CHEMICAL CHEMICAL BONDINGBONDING

Set 3Set 3

CHEMICAL CHEMICAL BONDINGBONDING

Set 3Set 3

Cocaine

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Credits

• Thank you to Mr. Neil Rapp who provided the bulk of this powerpoint on his website www.chemistrygeek.com

• Other information comes from Zumdahl, Steven, and Susan Zumdahl. Chemistry. Boston: Houghton Mifflin, 2003.

Lewis Lewis StructuresStructures

Lewis Lewis StructuresStructures

• Electron distribution Electron distribution is depicted withis depicted with Lewis (dot) Lewis (dot) structuresstructures

• This is how you This is how you decide how many decide how many atoms will bond atoms will bond covalently! covalently!

(In ionic bonds, it (In ionic bonds, it was decided with was decided with charges)charges)

G. N. Lewis G. N. Lewis 1875 - 19461875 - 1946

Bond and Lone Bond and Lone PairsPairs

Bond and Lone Bond and Lone PairsPairs

• Valence electrons are distributed as Valence electrons are distributed as shared orshared or BOND PAIRSBOND PAIRS and unshared and unshared oror LONE PAIRS.LONE PAIRS.

•

••

•

••

H Cllone pair (LP)

shared orbond pair

This is called a This is called a LEWIS LEWIS structure.structure.

Bond FormationBond FormationBond FormationBond Formation

A bond can result from anA bond can result from an overlapoverlap of of atomic orbitals on neighboring atoms.atomic orbitals on neighboring atoms.

ClH H Cl••

••

••

••

••

••

+

Overlap of H (1s) and Cl (2p)Note that each atom has a single, unpaired electron.Note that each atom has a single, unpaired electron.

Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

• Remember from the quantum chapter Remember from the quantum chapter that valence electrons are the that valence electrons are the electrons in the OUTERMOST energy electrons in the OUTERMOST energy level… that’s why we did all those level… that’s why we did all those electron configurations!electron configurations!

• B is 1sB is 1s22 2s 2s22 2p 2p11; so the outer energy ; so the outer energy level is 2, and there are 2+1 = 3 level is 2, and there are 2+1 = 3 electrons in level 2. These are the electrons in level 2. These are the valence electrons!valence electrons!

• Br is Br is [Ar] 4s[Ar] 4s22 3d 3d1010 4p 4p55

How many valence electrons are How many valence electrons are present?present?

Review of Valence ElectronsReview of Valence ElectronsReview of Valence ElectronsReview of Valence ElectronsNumber of valence electrons of a main (A) Number of valence electrons of a main (A)

group atom = Group numbergroup atom = Group number

Steps for Building a Dot StructureSteps for Building a Dot StructureSteps for Building a Dot StructureSteps for Building a Dot StructureAmmonia, NHAmmonia, NH33

1. Decide on the central atom; never H. 1. Decide on the central atom; never H. Why?Why?

If there is a choice, the central atom is If there is a choice, the central atom is atom of lowest affinity for electrons. atom of lowest affinity for electrons. (Most (Most of the time, this is the of the time, this is the least electronegative atomleast electronegative atom…in advanced …in advanced chemistry we use a thing called formal charge to determine the chemistry we use a thing called formal charge to determine the central atom. But that’s another story!)central atom. But that’s another story!)

Therefore, N is central on this oneTherefore, N is central on this one2. Add up the number of valence electrons that 2. Add up the number of valence electrons that

can be used.can be used. H = 1 and N = 5H = 1 and N = 5 Total = (3 x 1) + 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs= 8 electrons / 4 pairs

3.3. Form a single bond between Form a single bond between the central atom and each the central atom and each surrounding atom (each surrounding atom (each bond takes 2 electrons!)bond takes 2 electrons!)

H H

H

N

Building a Dot StructureBuilding a Dot Structure

H••

H

H

N

4.4. Remaining electrons form LONE PAIRS to Remaining electrons form LONE PAIRS to complete the octet as needed (or duet in the case of complete the octet as needed (or duet in the case of H).H).

3 BOND PAIRS and 1 LONE PAIR.3 BOND PAIRS and 1 LONE PAIR.

Note that N has a share in 4 pairs (8 electrons), while H shares 1 Note that N has a share in 4 pairs (8 electrons), while H shares 1 pair.pair.

5.5. Check to make sure there are 8 Check to make sure there are 8 electrons around each atom electrons around each atom except H. H should only have 2 except H. H should only have 2 electrons. This includes SHARED electrons. This includes SHARED pairs. pairs.

Building a Dot StructureBuilding a Dot Structure

6. 6. Also, check the number of electrons in your drawing with the number of Also, check the number of electrons in your drawing with the number of electrons from step 2. electrons from step 2. If you have more electrons in the drawing than in If you have more electrons in the drawing than in step 2, you must make double or triple bonds. If you have less electrons in step 2, you must make double or triple bonds. If you have less electrons in the drawing than in step 2, you made a mistake!the drawing than in step 2, you made a mistake!

H••

H

H

N

Carbon Dioxide, COCarbon Dioxide, CO22Carbon Dioxide, COCarbon Dioxide, CO22

1. Central atom = 1. Central atom =

2. Valence electrons =2. Valence electrons =

3. Form bonds.3. Form bonds.

O OC4. Place lone pairs on outer atoms.4. Place lone pairs on outer atoms.

This leaves 12 electrons (6 pair).This leaves 12 electrons (6 pair).

5. Check to see that all atoms have 8 electrons around it except for H, which can have 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.2.

C 4 e-C 4 e-O 6 e- X 2 O’s = 12 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electronsTotal: 16 valence electrons

Carbon Dioxide, COCarbon Dioxide, CO22Carbon Dioxide, COCarbon Dioxide, CO22

••O OC

•• ••

••••••

••O OC

•• ••

••••••

••O OC

•• ••

••

••O OC

•• ••

••

6. There are too many electrons in our drawing. We 6. There are too many electrons in our drawing. We must form DOUBLE BONDS between C and O. must form DOUBLE BONDS between C and O. Instead of sharing only 1 pair, a double bond shares 2 Instead of sharing only 1 pair, a double bond shares 2 pairs. So one pair is taken away from each atom and pairs. So one pair is taken away from each atom and replaced with another bond.replaced with another bond.

C 4 e-C 4 e-O 6 e- X 2 O’s = 12 e-O 6 e- X 2 O’s = 12 e-Total: 16 valence electronsTotal: 16 valence electrons

How many are in the drawing?How many are in the drawing?

Double and even Double and even triple bonds are triple bonds are commonly commonly observed for C, observed for C, N, P, O, and SN, P, O, and S

••O OC

•• ••

••

••O OC

•• ••

••

HH22COCO

SOSO33

CC22FF44

Now You Try One!Now You Try One!Draw Sulfur Dioxide, SODraw Sulfur Dioxide, SO22

Now You Try One!Now You Try One!Draw Sulfur Dioxide, SODraw Sulfur Dioxide, SO22

Homework Questions• 1) Does a Lewis structure tell which electrons come

from which atoms? Explain.• 2) Give a rationalization for the octet rule in terms of

orbitals.• 3) Write Lewis structures that obey the octet rule for

each of the following:A. HCN B. PH3 C. CHCl3 D. NH4

+

E. H2CO F. SeF2 G. CO2 H. O2

I. HBr– Except for HCN and H2CO, the first atom listed is the

central atom. For HCN and H2CO, carbon is the central atom.