each atom

Quadruple Bond = 8 electrons to each atom

Rules for Drawing Lewis Structures

Step 1: Count the total number of valance electrons.

Step 2: Identify the central atom (the first atom written unless that atom

is hydrogen). Place all terminal atoms around that atom.

Hydrogen atoms NEVER have more than one bond.

Step 3: Complete the octet for all atoms in the Lewis structure with

lone pairs of electrons (except hydrogen).

Step 4: Check your structure by counting the number of valance

electrons used (they will match step 1 if the structure is

correct). If your valance electrons dont match you will need

to tweak your structure as follows.

Step 1

Count the number of valence electrons available to each atom in the compound.

Example: CH4

C = 4 valence electrons

H = 1 valence electron -> 4 ve

Step 2

Identify the central atom. This is usually the first atom written.

Exception: Hydrogen. If that is the first atom written, then use the second atom.

Example: CH4

Central atom: C

Draw

Draw each component as a (Lewis) dot diagram.

H

H

H

H

C

Step 3: The Trading Game

Each atom has to put in one electron per bond, but it gets to count the bond as 2 electrons.

So by contributing an electron, it essentially doubles its investment.

Draw

Remember: Hydrogen is the exception to the octet rule. It only needs 2 electrons to be happy.

HH

H

HC

Step 4: Check Math

Make sure that each atom in your new lewis dot structure feels like it has eight electrons around it.

Remember: One bond feels like two electrons to EACH element it borders.

Math Check

HHHHC

22228

Check Math

If there are atoms whose octet rules are not satisfied, you may need to increase the number of bonds between atoms.

Tweaking Lewis Structures

Too Many Electrons Initially: Redraw the

Lewis structure from step 2 adding a double bond. If you still have too many electrons add another multiple bond and repeat.

Always add double bonds before triple bonds. Every double bonds effectively remove two electrons from the structure while triple bonds effectively remove 4 electrons.

Lewis Structures

CO2 carbon dioxide

O = C = O

8

8

8

Lewis Structures

CO carbon monoxide

: C O :

Rules for Molecules with an Overall Charge

When figuring out the number of electrons available, make sure to add or subtract as indicated by the charge.

Create the Lewis structure the same as always.

Put square brackets [ ] around the structure.

Write the charge in a superscript.

Called a Coordinate Covalent Bond

Lewis Structures Charged Species

A species that has a - charge has a shortage of bonds over the normal number

If a species has received electrons from elsewhere, it does not have to share as many electrons

Therefore less bonds have to be made

Lewis Structures Charged Species

A species that has a + charge has an excess of bonds over the normal number.

if a species has given up some electrons, it has to involve more of the electrons it has kept

Therefore more bonds have to be made

Lewis Structures Charged Species

The size of the - or + charge tells you the shortage or excess of bonds

+2 = 2 extra bonds; -3 = 3 bond shortage)

Coordinate Covalent Bond

ammonium ion

Coordinate Covalent Bond

hydronium ion

Exceptions to the Octet

Two Few Electrons: If you only have four or

six valance electrons initially you cant

possibly fill the octet rule (usually BeH2 or

BH3). Just place hydrogens around central

atoms and call it done.

Exceptions to the Octet Rule (That are not H)

There are two other exceptions to the Octet Rule (that are not Hydrogen)

PF5

SF6

Exceptions usually involve F

Exceptions to the Octet

Odd number of electrons: One atom will

have to have less than eight electrons.

Draw the Lewis structure as if it had one

more valance electrons than it actually does.

Then subtract one electron from the least

electronegative element (often the central

atom).

Exceptions to the Octect

Exceeding the Octet Rule: When you must break the octet rule draw the structure as you would in steps 1-4 and the place the extra electrons around the central atom. Even when breaking the octet rule no atom will ever have more than 12 electrons.

Exceptions to Octet Rule

PF5

expanded octet

Exceptions to Octet Rule

SF6

Expanded octet

Diatomic Molecules

In nature, the following elements are always found in a paired molecule. They are never found solo.

H2

O2

F2

Br2

I2

N2

Cl2

I Bring Clay For Our New House

Quiz on these 7 next class period. Memorize them.