Announcement

Comprehensive Final Exam

◦ March 25, 2015 Wednesday

◦ 3:30 – 5:30 PM

◦ B-209

Exam 4

1

Chapter 9 Models of Chemical Bonding

Chapter Outline

1. Atomic Properties and Chemical Bonds

2. The Ionic Bonding Model

3. The Covalent Bonding Model

4. Bond Energy and Chemical Change

5. Electronegativity and Bond Polarity

Excluded Topics: ◦ Ionic bonding model (p280-284)

◦ IR spectroscopy (p289)

◦ EN calculations and % ionic character

3

Metals vs Nonmetals

4

An ionic compound (salt)

results from chemical

reactions between a

metal and a non-metal.

A covalent compound

results from chemical

reactions between non-

metals.

Types of Chemical Bonding

5

Types of Chemical Bonding

6

Valence electrons are the electrons in the

outer shell (highest value of n quantum

number) of the atom that dictate chemistry

in bonding.

8

Group e- configuration # of valence e-

1A ns1 1

2A ns2 2

3A ns2np1 3

4A ns2np2 4

5A ns2np3 5

6A ns2np4 6

7A ns2np5 7

Lewis electron-dot symbols are used to

depict valence electrons and bonding

between atoms.

To draw the Lewis symbol for any main-

group element:

Example: Nitrogen, N, is in Group 5A and

therefore has 5 valence electrons.

9

Note A-group number: # of

valence electrons

Place one dot at a time on each of the four sides of the element

symbol.

Keep adding dots, pairing

them, until all are used up.

N ••

• • • N

•• •

• • N

•• • •

• N

••

•

• • or or or

Lewis electron-dot symbols are used to

depict valence electrons and bonding

between atoms.

The octet rule states that when atoms

bond, they lose, gain, or share electrons to

attain a filled outer level of 8 electrons

(or 2, for H and Li).

10

There are different ways to conceptualize

and draw ionic bonding.

1. Using spdf electron configurations

2. Orbital box diagrams

3. Lewis electron-dot symbols

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Li 1s22s1 + F 1s22p5 → Li+ 1s2 + F- 1s22s22p6

Learning Check

Use partial orbital diagrams and Lewis

symbols to depict the formation of Na+

and O2− ions from the atoms, and

determine the formula of the

compound formed.

12

Use partial orbital diagrams and Lewis

symbols to depict the formation of Na+ and

O2− ions from the atoms, and determine

the formula of the compound formed.

13

Covalent Bonding

Results from sharing of

one or more electrons

between nonmetals

14

15

Every covalent bond has a

characteristic bond length that

leads to maximum stability.

Distribution of electron density

in H2

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At some distance (bond

length), attractions

balance repulsions.

Electron density is high around

and between the nuclei.

We also use Lewis structures and the

“octet rule” to show covalent bonds and

bonding between atoms.

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7 e- 7 e- 8 e- 8 e-

8 e- or an “octet”

around each atom

single covalent bond

lone pairs lone pairs

single covalent bond

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Properties of a Covalent Bond

1. Bond Order: number of electron

pairs being shared by a given pair of

atoms

2. Bond Length: distance between the

nuclei of the bonded atoms

3. Bond Energy: energy needed to

overcome the attraction between

the nuclei and the shared electrons

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Bond order is the number of covalent bonds

between two atoms.

For a given pair of atoms, a higher bond

order results in a shorter bond length and

higher bond energy. 21

Comparing Bond Length and

Bond Strength Using the periodic table, rank the

bonds in each set in order of

decreasing bond length and decreasing

bond strength:

◦ (a) S–F, S–Br, S–Cl

◦ (b) C=O, C–O, C O

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Bond length: S–Br > S–Cl > S–F

Bond strength: S–F > S–Cl > S–Br

Bond length: C–O > C=O > CΞO

Bond strength: CΞO > C=O > C–O

Bond Energies and DHorxn

The bond energy, BE, is the amount

of energy required to break or make

one mole of bonds in a gaseous

covalent compound to form products

in the gaseous state at constant T

and P.

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25

DH°rxn = SBEreactant bonds broken- SBEproduct bonds formed

Bond Energies and DHorxn

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DH°rxn = SBEreactant bonds broken- SBEproduct bonds formed

H2(g) + F2(g)

2 HF(g)

Calculating Bond Energies in a

Reaction

Calculate the ΔHrxn for the

chlorination of methane to form

chloromethane gas using bond

energies.

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CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) ΔHrxn = ?

Bond Energies:

C – H 414 kJ/mol C – Cl 339 kJ/mol

Cl – Cl 243 kJ/mol H – Cl 431 kJ/mol

Calculating Bond Energies in a

Reaction 1. Write the balanced chemical equation and

setup the tables below:

2. Draw Lewis structures of reactants and

products.

3. Calculate for ΔHrxn.

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Calculate the ΔHrxn for the chlorination of methane

to form chloromethane gas using bond energies.

29

Practice Exercise

Calculate the ΔHrxn for the

hydrogenation of ethyne (acetylene)

to ethane.

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ΔHrxn = -296 kJ/mol

Electronegativity It is an element’s inherent ability to draw

electrons to itself when chemically bonded

to another atom in a molecule (relative to Li)

31

Differences in electronegativity between

bonding atoms blur the distinction between

covalent, polar covalent, and ionic bonding

types.

32

F – F H – F Li – F

Covalent

Bonding

Polar Covalent

Bonding

Ionic

Bonding

δ+ δ-

Dipole moment

Ionic Character and Melting

Point

Melting points of compounds decrease

across a period as compounds become less

ionic and more covalent.

33

Macroscopic properties change from

those of a solid consisting of ions to

those of a gas consisting of molecules.

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