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Lewis Dot Symbols for the Representative Elements &Noble Gases

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Li + F Li+ F -

The Ionic Bond

1s22s1 1s22s22p5 1s2 1s22s22p6

[He] [Ne]

Li Li+ + e-

e- + F F -

F -Li+ + Li+ F -

LiF

Ionic bond: the electrostatic force that holds ions together in an ionic compound.

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4

Born Haber Cycle

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6

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H2

2 x 1 = 2 v.e.

H. H.+ H.H.

by sharing each has an electronic configuration like [He]

This is called a covalent bond

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A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Why should two atoms share electrons?

F F+

7e- 7e-

F F

8e- 8e-

F F

F F

Lewis structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

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8e-

H HO+ + OH H O HHor

2e- 2e-

Lewis structure of water

Double bond – two atoms share two pairs of electrons

single covalent bonds

O C O or O C O

8e- 8e-8e- double bonds

Triple bond – two atoms share three pairs of electrons

N N8e- 8e-

N N

triple bond

or

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Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond

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The Electronegativities of Common Elements

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Variation of Electronegativity with Atomic Number

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9.2Classify the following bonds as ionic, polar covalent, or covalent:

(a)the bond in HCl

(b)The bond in KF

(c)the CC bond in H3CCH3

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Lewis Dot Structures: covalent molecules

CO2

2 x 6 = 121 x 4 = 4

16v.e.

“Nature likes symmetry”

CO O: :

..

..

......

C must followthe octet rule,

does it?

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Lewis Dot Structures: covalent molecules

CO2

2 x 6 = 121 x 4 = 4

16v.e.

“Nature likes symmetry”

CO O: :

..

..

......

Use multiple bonding to achieveoctets.

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NF3

18

HNO3

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CO32-

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SO42-

Formal Charges:

Next we show formal charges on the S and O atoms:

Note that we can eliminate some of the formal charges for by expanding the S atom’s octet as follows:

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Formal Charge and Lewis Structures

1. For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present.

2. Lewis structures with large formal charges are less plausible than those with small formal charges.

3. Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms.

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A resonance structure is one of two or more Lewis structures for a single molecule that cannot be represented accurately by only one Lewis structure.

O O O+ -

OOO+-

9.8

Draw three resonance structures for the molecule nitrous oxide, N2O (the atomic arrangement is NNO).

Indicate formal charges.

Rank the structures in their relative importance to the overall properties of the molecule.

9.8

Strategy The skeletal structure for N2O is

We follow the procedure used for drawing Lewis structures and calculating formal charges in Examples 9.5 and 9.6.

Solution The three resonance structures are

9.8

We see that all three structures show formal charges.

Structure (b) is the most important one because the negative charge is on the more electronegative oxygen atom.

Structure (c) is the least important one because it has a larger separation of formal charges. Also, the positive charge is on the more electronegative oxygen atom.

Check Make sure there is no change in the positions of the atoms in the structures. Because N has five valence electrons and O has six valence electrons, the total number of valence electrons is 5 × 2 + 6 = 16. The sum of formal charges is zero in each structure.

9.11

The question of which of these two structures is more important, that is, the one in which the S atom obeys the octet rule but bears more formal charges or the one in which the S atom expands its octet, has been the subject of some debate among chemists. In many cases, only elaborate quantum mechanical calculations can provide a clearer answer.

At this stage of learning, you should realize that both representations are valid Lewis structures and you should be able to draw both types of structures. One helpful rule is that in trying to minimize formal charges by expanding the central atom’s octet, only add enough double bonds to make the formal charge on the central atom zero.

9.11

Thus, the following structure would give formal charges on S(−2) and O(0) that are inconsistent with the electronegativities of these elements and should therefore not be included to represent the ion.

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SO2

2 x 6 = 12 1 x 6 = 6

18 v.e.

“Nature likes symmetry”

SO O....

..::

..

That’s 16 e-’s where do the other 2 go?

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SO2

2 x 6 = 12 1 x 6 = 6

18 v.e.

“Nature likes symmetry”

SO O....

..::

....

Does each atom have an octet?

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SO2

2 x 6 = 12 1 x 6 = 6

18 v.e.

“Nature likes symmetry”

SO O....

..::

....

Does each atom have an octet?

SO O....

:..

SO O..::

....:or :

:

These are called RESONANCE STRUCTURES

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What effect does the lone pair on the S atom have on geometry of the molecule?

SO O....

:..

SO O..::

....:or :

:These are called RESONANCE STRUCTURES

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S

O O....:

..S

O O.. ::..

..:

::

These are called RESONANCE STRUCTURES

What effect does the lone pair on the S atom have on geometry of the molecule?

120o

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Formal Charges

SO O.. ::

....

:

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XeF4

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The enthalpy change required to break a particular bond in one mole of gaseous molecules is the bond enthalpy.

H2 (g) H (g) + H (g) H0 = 436.4 kJ

Cl2 (g) Cl (g) + Cl (g) H0 = 242.7 kJ

HCl (g) H (g) + Cl (g) H0 = 431.9 kJ

O2 (g) O (g) + O (g) H0 = 498.7 kJ O O

N2 (g) N (g) + N (g) H0 = 941.4 kJ N N

Bond Enthalpy

Bond Enthalpies

Single bond < Double bond < Triple bond

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CH4 + 2O2

CO2 + 2H2O

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