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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.

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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.

3Review 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?

4Steps 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

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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.

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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

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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

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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?

9Double 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

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Now You Try One!Now You Try One!Draw PH3Draw PH3

Now You Try One!Now You Try One!Draw PH3Draw PH3

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Sigma () and Pi () bond

sigma bond - direct orbital overlap between the two nuclei.

pi bond has orbital overlap off to the sides of the line joining the two nuclei.

Sigma bonds are stronger than pi.

QuickTime™ and aTIFF (Uncompressed) decompressor

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12Violations of the Octet Violations of the Octet RuleRule

Violations of the Octet Violations of the Octet RuleRule

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

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MOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRYMOLECULAR GEOMETRY

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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

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Some Common GeometriesSome Common Geometries

LinearLinear

Trigonal PlanarTrigonal Planar TetrahedralTetrahedral

16VSEPR 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)

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Other VSEPR chartsOther VSEPR charts

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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

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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..

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Hybridization

• The orbitals mix to give hybrid orbitals

• The number of atomic orbitals that mix and form the hybrid orbitals are equal to the total number of electron pairs

• 2 electron pairs=sp hybrid

• 3 electron pairs=sp2 hybrid (one s, 2 p orbitals)

• 4 electron pairs =sp3 hybrid (one s, 3 p)

• 5 electron pairs= sp3d hybrid

• 6 electron pairs = sp3d2 hybrid

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Molecular Models Lab Chemical formulas

Lewis Structure

Effective pairs

Bond pairs

Lone pairs

Molecular Geometry(Draw)

Hybridization

Name of Shape

H2

O2

N2

H2O

PH3

CF4

CO2

PCl5

HCN

CO

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

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 (+ (+ ) - polar covalent ) - polar covalent bondbond

H Cl••

••

+ -••H Cl

••

••

+ -••

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Polar and Non-polar molecules

• Bond can be polar but because of symmetry of molecule, polarity cancels out. Ex- CCl4

• Bond is polar but molecule is not symmetrical- then molecule stays polar. Ex- H2O

• Bonds are not polar due to same electronegativity- molecule is nonpolar. Ex- CH4.

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QuickTime™ and aTIFF (Uncompressed) decompressorare needed to see this picture.

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Electronegativity difference and bond polarity

Electronegativity difference

Bond character

>1.7 Mostly ionic

0.4- 1.7 Polar covalent

<0.4 Mostly covalent

0 Nonpolar covalent

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Like dissolves like

• Polar solvents dissolve polar solutes-

• Example: Salt in water= Na+ Cl-

• Non-polar solutes dissolve in nonpolar solvents

• Example: Oil in CCl4

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• 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

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Bond PolarityBond PolarityBond PolarityBond Polarity

• ““Like Dissolves Like”Like Dissolves Like”

–Polar dissolves PolarPolar dissolves Polar

–Nonpolar dissolves Nonpolar dissolves NonpolarNonpolar

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Polarity Practice Worksheet• For each of the following pairs,

determine which is most polar based on Lewis structure and electronegativity.

• 1. CHCl3 or CHBr3

• 2. H2O or H2S• 3. HCl or HI• 4. C2HBr or C2HCl• 5. CH3OH or CH3OCH3

• 6. CH3C=OCH3 or C3H8O

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Molecular Bonding Practice

• For each of the following molecules:» 1. H2S 2. NCl3 3. CHF3

»a. Draw the Lewis dot structure and the geometry of the molecule

»B. Draw the dipoles and determine the overall polarity of the molecule

»C. What is the hybridization of the central atom?

»D. Give the number of sigma and pi bonds

»E. Build the molecule using your model kits.

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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