1

CHEMICAL CHEMICAL BONDINGBONDINGCHEMICAL CHEMICAL BONDINGBONDING

Cocaine

SAVE PAPER AND INK!!! When you print out the notes on PowerPoint,

print "Handouts" instead of "Slides" in the print setup. Also,

turn off the backgrounds (Tools>Options>Print>UNcheck

"Background Printing")!

2Chemical Chemical BondingBonding

Problems and questions —Problems and questions —

How is a molecule or How is a molecule or polyatomic ion held polyatomic ion held together?together?

Why are atoms distributed Why are atoms distributed at strange angles?at strange angles?

Why are molecules not flat?Why are molecules not flat?

Can we predict the Can we predict the structure?structure?

How is structure related to How is structure related to chemical and physical chemical and physical properties?properties?

3

Review of Chemical BondsReview of Chemical Bonds• There are 3 forms of bonding:There are 3 forms of bonding:• __________________—complete —complete transfer transfer of 1 or more electrons from of 1 or more electrons from one atom to another (one one atom to another (one loses, the other gains) loses, the other gains) forming oppositely charged forming oppositely charged ions that attract one anotherions that attract one another

• __________________——some valence some valence electrons electrons sharedshared between between atomsatoms

• __________________ – holds atoms of a – holds atoms of a metal togethermetal together

Most bonds are Most bonds are somewhere in somewhere in between ionic between ionic and covalent.and covalent.

4The type of bond can The type of bond can usuallyusually be be

calculated by finding the difference calculated by finding the difference in electronegativity of the two in electronegativity of the two atoms that are going together.atoms that are going together.

5Electronegativity Difference

• If the difference in electronegativities is between:

– 1.7 to 4.0: Ionic

– 0.3 to 1.7: Polar Covalent

– 0.0 to 0.3: Non-Polar CovalentExample: NaClNa = 0.8, Cl = 3.0Difference is 2.2, sothis is an ionic bond!

6

Ionic BondsIonic BondsIonic BondsIonic BondsAll those ionic compounds were All those ionic compounds were made from ionic bonds. We’ve made from ionic bonds. We’ve been through this in great been through this in great detail already. Positive detail already. Positive cations and the negative cations and the negative anions are attracted to one anions are attracted to one another (remember the another (remember the Opposites Attract!)Opposites Attract!)

Therefore, ionic Therefore, ionic compounds are usually compounds are usually between metals and between metals and nonmetals (opposite ends nonmetals (opposite ends of the periodic table).of the periodic table).

7

Electron Electron Distribution Distribution in Moleculesin Molecules

Electron Electron Distribution Distribution in Moleculesin Molecules

• Electron distribution is Electron distribution is

depicted withdepicted with Lewis Lewis (electron dot) (electron 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

8

Bond and Lone Bond and Lone PairsPairs

Bond and Lone Bond and Lone PairsPairs

• Valence electrons are distributed Valence electrons are distributed as shared oras shared or BOND PAIRSBOND PAIRS and and unshared orunshared or LONE PAIRS.LONE PAIRS.

•

••

•

••

H Cllone pair (LP)

shared orbond pair

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

9

Bond FormationBond FormationBond FormationBond FormationA 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, Note that each atom has a single, unpaired electron.unpaired electron.

10Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

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

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

11Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

Number of valence electrons of a Number of valence electrons of a main (A) group atom = Group numbermain (A) group atom = Group number

12Steps for Building a Dot Steps for Building a Dot StructureStructureSteps for Building a Dot Steps for Building a Dot StructureStructureAmmonia, 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 of the time, this is the (Most of the time, this is the least least electronegative atomelectronegative atom…in advanced chemistry we use …in advanced chemistry we use a thing called formal charge to determine the 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 one

2. Add up the number of valence 2. Add up the number of valence electrons that can be used.electrons that 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

13

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

H H

H

N

Building a Dot Building a Dot StructureStructure

H••

H

H

N4.4. Remaining electrons form Remaining electrons form LONE PAIRS to complete the LONE PAIRS to complete the octet as needed (or duet in octet as needed (or duet in the case of H).the case of H).3 BOND PAIRS and 1 LONE 3 BOND PAIRS and 1 LONE PAIR.PAIR. Note that N has a share in 4 pairs Note that N has a share in 4 pairs (8 electrons), while H shares 1 (8 electrons), while H shares 1 pair.pair.

14

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

Building a Dot Building a Dot StructureStructure

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

H••

H

H

N

15

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 4. Place lone pairs on outer atoms.atoms.

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

5. Check to see that all atoms have 8 electrons 5. Check to see that all atoms have 8 electrons around it except for H, which can have 2.around it except for H, which can have 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

16

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

17Double and Double and even triple even triple bonds are bonds are commonly commonly observed for observed for C, N, P, O, C, N, P, O, and Sand S

••O OC

•• ••

••

••O OC

•• ••

••

HH22COCO

SOSO33

CC22FF44

18

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

19Violations of the Octet Violations of the Octet RuleRule

(Honors only)(Honors only)

Violations of the Octet Violations of the Octet RuleRule

(Honors only)(Honors only)Usually occurs with B and elements Usually occurs with B and elements of higher periods. Common of higher periods. Common exceptions are: Be, B, P, S, and Xe. exceptions are: Be, B, P, S, and Xe.

BF3BF3

SF4SF4

Be: 4Be: 4

B: 6B: 6

P: 8 OR 10P: 8 OR 10

S: 8, 10, OR 12S: 8, 10, OR 12

Xe: 8, 10, OR 12Xe: 8, 10, OR 12

20

MOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRY

21

VSEPRVSEPR •VValence alence SShell hell EElectron lectron

PPair air RRepulsion theory.epulsion theory.

• Most important factor Most important factor in determining geometry in determining geometry is relative is relative repulsion repulsion between electron pairs.between electron pairs.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

Molecule adopts Molecule adopts the shape that the shape that minimizes the minimizes the electron pair electron pair repulsions.repulsions.

MOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRY

22

Some Common GeometriesSome Common Geometries

LinearLinear

Trigonal PlanarTrigonal Planar TetrahedralTetrahedral

23VSEPR chartsVSEPR charts

• Use the Lewis structure to determine the Use the Lewis structure to determine the geometry of the moleculegeometry of the molecule

• Electron arrangement establishes the bond Electron arrangement establishes the bond anglesangles

• Molecule takes the shape of that portion of the Molecule takes the shape of that portion of the electron arrangementelectron arrangement

• Charts look at the CENTRAL atom for all data!Charts look at the CENTRAL atom for all data!

• Think REGIONS OF ELECTRON DENSITY rather Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would than bonds (for instance, a double bond would only be 1 region)only be 1 region)

24

25

Other VSEPR chartsOther VSEPR charts

26

Structure Determination by Structure Determination by VSEPRVSEPR

Structure Determination by Structure Determination by VSEPRVSEPR

Water, HWater, H22OOThe electron pair The electron pair geometry is geometry is TETRAHEDRALTETRAHEDRAL

The electron pair The electron pair geometry is geometry is TETRAHEDRALTETRAHEDRAL

The molecular The molecular geometry is geometry is BENTBENT..

The molecular The molecular geometry is geometry is BENTBENT..

H O H••

••

H O H••

••

2 bond 2 bond pairspairs

2 lone 2 lone pairspairs

27

Structure Structure Determination by VSEPRDetermination by VSEPR

Structure Structure Determination by VSEPRDetermination by VSEPRAmmonia, NHAmmonia, NH33

The electron pair geometry is The electron pair geometry is tetrahedral.tetrahedral.

H

H

H

lone pair of electronsin tetrahedral position

N

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

The The MOLECULAR GEOMETRYMOLECULAR GEOMETRY — the — the positions of the atoms — is positions of the atoms — is TRIGONAL TRIGONAL PYRAMIDPYRAMID..

28

Bond PolarityBond PolarityBond PolarityBond PolarityHCl is HCl is POLARPOLAR because because it has a positive end it has a positive end and a negative end. and a negative end. (difference in (difference in electronegativity)electronegativity)

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has a greater share in Cl has a greater share in bonding electrons than bonding electrons than does H.does H.

Cl has slight negative charge Cl has slight negative charge (-(-)) and H has and H has slight positive charge slight positive charge (+ (+ ))

H Cl••

••

+ -••H Cl

••

••

+ -••

29

• This is why oil and water will not mix! Oil This is why oil and water will not mix! Oil is nonpolar, and water is polar.is nonpolar, and water is polar.

• The two will repel each other, and so you The two will repel each other, and so you can not dissolve one in the othercan not dissolve one in the other

Bond PolarityBond PolarityBond PolarityBond Polarity

30

Bond PolarityBond PolarityBond PolarityBond Polarity

• ““Like Dissolves Like”Like Dissolves Like”

–Polar dissolves PolarPolar dissolves Polar

–Nonpolar dissolves Nonpolar dissolves NonpolarNonpolar

31

Diatomic ElementsDiatomic Elements

• These elements do not exist as a single atom; they always appear as pairs

• When atoms turn into ions, this NO LONGER HAPPENS!

– Hydrogen

– Nitrogen

– Oxygen

– Fluorine

– Chlorine

– Bromine

– Iodine

Remember: Remember: BrINClHOFBrINClHOF