Chemical BondingChemical Bonding
Chapter 6Chapter 6
Sections 1, 2, and 5Sections 1, 2, and 5
Chemical Bonds
A chemical bond is the mutual electrical attractionbetween the nuclei and valence electrons ofdifferent atoms that bind the atoms together
Noble gases tend not to do this because of their filleds and p orbitals.
They have a stable octet: outer s and p orbitals arecompletely filled with e-’s ( 8 total)
Chemical Bonds
Atoms that don’t have a stable octet are morereactive because their potential energy is higher.They become more stable by decreasing theirpotential energy.
Octet Rule: chemical compounds tend to form so thateach atom has an octet of e-’s in its highestoccupied energy level
How to do this? Gain , lose or share electrons betweenatoms
Chemical Bonds
By forming a chemical bonds, atoms gain stability!
Chemical changes always involve energy
What type of bonds can be formed?
Ionic bond
Covalent bond
Nonpolar covalent
Polar covalent
Chemical Bonds
Ionic bonding: bonds that result from electricalattractions between cations and anions
Covalent bonding: sharing of electron pairs between 2 ormore atoms
*** In reality, bonding is often somewhere between thetwo extremes***
Two types of Covalent Bonds
Nonpolar –covalent: equal sharing of electron pairs
Polar-covalent: unequal attraction for the sharedelectrons
How can we determine the type of bond?
Knowing how strong an atom’s ability is to attractelectrons (aka electronegativity), helps usdetermine if it will form a ionic or covalent bondwith another atom.
A large difference in E.N. between atom’s will result inan ionic bond
A small difference between atom’s will result in aform of covalent bonding
Nonpolar
Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
What type of Bond is it?
ElectronegativityDifference
Bond Type
0 to 0.3
Nonpolar Covalent
0.4 to 1.7
Polar Covalent
1.7
Ionic
Do you see any trends?
A metal and nonmetal tend to form ionic compounds
Nonmetal and nonmetal tend to form polar-covalent ornonpolar- covalent compounds
H
F
F
H
Polar covalent bond or polar bond :
covalent bond with greater electron densityaround one of the two atoms
electron rich
region
electron poor
region
e- rich
e- poor
+
-
Classify the following bonds as ionic, polarcovalent,or covalent:
Cs – 0.7
Cl – 3.0
3.0 – 0.7 = 2.3
Ionic
H – 2.1
S – 2.5
2.5 – 2.1 = 0.4
Polar Covalent
N – 3.0
N – 3.0
3.0 – 3.0 = 0
Nonpolar
Covalent
CsCl
H2S
N2
Properties of MolecularCovalent CompoundsProperties of MolecularCovalent Compounds
Not very soluble in waterNot very soluble in water
Do not conduct electricityDo not conduct electricity
Low melting pointsLow melting points
Low boiling pointsLow boiling points
Can be solids, liquids and gases at room temperatureCan be solids, liquids and gases at room temperature
Comparison of Ionic and Covalent Compounds
Types of Crystals
Lewis StructuresLewis Structures
Review:What are valence electrons?Review:What are valence electrons?
Lewis Dot DiagramsLewis Dot Diagrams
−an electron-configuration notation with only the valenceelectrons of an element are shown, indicated by dotsplaced around the element’s symbol.−an electron-configuration notation with only the valenceelectrons of an element are shown, indicated by dotsplaced around the element’s symbol.
−the inner core electrons are not shown.−the inner core electrons are not shown.
•Eight electrons in the valence shell (filling s and porbitals) make an atom STABLE
s2p6
This is called the octet rule
The Octet Rule
•Bond formation follows the octet rule: Chemicalcompounds tend to form so that each atom:
by gaining, losing, or sharing electrons, has an octetof electrons in its valence energy level.
Lewis Structures for Compounds
•The pair of dots between two symbolsrepresents the shared pair of a covalent bond.
•Each fluorine atom is surrounded by three pairsof electrons that are not shared in bonds.
•An unshared pair, also called a lone pair, is a pairof electrons that is not involved in bonding andthat belongs exclusively to one atom.
Lewis Structures
•The pair of dots representing a shared pair ofelectrons in a covalent bond is often replaced bya long dash.
covalent bond : is a chemical bond in whichtwo or more electrons are shared by twoatoms.
Why should two atoms share electrons?
F
F
+
7e-
7e-
F
F
8e-
8e-
F
F
F
F
Lewis structure of F2
lone pairs
lone pairs
lone pairs
lone pairs
single covalent bond
single covalent bond
Multiple Covalent Bonds
•double covalent bond or double bond :
covalent bond in which two pairs of electronsare shared between two atoms
•shown by two side-by-side pairs of dots or bytwo parallel dashes
Multiple Covalent Bonds
•triple covalent bond or triple bond :
covalent bond in which three pairs of electronsare shared between two atoms.
8e-
H
H
O
+
+
O
H
H
O
H
H
or
2e-
2e-
Single Bond – two atoms share one pair of electrons
Double Bond – two atoms share two pairs of electrons
single covalent bonds
O
C
O
or
O
C
O
8e-
8e-
8e-
double bonds
double bonds
Triple Bond – two atoms share three pairs of electrons
N
N
8e-
8e-
N
N
triple bond
triple bond
or
BondType
BondLength
(pm)
C-C
154
CC
133
CC
120
C-N
143
CN
138
CN
116
Lengths of Covalent Bonds
Bond Lengths
Triple bond < Double Bond < Single Bond
1.Draw skeletal structure of compound showingwhat atoms are bonded to each other. Put leastelectronegative element in the center.
2.Count total number of valence e-. Add 1 foreach negative charge. Subtract 1 for eachpositive charge.
3.Complete an octet for all atoms excepthydrogen
4.If structure contains too many electrons, formdouble and triple bonds on central atom asneeded.
Writing Lewis Structures
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
F
N
F
F
Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)
5 + (3 x 7) = 26 valence electrons
Step 3 – Draw single bonds between N and F atoms and complete
octets on N and F atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
O
C
O
O
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4)
-2 charge – 2e-
4 + (3 x 6) + 2 = 24 valence electrons
Step 3 – Draw single bonds between C and O atoms and complete
octet on C and O atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Step 5 - Too many electrons, form double bond and re-check # of e-
2 single bonds (2x2) = 4
1 double bond = 4
8 lone pairs (8x2) = 16
Total = 24
resonance structure: one of two or more Lewis structures fora single molecule can be drawn to represent a molecule
O
O
O
+
-
O
O
O
+
-
O
C
O
O
-
-
O
C
O
O
-
-
O
C
O
O
-
-
What are the resonance structures of the
carbonate (CO32-) ion?
Exceptions to the Octet Rule
The Incomplete Octet
H
H
Be
Be – 2e-
2H – 2x1e-
4e-
BeH2
BF3
B – 3e-
3F – 3x7e-
24e-
F
B
F
F
3 single bonds (3x2) = 6
9 lone pairs (9x2) = 18
Total = 24
Exceptions to the Octet Rule
Odd-Electron Molecules
N – 5e-
O – 6e-
11e-
NO
N
O
The Expanded Octet (central atom with principal quantum number n > 2)
SF6
S – 6e-
6F – 42e-
48e-
S
F
F
F
F
F
F
6 single bonds (6x2) = 12
18 lone pairs (18x2) = 36
Total = 48
Molecular GeometryMolecular Geometry
Chapter 6.5Chapter 6.5
VSEPR THEORYVSEPR THEORY
Lewis Dot Diagrams are 2D but we live in a 3D world.Lewis Dot Diagrams are 2D but we live in a 3D world.
How are molecules actually arranged??How are molecules actually arranged??
Follows the Valance Shell Electron Pair Repulsion Theory orVSEPRFollows the Valance Shell Electron Pair Repulsion Theory orVSEPR
AB2 – LinearAB2 – Linear
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
2
0
180˚
Cl
Cl
Be
AB3 – Trigonal PlanarAB3 – Trigonal Planar
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
3
0
120˚
AB2E1 – BentAB2E1 – Bent
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
2
1
<120˚
AB4 – TetrahedralAB4 – Tetrahedral
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
4
0
109.5˚
AB3E1 – TrigonalPyramidalAB3E1 – TrigonalPyramidal
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
3
1
107˚
AB2E2 – BentAB2E2 – Bent
Number of SurroundAtoms
Number of Lone Pairs
Bond Angle
2
2
104.5˚
Predicting MolecularGeometryPredicting MolecularGeometry
1.Draw Lewis structure for molecule.
2.Count number of lone pairs on the central atom andnumber of atoms bonded to the central atom.
3.Use VSEPR to predict the geometry of the molecule.
What are the molecular geometries of SO2 and SF4?
S
O
O
AB2E
bent
S
F
F
F
F
AB4E
distorted
tetrahedron
H
F
electron rich
region
electron poor
region
The electronegativity of an atom willcreate a dipole, or polar molecule.The electronegativity of an atom willcreate a dipole, or polar molecule.
Which of the following molecules have a dipole moment?
H2O, CO2, SO2, and CH4
O
H
H
dipole moment
polar molecule
S
O
O
C
O
O
no dipole moment
nonpolar molecule
dipole moment
polar molecule
C
H
H
H
H
no dipole moment
nonpolar molecule
Intermolecular forces: attractive forcesbetween molecules.
Intramolecular forces: hold atoms together,attractive forces within a molecule.
Generally, intermolecular forces are muchweaker than intramolecular forces.
Properties of IonicCompoundsProperties of IonicCompounds
Combination of ions (cation/anion)Combination of ions (cation/anion)
Hard and BrittleHard and Brittle
Tightly packed solids in a crystal latticeTightly packed solids in a crystal lattice
Usually soluble in waterUsually soluble in water
Conducts electricity when dissolvedConducts electricity when dissolved
High melting pointsHigh melting points
Breaking Ionic BondsBreaking Ionic Bonds
Ionic Bonds are very tightly boundIonic Bonds are very tightly bound
positive and negative attractionpositive and negative attraction
A LOT of energy needs to be put in to break an ionic bondA LOT of energy needs to be put in to break an ionic bond
How does this affect the melting point?How does this affect the melting point?