Chapter

8

Covalent Bonding

Section 8.1Molecular Compounds

OBJECTIVES: Distinguish between the melting points and boiling points of molecular compounds

and ionic compounds. Describe the information provided by a molecular formula.

Bonds are…

Forces that hold groups of atoms together and make them function as a unit. Two types:

1) Ionic bonds – transfer of electrons (gained or lost; makes formula unit)

2) Covalent bonds – ________________________ of electrons. The resulting particle is called a ___________________________________

Covalent Bonds

The word covalent is a combination of the prefix co- (from Latin com, meaning “with” or “together”), and the verb valere, meaning “to be strong”.

Two electrons shared together have the strength to hold two atoms together in a bond.

Molecules

Many elements found in nature are in the form of molecules: a ______________________________________________________________ joined

together by covalent bonds. For example, air contains oxygen molecules, consisting of two oxygen atoms joined

covalently Called a “_______________________________________________________” (O2)

How does H2 form?(diatomic hydrogen molecule)

The nuclei repel each other, since they both have a positive charge (like charges repel).

But, the nuclei are attracted to the electrons They _______________________ the electrons, and this is called a

“_____________________________________________”, and involves only ____________________________________

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+ +

Covalent bonds

Nonmetals hold on to their valence electrons. They can’t give away electrons to bond. But still want noble gas configuration. Get it by _______________________________________________with each other =

___________________________________________ By sharing, both atoms get to count the electrons toward a noble gas configuration.

Fluorine has seven valence electrons (but would like to have 8) A second atom also has seven By sharing electrons… …both end with full orbitals

Molecular Compounds

Compounds that are bonded covalently (like in water, or carbon dioxide) are called ________________________________________________

Molecular compounds tend to have relatively __________________________ melting and boiling points than ionic compounds – this is not as strong a bond as ionic

Thus, molecular compounds tend to be _____________________________________ at room temperature

• Ionic compounds were solids A molecular compound have a ___________________________________________

o Shows how many atoms of each element a molecule contains

The formula for water is written as H2O The ___________________________________ behind hydrogen means there are 2

atoms of hydrogen; if there is only one atom, the subscript 1 is omitted Molecular formulas do not tell any information about the structure (the arrangement

of the various atoms).

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Section 8.2The Nature of Covalent Bonding

OBJECTIVES: Describe how electrons are shared to form covalent bonds, and identify

exceptions to the octet rule. Demonstrate how electron dot structures represent shared electrons. Describe how atoms form double or triple covalent bonds. Distinguish between a covalent bond and a coordinate covalent bond, and

describe how the strength of a covalent bond is related to its bond dissociation energy.

Describe how oxygen atoms are bonded in ozone.

A Single Covalent Bond is...

A sharing of two valence electrons. Only _____________________________ and ____________________________ Different from an ionic bond because they actually form molecules. Two specific atoms are joined. In an ionic solid, you can’t tell which atom the electrons moved from or to

How to show the formation…

It’s like a jigsaw puzzle. You put the pieces together to end up with the right formula. Carbon is a special example - can it really share 4 electrons: 1s22s22p2?

o Yes, due to electron promotion! Another example: lets show how water is formed with covalent bonds, by using

an electron dot diagram

Water

Put the pieces together The first hydrogen is happy The oxygen still needs one more So, a second hydrogen attaches Every atom has full energy levels

Multiple Bonds

Sometimes atoms share more than one pair of valence electrons. A ____________________________________________ is when atoms share

two pairs of electrons (4 total)

H

O

Each hydrogen has 1 valence electron- Each hydrogen wants 1 more

The oxygen has 6 valence electrons- The oxygen wants 2 more

They share to make each other complete

A ____________________________________________ is when atoms share three pairs of electrons (6 total)

Table 8.1, p.222 - Know these 7 elements as _____________________________:

Dot diagram for Carbon dioxide

CO2 - Carbon is central atom ( more metallic ) Carbon has 4 valence electrons Wants 4 more Oxygen has 6 valence electrons Wants 2 more

C

O

Carbon dioxide

Attaching 1 oxygen leaves the oxygen 1 short, and the carbon 3 short

Attaching the second oxygen leaves both of the oxygen 1 short, and the carbon 2 short

The only solution is to share more Requires two double bonds Each atom can count all the electrons in the bond

How to draw them?

Use these guidelines:1) Add up all the valence electrons.2) Count up the total number of electrons to make all atoms happy.3) Subtract; then Divide by 24) Tells you how many bonds to draw5) Fill in the rest of the valence electrons to fill atoms up.

Example

NH3, which is ammonia N – central atom; has 5 valence electrons, wants 8 H - has 1 (x3) valence electrons, wants 2 (x3) NH3 has 5+3 = _______ NH3 wants 8+6 = 14 (14-8)/2= ___________________ 4 atoms with _________ bonds

Draw in the bonds; start with singles All 8 electrons are accounted for Everything is full – done with this one

Example: HCN

HCN: C is central atom N - has _____ valence electrons, wants _____ C - has _____ valence electrons, wants _____ H - has _____ valence electron, wants _____ HCN has _____+_____+_____ = _____ HCN wants _____+_____+_____=_____ (_____-_____)/2= _____ bonds 3 atoms with _____ bonds – this will require multiple bonds - not to H however

HCN

Put single bond between each atom Need to add 2 more bonds Must go between C and N (Hydrogen is full)

Uses 8 electrons – need 2 more to equal the 10 it has

Must go on the N to fill its octet

Another way of indicating bonds

Often use a line to indicate a bond

Called a ________________________________________________ Each line is ________ valence electrons

Other Structural Examples

A Coordinate Covalent Bond...

When one atom donates both electrons in a covalent bond. Carbon monoxide (CO) is a good example:

Coordinate covalent bond

Most polyatomic cations and anions contain covalent and coordinate covalent bonds

Table 16.2, p.445 Sample Problem 16.2, p.446 The ammonium ion (NH4

1+) can be shown as another example

Bond Dissociation Energies...

____________________________________________________________________________________________________________________________________

High dissociation energy usually means the chemical is relatively _________________________________, because it takes a lot of energy to break it down.

Resonance is...

When ____________________________________ valid dot diagram is possible. Consider the two ways to draw ozone (O3) Which one is it? Does it go back and forth? It is a hybrid of both, like a mule; and shown by a double-headed arrow found in double-bond structures!

Resonance in Ozone

Neither structure is correct, it is actually a hybrid of the two. To show it, draw all varieties possible, and join them with a double-headed arrow.

Resonance

Occurs when more than one valid Lewis structure can be written for a particular molecule (due to position of double bond)

These are resonance structures of benzene.

The actual structure is an average (or hybrid) of these structures.

The 3 Exceptions to Octet rule

For some molecules, it is impossible to satisfy the octet rule

#1. usually when there is an ________________________________ of valence electrons– NO2 has 17 valence electrons, because the N has 5, and each O contributes

6. It is impossible to satisfy octet rule, yet the stable molecule does exist

Another exception: Boron• Page 451 shows boron trifluoride, and note that one of the fluorides might

be able to make a coordinate covalent bond to fulfill the boron#2 -But fluorine has a ________________________________________ (it is greedy), so this coordinate bond does not form

• #3 -Top page 229 examples exist because they are in period 3 or beyond