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Chem 101 1

Reading assignment: 8.1-8.4

As you read ask yourself:

What determines whether any two atoms react?

What are the forces that hold atoms together in molecules and ions together in ionic compounds?

How do the characteristics of these forces and the bonds that are formed determine the physical and chemical properties of the compounds?

How does the unequal sharing of an electron pair affect the bond? What determines unequal sharing?

Chapter 8 :Basic concepts of chemical bonding8.1 Chemical Bonds, Lewis Symbols, and the Octet Rule

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K+: [Ar]

The Octet Rule

Chem 101 4

tend to gain electronstend to lose electrons

transition elements (and lanthanides and actinides)form positive ions

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495 kJ mol-1 to remove electron

from sodium

8.2 Ionic bonding

349 kJ mol-1 back by giving electron

to chlorine

yet reaction of sodium metal and chlorine gas to form sodium chloride is violently exothermic

Also need to account for: electrostatic attraction between the newly formed sodium cation and chloride anion

Formation of NaCl from Na and Cl2

Chem 101 6

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7

Energetics of Ionic bonds:Lattice energy

Chem 101 8

Lattice energy

dQQEel

21

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Lewis structures

8.3 Covalent Bonding

Electron pairs are usually unequally shared between different atoms:

8.4 Bond Polarity and Electronegativity

F2 HF

nonpolarcovalent

bond

polarcovalent

bondelectrons

shared equally

one atom attracts bonding electrons

more than the other

high electron density

low

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Chem 101 11

Ionic and covalent bonding

Chem 101 12

represent the two extremes of the continuum

ionic covalent

extended lattice structures

molecules

Properties:

o high melting solids, brittle

o soluble in water

o solutions and melts conductelectricity

o low melting and boiling points (often gases, liquids at room temp.)

o most are insoluble in water

o solutions and melts are non-conducting

Properties:

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8.5 Lewis Structures

Lewis structures

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1. Find the total number of valence electrons (account for any charges) = TOTAL

2. decide connection between atoms, draw a line to represent 1 electron pair for each connection, count the electrons SHARED

3. calculate the remaining electrons = TOTAL – SHARED, assign these to the terminal atoms to make octet (or 2 for H atom)

4. any electrons left? – put them on the central atom

5. if central atom doesn’t have an octet, make multiple bonds from nonbondedelectron pairs on terminal atoms

Choose central atom correctly

least electronegative atom(not H)

oxygen rarely bonds to itself

PCl3

For PCl3: 5 (for P) + (3 × 7) (for Cl3) = 26

26 – 6 = 20

3 bonds= 6

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Multiple Bonds

Chem 101 15

Formal charges

Chem 101 16

often can make more than one Lewis structure

bookkeeping of electrons

which one is correct?

calculate the charge on atom IF all bonding electrons shared equally

assign to the atom all unshared (nonbonding) electrons

½ of all bonding (shared) electrons+

formal charge = number of valence electrons – total assigned electrons

Evaluate Lewis structures

more stable if there are small (or no) formal charges

the most electronegative atom has the most negative formal charge

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How many Lewis structures are possible for the unstable fulminate ion (CNO-) with the N in the center?

a) 0b) 1c) 2d) 3e) 5

-C N O

-B.D:3

C

C N O-

A.

How many Lewis structures are possible for the unstable fulminate ion (CNO-) with the N in the center?

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-C N O

-B.

A

C

C N O-

A.

The unstable fulminate ion (CNO-) has three possible resonance structures. Based on formal charge considerations, which is the better structure?

B

C

-C N O

-B.

A

C

C N O-

A.

The unstable fulminate ion (CNO-) has three possible resonance structures. Based on formal charge considerations, which is the better structure?

B

C

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Chem 101 21

Reading assignment: 8.5-8.8

As you read ask yourself:

How can I use Lewis structures to account for bonding in covalent molecules?

What are the differences between single, double and triple bonds in terms of bond distance and strength?

When is it useful to assess the formal charges on atoms in a Lewis structure?

How does a resonance hybrid structure differ from a regular Lewis structure? How do resonance structures affect the predicted bond distances?

What are the situations when the octet rule is disobeyed? How will I recognize molecules that disobey the octet rule?

How can I make use of average bond enthalpies?

Resonance hybrids

Chem 101 22

hybrid is intermediate between the two “parent” structures

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Resonance has impact on bond lengths and strengths

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compare NO+, NO2- and NO3

-

NO+ : share = 2 N O N O

NO2- : electrons: 5 + 6 +6+1 = 18 share = 4 N OO

N OO-

N OO-

NO3- :

electrons 5+6+6+6+1=24

share = 6

electrons: 5 + 6-1 = 10

N-O bond average = 1.5

N-O bond average = 1.3

N-O bond = 3

aromatic compounds show resonance

Chem 101 24

benzene

shorthand notation omits H atoms

C-C bond average = 1.5

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Exceptions to the octet rule

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1. less than an octet small atoms that are too crowded with an octet

2. odd number of electrons octet impossible, small number of stable molecules

or?

3. more than an octet elements in third period (and beyond) can expand valence shell

Exceptions to the octet rule…

Chem 101 26

can expand valence shell to make a Lewis structure with lower formal charge

SO42-

experimental info:

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Notices Midterm #1 is TONIGHT at 5 pm, right here. Bring your student photo-ID. Place it on your desk for

the duration of the exam. Bring your Sharp El-510R calculator. Bring a pencil to fill out the dot form for the multi-choice. Bring a pen for your written answers. You will be provided with a data sheet and you will write

on your exam paper. Turn off your cell phone and leave it in your bag. Do not bring any device with earphones.