Drawing Lewis Structures

1

Drawing Lewis Structures

A step-by-step guide

See page 305

2


1) Count the total valence electrons for the molecule: To do this, find the number of valence electrons for each atom in the molecule, and add them up.

Example: CO2

Valence electrons: C = 4, (O = 6)x2 16Total number of valence electrons is important, not

where they came from.

To start: Draw Lewis symbols of all individual atoms in formula

3


2a) Write the symbols for each atom

b) show which atom is attached to which • 2 elements: central atom is first• more than 2: order of connected atoms as written in

formula

c) Use a single dash to show bonds between atoms:

CO2: 2 atoms, C is centrals, both O are attached to C by single bond

O—C—O

4


3) Complete octets around all atoms bonded to central atom by adding electrons.

(H does not get an octet!)

CO2: .. ..

: O – C – O :

.. ..

Count electrons now: 16 valence electrons shown, both O have octet, C?

5


4) Any leftover valence electrons after step 3?

Place them on the central atom.

Even if doing so gives that atom more than 8 electrons (there are exceptions to octet rule)

CO2: no leftover valence electrons

6


5) If central atom does not have an octet of electrons after step 4:

Try multiple bonds!

Use unshared electrons already shown in step 3 and 4 and move them between central atom and other atoms.

CO2O = C = O¨¨

¨¨

7


6)Some handy bond rules to remember for molecules:

– Hydrogen and the halogens (F, Cl, Br, I) form a single bond.– The family oxygen (O, S, Se) forms 2 single bonds (or 1 double

bond)– The family nitrogen (N, P, As) forms 3 single bonds, or 1 single

bond and a double bond or a triple bond. So does boron. – The family carbon (C, Si) forms 4 single bonds, or 2 double

bonds, or…..

• A good thing to do is to bond all the atoms together by single bonds, and then add the multiple bonds until the rules above are followed.

8

Drawing Lewis StructuresDraw the Lewis structure for CHCl3.•# of Valence electrons = 4 + 1 + 3x(7) = 26•Draw all atoms, connected by single dash (single bond, 2 electrons).•Place electrons around atoms bonded to central atoms for octet.

C = central atom

Cl

H C Cl

Cl

9

Drawing Lewis StructuresDraw the Lewis structure for PCl3•# valence electrons = 5 + 3x(7) = 26•Draw all atoms, connected by single dash (single bond, 2 electrons). 2 atoms: P is central.•Place electrons around atoms bonded to central atoms giving them a total of 8 electrons.•Count electrons (have 24 now), place any extra valence electrons on central atom

Cl

Cl—P—Cl

10

Lewis Structures of Ions

1) Count the total valence electrons for the molecule: To do this, find the number of valence electrons for each atom in the molecule, and add them up. For polyatomic anions, add the charge of the ion to the number of valence electrons. For polyatomic cations, subtract the charge of the ion from the number of valence electrons.

PO43- # valence electrons = 5 + 4x(6) + 3 = 32

11

Lewis Structures of Ions

2) Draw an arrangement of the atoms for the molecule that follows the rules above, central atom, surrounded by others, connected by single bonds.– Hydrogen and the halogens bond once. – The family oxygen is in bonds one, two, or three

times. – The family nitrogen is in bonds two, three, or four

times – Boron usually bonds four times. – The family carbon is in bonds four times.

12


Draw the Lewis structure for PO43-.

# valence electrons = 5 + 4(6) + 3 = 32

P = central atom

O

O P O

O

3-Don’t forget to show the charge of the ion, too.

13


Example: Draw the Lewis structure for NO2-

• Valence electrons: 5 + 2x6 + 1(for 1- charge) = 18• Central atom is:• Connect with single bonds, add octet electrons to

O:

• Count electrons, add extra to central atom.• Check for octets on all, form double bonds if

needed.

:O-N-O:...... ..

14


Draw the Lewis structure of formic acid, HCO2H. (Hint: C is the central atom. One of the H is bonded to an O.)

15


• How do we know which of the possible structures for HCO2H is more likely to be correct?

Formal Charge: a calculated value that is used to compare the number of valence electrons to the # of electrons assigned in a Lewis structure

16

Writing Lewis Structures

• assign 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.

17

Writing Lewis Structures

• The best Lewis structure…– …is the one with the fewest charges.– …puts a negative charge on the most

electronegative atom.

18


Example: Assign formal charges to each of the Lewis structures drawn for HCO2H.

H C O H

O

H C O H

O

19


• Some compounds cannot be adequately described by a single Lewis structure.

• Ozone (O3):

O

O O

This single structure implies that the two O-O bonds should be different.

They are not!

20

Resonance

This is the Lewis structure we would draw for ozone, O3.

-

+

21

Resonance

• But this is at odds with the true, observed structure of ozone, in which…– …both O-O bonds are

the same length.– …both outer oxygens

have a charge of -1/2.

22

Resonance

• One Lewis structure cannot accurately depict a molecule like ozone.

• We use multiple structures, resonance structures, to describe the molecule.

23

Resonance

Just as green is a synthesis of blue and yellow…

…ozone is a synthesis of these two resonance structures.

24


• These two structures are equivalent except for the placement of electrons.– Resonance structures

• Resonance structures for ozone:

O O

O O O O

25


• Resonance structure: one of a group of Lewis structures used to describe a molecule that cannot be accurately depicted using a single Lewis structure

• NOTE: The real molecule is a “hybrid” or average of the resonance structures. It does not “flip” back and forth between the possible structures.

26


Example: Draw all possible resonance structures for NO3

-.

# valence electrons = 5 + 3(6) + 1 = 24

O

N

O O

O

N

O O

- - -O

N

O O

- -

27


Example: Draw all possible resonance structures (that follow the octet rule) for SO2.

28

Resonance

• 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.

29


• Not all compounds follow the octet rule!

• Exceptions to the octet rule:

– Molecules with an odd # of electrons• NO (5 + 6 = 11 electrons)

30

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– Ions or molecules with more than eight

valence electrons (an expanded octet)

31

Odd Number of Electrons

Though relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of electrons.

e.g.: NO (5 + 6 = 11 valence electrons)

Look at formal charges to find preferred structure (least number of formal charges

32



– Molecules in which an atom has less than an octet.

F

B

F F

33

Fewer Than Eight Electrons

• Consider BF3:– Giving boron a filled octet places a negative

charge on the boron and a positive charge on fluorine.

– This would not be an accurate picture of the distribution of electrons in BF3.

34

Fewer Than Eight Electrons

Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.

35



– Molecules in which an atom has more than an octet.

Cl

Cl P Cl

Cl Cl

36

More Than Eight Electrons

• 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.

37


Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.

38


• This eliminates the charge on the phosphorus and the charge on one of the oxygens.

• The lesson is: when the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.

Drawing Lewis Structures

Documents

Transcript of Drawing Lewis Structures