Chapter 8Chapter 8Covalent BondingCovalent Bonding

Sec. 8.1: The Covalent Bond

The Covalent BondThe Covalent Bond

Objectives– Apply the octet rule to atoms that bond

covalently.– Describe the formation of single, double, and

triple covalent bonds.– Compare & contrast sigma & pi bonds– Relate the strength of covalent bonds to bond

length and bond dissociation energy

Most compounds, including those in living organisms are covalently bonded.

Why do atoms bond?Why do atoms bond?

The stability of a substance is related to its energy: lower energy states are more stable.

Metals and nonmetals gain stability and a lower energy state by transferring electrons. The ions that form have stable noble-gas configurations.

(Ions of opposite charge are then attracted to each other in an ionic bond.)

•According to the octet rule, there is another way atoms can gain stability. They can SHARE valence electrons to achieve a noble-gas configuration.

The Covalent BondThe Covalent Bond

The chemical bonds that results from the sharing of valence electrons is a covalent bond.

The Covalent BondThe Covalent Bond

In a covalent bond, the shared electrons are considered to be part of the complete outer energy level of both atoms involved. – Covalent bonding occurs between

NONMETAL ATOMS.– The elements are generally relatively close to

each other on the periodic table.

What is happening in that What is happening in that covalent bond??covalent bond??

The nucleus of 1 atom is attracted to the electrons

of another atom and vice versa.

The Covalent BondThe Covalent Bond

A distance between 2 atoms is reached where the attraction-repulsion forces are balanced.

This is a point of maximum stability for the atoms.

This is the point of covalent bond formation.

The Covalent BondThe Covalent BondA molecule is formed

when two or more atoms bond covalently.

Compounds with covalent bonds are called covalent or molecular compounds.

The Covalent BondThe Covalent Bond

Hydrogen, Nitrogen, Oxygen, Fluorine, Chlorine, Bromine, and Iodine occur in nature as diatomic molecules because these 2-atom molecules are more stable than single atoms.

The Covalent BondThe Covalent Bond When a single pair of electrons is shared, a single

covalent bond is formed. H2 has a single covalent bond.

The Single Covalent BondThe Single Covalent BondThe diatomic molecules of Group 7A elements also

have single covalent bonds because they only need 1 more electron to attain noble gas configurations.– Example:

Draw the Lewis dot structure of F2

Note that it has 3 unshared pairs (also called lone pairs) and 1 shared (or bonding) pair of valence electrons.

In a diagram, the shared pair can be represented by a single dash

What about oxygen?What about oxygen? Each O atom has 2 unshared

pairs of valence electrons. Therefore, O2 has 2 bonding

pairs. O2 has a double covalent

bond. Double bonds are

represented in diagrams by a double dash: =

O = O

Nitrogen?Nitrogen?

3 shared pairsa triple covalent bondEach atom has one unshared or lone

pair

..

WaterWater

Recall oxygen and all group 6A atoms have 6 valence electrons.

There are 2 electrons available to form covalent bonds.

Oxygen will share 1 with each hydrogen.

2 single covalent bonds form - 2 lone pairs remain

AmmoniaAmmonia Recall nitrogen and all

group 5A atoms have 5 valence electrons.

N has 3 electrons that are available to form covalent bonds.

Nitrogen will share one with each hydrogen.

3 single covalent bonds form with one lone pair

Group 4AGroup 4A

Will form 4 covalent bonds

For example, methane (CH4) forms 4 C-H covalent bonds

The Covalent BondThe Covalent Bond

Draw the Lewis structure for each of these molecules:

PH3

H2S

CCl4

Sigma (Sigma (σ) BondsSingle covalent bonds are also called sigma

bondsThe shared electron pair is in an area

centered between the 2 atomsAtomic orbitals overlap and merge

– “s” with another “s”– “s” with a “p”– “p” with another “p”

A new, hybridized orbital is formed. It is called abonding orbital – it is a localized regionwhere bondingelectrons will mostlikely be found.

Illustrated are thesigma bonds inmethane (text p.245).

Multiple Covalent bondsMultiple Covalent bondsA miltiple bond consists of one sigma (σ)

bond and at least one pi (π) bond.– The pi bond is formed when parallel orbitals

overlap and share electrons. – The pi bond occupies the space above and below

the line that represents where the two atoms are joined together.

The Pi bond in Ethene (C2H4)

Strength of covalent bondsStrength of covalent bonds

Strength depends on how much distance there is between the nuclei of bonded atoms.

BOND LENGTH is the distance from the center of one nucleus to the center of the other nucleus of 2 bonded atoms at the point of maximum attraction.

Strength of covalent bondsStrength of covalent bonds

The shorter the bond length, the stronger the bond

As the number of shared pairs increases, the bond length decreases, so bond strength increases.– the bonds in F2 are weaker than those in O2

– the bonds in O2 are weaker than those in N2

Energy is released when Energy is released when bonds formbonds form

The bond dissociation energy is the energy added to break bonds.

It is “+” in valueBond strength is described in terms of bond

dissociation energy.– for F2, BDE equals 159 kJ/mol

– for N2, BDE equals 945 kJ/mol; N2 has more shared pairs, so it has a shorter and stronger bond!

The sum of the BDE’s for all The sum of the BDE’s for all the bonds in a compound give the bonds in a compound give an indication of the an indication of the potential potential

energyenergy available in one available in one molecule of the compound.molecule of the compound.

Chemical ReactionsChemical Reactions(an overall view)(an overall view)

In all chemical reactions, 2 changes in bonds must occur– Bonds between reactants MUST break; requires

energy.– Bonds between products MUST form; releases

energy.

Determining whether a reaction is endothermic or exothermic depends on the net energy change of the reaction.

Chemical ReactionsChemical Reactions A reaction is

EXOTHERMIC if more energy is released when new bonds form than is required to break bonds.

A reaction is ENDOTHERMIC if more energy is required to break bonds than is released when new bonds form.

Either way, the net energy change is called the HEAT OF REACTION.