chapter 8 covalent bonding sec. 8.1: the covalent bond
TRANSCRIPT
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.