1

Bonding: Part TwoThree types of bonds:

•Ionic Bond transfer valence e- (NaCl)

•Metallic bondmobile valence e- (Fe)

•Covalent bond shared valence e- (H2O)

2

Single Covalent Bond

+H H

H-atoms

H H

H2 molecule

Electrons are sharedby the two H atoms

3

Single Covalent Bond

“Structural” Formula

H—H

Single covalent bond (2 shared e-)

4

Ionic vs. Covalent Compounds

Covalent: discrete molecules

H2O, CH4, CO2

Ionic: cations and anions (occupy crystal lattice points)

”formula unit” NaCl, MgI2

5

Covalent BondCovalent bonds usually form between nonmetal atoms (Groups 14, 15, 16, and 17).

Each atom tries to attain the e-

configuration of a noble gas by sharing electrons. (“octet rule”)

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Back to H2

+H H

H-atoms

H H

Each H has e-

config. of He

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Fluorine: F2 or F—F

F F+ F F

Each F has e-

config of [Ne]

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Lone Pairs

F F Each F has an octet

“lone pairs” or “nonbonding pairs” of e-

Paired valence e- not in the bond

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Water (H2O)

H

H

O O H

H

Each atom has Noble Gas configuration

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You Try It!

1.Ammonia (NH3)

2.Chlorine gas (Cl2)

3.Methane (CH4)

Draw dot structures for:

11

Multiple Bonds

Sometimes more than one pair of bonding electrons are needed in a bond to attain a noble gas configuration.

Double bond: two pairs of e-

Triple bond: three pairs of e-

12

Double Bond: O2

O+O

Still no octet, so form a double bond

O O

OO OOor

13

Triple Bond

N N N N

Try nitrogen (N2)

14

Coordinate Covalent Bond

Sometimes both e- in the bond come from just one of the atoms.

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Coordinate Covalent Bond

CO + O C

no octet

O C

e.g. CO

e- pair came from oxygen

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Coordinate Covalent Bond

The ammonium ion, NH4+

NH+ +

H

H

H

N

H

H

H

H

+

no e-

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Dot Structure Rules

e.g. NF31.Arrange the atoms with least

electronegative element in center.The central atom is never hydrogen.

F N FF

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Dot Structure Rules2.Count total valence electrons.

Account for charges in polyatomic ions.

F N FF

5 + 3(7) = 26

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Dot Structure Rules3.Connect atoms with single

covalent bonds. Then complete octets (H has only 2 not 8).

F N FF

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Dot Structure Rules4.If octet rule is not satisfied for the

central atom, try double or triple bonds, using lone pairs from surrounding atoms.

OKF N FF

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Dot Structure HNO3

Step 1. Skeletal structure

O N O H

O

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HNO3Step 2. Number of valence e-

O N O H

O

N, O, H 5 + 3(6) + 1 = 24

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HNO3Step 3. Add bonds and

complete octets

Out of e-, but no octet for N

O N O H

O

24

HNO3Step 4. Add multiple bonds

O N O H

O

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You Try It !!!

Draw the e- dot structures for:1.Hydrogen chloride HCl

2.Hydrogen peroxide H2O2

3.Hydronium ion H3O+

4.Ozone O3

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Lewis Structures

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Resonance: Ozone O3

O O O

O O OEach is called a “resonance structure”. The bonds are equal (~1.5 bond)

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Resonance: Try It !!!

Draw resonance structures for SO2

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Exception to the Octet Rule: Odd # e-

Try to write the dot structure of nitrogen monoxide.

With an odd number of valence electrons (11), it is impossible to have octets around both atoms.

NO is “paramagnetic.”

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Predicting Molecular Shapes

linear triatomic trigonal planar

bent triatomic

trigonalpyramid tetrahedral others

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VSEPR Theory

“Valence-shell e- pair repulsion”

•All valence electron pairs

(bonding & nonbonding pairs)

repel each other.

• Predicts geometry.

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VSEPR Theory

Methane (CH4) is drawn as:

H—C—H

H

H

or C

H

H

H

H

Actually CH4 is 3-D

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Maximum

repulsion of

e- pairs

tetrahedron

C

109.5o

VSEPR Theory

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VSEPR Theory

Ammonia (NH3)

H—N—H

H 3 bonding e- pairs

1 nonbonding e- pair

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Ammonia

tetrahedral

electrons

trigonal pyramidal

atoms

VSEPR Theory

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VSEPR Theory

Water (H2O)

H—O—H

2 bonding e- pairs

2 nonbonding e- pairs

37e-: tetrahedral atoms: bent linear

VSEPR Theory

Water

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1.Draw the Lewis dot structure.

2.Move e- pairs (bonding and lone pairs) as far apart as possible.

3.Treat double and triple bonds as if they were single bonds.

4.Distinguish between shape of e-

pairs and molecular shape

VSEPR: Rules

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Possible Molecular Shapes

See problem set.

linear bent

tetrahedron trigonal pyramid trigonal planar

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Predict the Molecular Shape Hydrogen sulfide: H2S

Carbon tetrachloride: CCl4

Sulfur dioxide: SO2

Sulfur trioxide: SO3

Nitrogen tribromide: NBr3

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Is Breaking a Bond Endo- or Exothermic?

Endothermic! It takes energy to break a bond.

Energy is given off when bonds form (exothermic).

molecule

atoms

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Bond Energy

DH = +435 kJ (per mol)

H—H H + H

The energy needed to break a bond is called “bond dissociation energy” or “bond energy”.

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Bond Bond Energy (kJ)

Bond Length (pm)

H—H 435 74

C—H 393 109

C—C 347 154

C C 657 133

C C 908 121

Bond Dissociation Energy

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Bond Strength vs. Length

Multiple bonds have shorter bond lengths! Why?

C—C C C C Cweakest, strongest,longest shortest

Multiple bonds are stronger than single bonds.

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Bond PolarityCovalent bonds involve sharing e-, however the two bonded atoms don’t always share equally.

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Bond PolarityHow do you know which atom wins the “tug-of war” for the bonding electrons?

The more electronegative one.

Regents Table ‘S’

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Bond PolarityIn some cases neither atom wins.Both atoms have same electronegativity.

Hydrogen

Nitrogen

Oxygen

Chlorine

“Nonpolar”

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Polar Molecules

FH

Bonding electrons shift toward F.

Thus HF is polar.

d+ d-

EN = 2.2 EN = 4.0

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Water

O

H HEN = 2.2

EN = 3.4Each bond in water is polar, and the overall molecule is polar because of its shape(nonsymmetrical).

Distinguish between bond polarity and molecular polarity.

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Nonpolar MoleculesO C O

Even though CO2 has polar bonds,

it is a nonpolar molecule.

Where is the average center of positive charge, and where is the center of negative charge?

CO2

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Summary: Polar Molecules

If only 2 atoms in the molecule: the molecule is polar if the atoms have different electronegativities.

H-Clpolar

Br-Brnonpolar

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Summary: Polar MoleculesIf more than 2 atoms in molecule:

Draw dot structure

If the central atom has• a lone pair or• if the outside atoms are

different, the molecule is polar (not symmetrical).

Summary: Polar Molecules

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CH4 is nonpolar (symmetrical)

CH2F2 is polar (nonsymmetrical)

NH3 is polar (nonsymmetrical)

CH4 CH2F2 NH3

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Polar Molecule: Yes or No?

CHCl3 HI

NI3 Br2

SO3 CI4

Symmetrical nonpolarNonsymmetrical polar

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Bond Polarity & Bond TypeIf the difference in electronegativityis greater than ~2.0, one atom takes all the bonding e- and the compound is ionic.

What types of elements would have a big difference in EN?

Difference in Electronegativity

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0 1.0 2.0 3.0

more ionic

more covalent

H2 HCl NaCl CsFnonpolar polar

covalent ionic

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Identify Bond Types nonpolar covalent polar covalent ionic for these pairs of atoms:

Cs & F P & O H & Br

http://www.wimp.com/chemistrydogs/

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Ionic vs. Covalent Compounds

Ionic Covalent

Unit formula unit molecule

Bond e- transfer e- sharing

Elements metal + nonmetal

nonmetals

D EN > 2.0 < 2.0

State @STP solid S, L, G

M. P. high low

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Warm-up

60

Write the e- dot symbol of calcium chloride.

Define the metallic bond. How does this bonding explain why metals conduct electricity?

Draw dot structure for methane, CH4.

Warm-up

61

What is the Lewis dot structure of PH3 ?

Warm-up:

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Write dot structure:

• ammonium ion

• nitrate ion

Warm-up

63

Draw the e- dot structure for NO2and draw its resonance structures.

Warm-up

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Predict the shape of: SeO3 CS2 PCl3 NO2

Warm-up

65

What is the molecular shape of:NBr3

SCl2

CO3-2

Warm-up

66

Write the dot formula for the bromine atom and the bromide ion.

What is the shape of the nitrate ion?

Warm-up

67

Draw the correct dot structure and state whether the molecule is polar or nonpolar?

• SO2

• PBr3

• CH2Br2

Warm-up

68

Draw a diagram showing at least 12 atoms of metallic potassium in the solid state.

Write the dot formula for sodium oxide.