1BONDING & BONDING & MolecularGeometMolecularGeomet

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BONDING & BONDING & MolecularGeometMolecularGeomet

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Chemistry I – Chapter 8Chemistry I – Chapter 8

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2Bonding & Bonding & GeometryGeometry

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

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

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

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

3Review of Chemical Review of Chemical BondsBonds

• There are 2 forms of bonding:There are 2 forms of bonding:

• __________________—complete —complete transfer transfer of 1 or more electrons from one of 1 or more electrons from one atom to another (one loses, the atom to another (one loses, the other gains) forming oppositely other gains) forming oppositely charged ions that attract one charged ions that attract one anotheranother

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

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 calculated by be calculated by finding the difference in electronegativity of finding the difference in electronegativity of

the two atoms that are going together.the two 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 Covalent

Example: NaClNa = 0.9, Cl = 3.0Difference is 2.1, sothis is an ionic bond! HCl is polar covalentCl2 is n-p covalent

6

Ionic BondsIonic BondsIonic BondsIonic BondsAll those ionic compounds were made All those ionic compounds were made

from ionic bonds. We’ve been from ionic bonds. We’ve been through this in great detail already. through this in great detail already. Positive cations and the negative Positive cations and the negative anions are attracted to one another anions are attracted to one another (remember the Paula Abdul Principle (remember the Paula Abdul Principle of Chemistry: Opposites Attract!)of Chemistry: 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

8Review of Valence Review of Valence ElectronsElectrons

Review of Valence Review of Valence ElectronsElectrons

• Remember from the electron chapter Remember from the electron 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?

9

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.

11Steps for Building a Dot Steps for Building a Dot StructureStructureSteps for Building a Dot Steps for Building a Dot StructureStructure

Ammonia, NHAmmonia, NH33

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

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

Therefore, N is central on this oneTherefore, N is central on this one

Then, Go to the CAR!Then, Go to the CAR!

1. Count valence e-1. Count valence e-

2. Arrange octets2. Arrange octets

3. Recount electrons3. Recount electrons

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Count Valence Electrons

Add up the number of valence electrons that Add up the number of valence electrons that can be used. can be used. NHNH33

N = 5N = 5

H = 1 times 3 H = 1 times 3

Total = 5 + (3 x 1) Total = 5 + (3 x 1)

= 8 electrons / 4 pairs= 8 electrons / 4 pairs

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Arrange OctetsRemember Hydrogen only needs 2!

H••

H

H

N

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Recount Electrons

2, 4, 6, 8 electrons total. It works!

CCheck the number of electrons in your heck the number of electrons in your drawing with the number of electrons drawing with the number of electrons from step 2. If you have more electrons from step 2. If you have more electrons in the drawing than in step 2, you must in the drawing than in step 2, you must make double or triple bonds.make double or triple bonds. H

••

H

H

N

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In Other Words, Go To The CAR

• Count valence electrons

• Arrange octets

• Recount electrons to make sure they match the valence electrons

If you go to the CAR, you will be in the driver’s seat!

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

1. Count valence e-1. Count valence e-

2. Arrange electrons2. Arrange electrons

3. Recount electrons3. Recount electrons

O OC Place lone pairs on outer atoms.Place lone pairs on outer atoms.

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

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

18Double and Double and even triple even triple bonds are 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

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

B: 6B: 6((remember this)remember this)

Be: 4Be: 4

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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Lewis Dot Structures

HOT or not?

21

Definitely HOT!

22

MOLECULAR MOLECULAR GEOMETRYGEOMETRY

MOLECULAR MOLECULAR GEOMETRYGEOMETRY

23

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

SOSO22

(hint: go to the CAR!)(hint: go to the CAR!)

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

SOSO22

(hint: go to the CAR!)(hint: go to the CAR!)

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SOSO221. Count valence electrons

S = 6 O = 6(2)

6 + 12 = 18 electrons

2. Arrange octets

O S O →

3. Recount electrons

18 = 18

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VSEPRVSEPR • VValence alence SShell hell EElectron lectron PPair air

RRepulsion theory.epulsion theory.

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

26

Some Common GeometriesSome Common Geometries

LinearLinear

Trigonal PlanarTrigonal Planar TetrahedralTetrahedral

27VSEPR chartsVSEPR charts

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

• Electron arrangement establishes the bond anglesElectron arrangement establishes the bond angles

• Molecule takes the shape of that portion of the electron Molecule takes the shape of that portion of the electron arrangementarrangement

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29Molecular Geometry  

 Recall that atoms share electrons with other atoms, they do so in pairs. A pair of electrons (a bond) in overlapping orbitals holds two atoms together. The bond between two nitrogen atoms creates a diatomic nitrogen molecule.

(Linear geometry)

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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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Determine the Shape by Determine the Shape by Using VSEPRUsing VSEPR

Determine the Shape by Determine the Shape by Using VSEPRUsing VSEPR

Ammonia, NHAmmonia, NH33

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

32

Boron Trichloride

Because boron is stable with 6 valence electrons and the electrons repel each other it makes a trigonal planar geometry.

33

Methane CH4in 3 dimensions methane is a tetrahedral geometry

34

Diatomic ElementsDiatomic Elements

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

–Hydrogen - HH22

–Nitrogen - NN22

–Oxygen

–Fluorine

–Chlorine

–Bromine

– Iodine

– Are these molecules polar, nonpolar or ionic? Why?

35

Bond PolarityBond PolarityBond PolarityBond PolarityHCl is HCl is POLARPOLAR because it because it

has a positive end and a has a positive end and a negative end. (difference negative end. (difference in electro-negativity of .3 in electro-negativity of .3 or more)or more)

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

••

••

+ -••

36

Bond Polarity and Molecular Polarity

Some molecules have polar bonds but are part of nonpolar molecules. And there are some nonpolar molecules with polar bonds.

Molecular polarity is important because it predicts how molecules will behave, since the charges influence how the molecules will interact with each other.

37Molecular PolarityBonds are polar when one atom is positive and the other negative. Molecules with many atoms have polarity, with one end positive, the other negatively charged.

You can predict the polarity of the molecule by looking at the ends of the molecule to see if it has a positive end and a negative end. Lone pairs of electrons are negative while hydrogen atoms or other low electronegativity atoms tend to be slightly positive.

Polar = positive end and negative end

Nonpolar = same charge at both ends

38Molecular Molecular PolarityPolarity

How can you determine if a molecule is polar?

Look for lone pairs of electrons at one end (-)

And hydrogens (+) at the other end.

39

 

Molecular Polarity 

In water, the electron dot formula shows the 6 oxygen valence electrons and the 2 hydrogen electrons bonding to make a bent molecule. In three dimensions, this molecule has length, width and depth. The electrons repel each other (valence shell electron pair repulsion, VSEPR theory) into a bent shape. Because one end is positive and the other end is negative, it is polar.

40Molecular Polarity of Methane CH4

The Lewis dot structure shows the carbon atom surrounded by eight electrons, and baby hydrogen with 2 electrons each. The 3-D geometry looks very different with the electron pairs repelling into a tetrahedron. Because there are hydrogens at each end, it is nonpolar. There is no negative- positive dipole.

41

• 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

Molecular Molecular PolarityPolarity

Molecular Molecular PolarityPolarity

42Molecular Polarity Molecular Polarity ExamplesExamples

Molecular Polarity Molecular Polarity ExamplesExamples

• ““Like Dissolves Like”Like Dissolves Like”

–Polar dissolves PolarPolar dissolves Polar

–Nonpolar dissolves NonpolarNonpolar dissolves Nonpolar

43

Hot or Not?

44

Totally Sizzling!

45

The End