Lewis Dot Structures: Electron dot structures of compounds

Q: What is the formula for water? H2O

Q: How many valence electrons does each hydrogen have?1

Q: How many valence electrons does the oxygen have? 6

Q: How many valence electrons are there in each water molecule?#VE = 2H + 1 oxygen =2(1) + 6 = 8

More examples

1. How many VE in H2O2? #VE = 2(1) + 2(6) = 14H O

2. How many VE in H2SO4? #VE = 2(1) + 6 + 4(6) = 32H S O

Simple Lewis Dot Structures1. Determine the central atom. This is usually the atom that you

have the least of in the formula.

H2O BF3 NCl3 SiF4C2H6Ex:

2. Draw the electron dot structure of the central atom.

•N•••

••B•

••O•••

•••C C•

• •

• •

•• •Si•

•

•

3. Wherever there are unpaired electrons on the central atom, pair them up with unpaired electrons on the attached atoms.

O••

••H●H●• ●H●H• B

••••●F●●F●•

••••••••••

●F●●F●••••••

•••

●F●●F●••••••

••N••

••••

••●Cl●●Cl●•• ••

●Cl●●Cl●••••

•••••• •

••••

••••●Cl●●Cl●•• ••

C C

••H●H● ●H●H••

••HH

••HH

••HH ••HH Si••••

●F●●F●•••••

••••••●F●●F●••

•••••••

●F●●F●••••••

••

••●F●●F●

••••••••

4. Finally, check your electron count.

H2O: 2(1)+6 = 8

8

BF3: 3 + 3(7) = 24

24

NCl3: 5 + 3(7) = 26

26

C2H6: 2(4) + 6(1) = 14

SiF4: 4 + 4(7) = 32

14 32

What to do when you end up with unpaired electrons:

Example: Draw the Lewis structure for O2

•O••

•••

•O•••

• •

#VE = 2(6) = 12

Pair up the remaining unpaired electrons between the two oxygen atoms.

Because each shared pair of electrons represents a chemical bond, there is a double bond between the oxygens in O2.

What would the Lewis structure of N2 look like?

•N•••

•N•••

•• N•

••• N

•••• •• N

••N••

Note: 1 stick = 2 electrons in a bond

••O

•• O

••

••

The ‘Keep ‘em Happy’ approach to Lewis structures:

1. Add up the number of valence electrons present

2. Draw the stick-skeleton of the molecule

3. Satisfy the octet rule for all atoms in the molecule Exceptions: H only needs 2e- and B only needs 6e-

4. Count up the number of electrons present in the Lewis structure

- If there aren’t enough e- (the molecule is ‘unhappy’), add the missing electrons to the central atom

- If there are too many e- (the molecule is ‘too happy’), take the excess away from the central atom and then form double bonds with the terminal atoms to satisfy the octet.

Examples:

SF4

#VE = 34 SF F

F F

32 34

This is the only possible structure because Fluorine never forms double bonds

SO2

SO O#VE = 18

20 18

Note: 1 stick = 2 electrons in a bond

This molecule is too happy, take away an electron pair from the central atom… but now sulfur is unhappy with only 6 e-

Make sulfur happy by using one of the pairs on oxygen to form a double bond.

This molecule isn’t ‘happy’ because it doesn’t have enough electrons.