chapter 2 notes lecture

8/14/2019 Chapter 2 Notes Lecture

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ChemistryOnline, 2009-2011

Chapter 2

Molecular Substance

and Lewis Structures


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ANNOUNCEMENTS

HW#1: Chapter1: due (09/13/13)

HW#13.3 (Electron Config.), due:09/18/13

HW#2: Chapter 2: due (09/23/13)

HW#3: Chapter 3: due (09/23/13)

Exam#1: 09/23/13

(We will have it indifferent rooms)


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A polyatomic ionis a group of atoms that are bonded

together by covalent bonds (a molecular compound)that has a net ionic charge. These combine with simple

ions to give more complex ionic compounds.

POLYATOMICIONS


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Draw the Lewis structure for chlorate anion(ClO3-).

Cl OO

O

IN-CLASSPROBLEM

Draw the atoms and their

associated electrons.

I VIII

II III IV V VI VII1

2

6

4

5

3


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Cl OO

O

IN-CLASSPROBLEM



I VIII

II III IV V VI VII1

2

6

4

5

3


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IN-CLASSPROBLEM



Next, add an extra electron

to the chlorine to give the

negative charge.


Cl OO

O


7/26 ChemistryOnline, 2009-2011

Group the atoms and their

associated electrons so that

each oxygen has eight

electrons.

O Cl O

OThis also leaves eight electrons

around the chlorine.

IN-CLASSPROBLEM


O Cl O

O



Draw the Lewis structure for nitrate anion(NO3-).


associated electrons.N OO

O

IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3





associated electrons.N OO

O

IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3


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associated electrons. Next add an

extra electronto the nitrogen to

give the negative charge.


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

N OO

O



Group the atoms and their


each oxygen has eight

electrons.

O N O

O


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

This is not good, this is

not right, nitrogen only

has six electrons!


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We can fix this, however, by simply

moving a pair of electrons from

one of the oxygens to form a

double bond to the nitrogen.


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

O N O

O


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This is a correct structure for

the nitrate anion...but is it

the only correct structure?

N OO

O


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3


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What if we had chosen to form

the double bond with the

oxygen on the left?


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

N OO

O


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How about the oxygen on the

bottom?N OO

O


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3


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So which one is correct?

All of them!


IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

N OO

O


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These structures are

called resonance forms

and because they differ

only in the arbi trary

placement of electrons,

they are all equivalent.

N OO

O

N OO

O

N OO

O

IN-CLASSPROBLEM


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To indicate that they are resonance

forms, a special double headed

arrowis used to connect them. It

is important to note that theseforms do not represent distinct

structures, but that the nitrate

anion is a hybridof all of these

forms.

N OO

O

N OO

O

N OO

O

IN-CLASSPROBLEM


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The hybrid looks something

like this.

N OO

O

N OO

O

N OO

O

N OO

O

IN-CLASSPROBLEM


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Nobody is perfect not even Lewis. As it turns out, the

octet ruleis a good guide, but there are manyexceptions. Many of these involve elements in the third

period and beyond

EXCEPTIONSTOTHEOCTETRULE

F

FF

F 120

0

S


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IN-CLASSPROBLEM

I VIII

II III IV V VI VII1

2

6

4

5

3

Draw the Lewis structure for sulfur tetrafluoride(SF4).



we can make four bonds.

S F

F

FF


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S F

F

FF

We seem to have a problem.

In order to form four bonds,

we must move one electron

pair on sulfur into storage

and then split the remaining

pair into single electrons.

IN-CLASSPROBLEM




we can make four bonds.

storage

S F

F

FF


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Finally, we must put our pair of

electrons back on the sulfur.

S F

F

FF

This gives the sulfur ten valence

electrons. This is called valence

expansion. Elements above the

second period may undergo

valence expansionif needed.

IN-CLASSPROBLEM


Now we can form fourbonds, giving the fluorines

eight electrons each.


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Ch i t O li 2009 2011

Group 4A Group 5A Group 6A Group 7A

S

F

F

F

F

Cl

F

F

F

Xe

F

F

P

F

F

F

F

F

Si

F

F

F

F

F

S

F

F

F

F

F

F

P

F

F

F

F

F

F

Si

F

F

F

F

F

F

2

Br

F

F

F

F

F

Xe

FF

F F

Group 8A

The table below shows several other examples of

valence expansion. These molecules and theirshapeswill be addressed further next semester.

EXCEPTIONSTOTHEOCTETRULE

chapter 2 notes lecture

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