Chapter 8 – Covalent Bonding The unspoken hero: “Covalent Bond”
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Transcript of Chapter 8 – Covalent Bonding The unspoken hero: “Covalent Bond”
Chapter 8 – Covalent Bonding
The unspoken
hero: “Covalent
Bond”
Review of Chapter 7• In Chapter 7, we learned about electrons being
transferred (“given up” or “stolen away”)• This type of “tug of war” between a METAL and NONMETAL is called an IONIC BOND,which results in a SALT being formed
Chapter 8.1 – Molecular Compounds
• Covalent Bonds - atoms held together by SHARING electrons between NONMETALS
Salt versus Molecules
•Molecule - A group of atoms joined together by a covalent bond•Compound - a group of two or more elements bonded together (Ionic or Covalent).
Monatomic vs. Diatomic Molecules• Most molecules can be monatomic or diatomic
•Diatomic Molecule - molecule consisting of two atoms•There are 7 diatomic molecules (SUPER 7) –
•H2, O2, N2, Cl2, Br2, I2, F2
Properties of Molecular Compounds
• Lower Melting Points than Ionic Compounds (means weaker bonds than ionic)
• Liquids or gases at room temperature
Molecular Formulas
• Molecular Formula – formula of a molecular compound
• Shows how many atoms of each element a molecule contains
•ExampleH2O contains 3 atoms (2 atoms of H, 1 atom of O)
C2H6 contains 8 atoms (2 atoms of C, 6 atoms of H)
Practice
How many atoms total and of each do the following molecular compounds contain?
1. H2
2. CO
3. CO2
4. NH3
5. C2H6O
2
2
3
4
9
Practice: True or False
1. All molecular compounds are composed of atoms of two or more elements.
2. All compounds are molecules.
3. Molecular compounds are composed of two or more nonmetals.
4. Atoms in molecular compounds exchange electrons.
5. Molecular compounds have higher melting and boiling points than ionic compounds.
Share
Lower
Ionic versus Covalent
IONIC COVALENT
Bonded Name Salt Molecule
Bonding Type Transfer e- Share e-
Types of Elements Metal & Nonmetal Nonmetals
Physical State Solid Solid, Liquid, or Gas
Melting Point High (above 300ºC) Low (below 300 ºC)
Solubility Dissolves in Water Varies
Conductivity Good Poor
Chapter 8.2 – Covalent Bonding• Remember that ionic compounds
transfer electrons in order to attain a noble gas electron configuration
• Covalent compounds form by sharing electrons to attain a noble gas electron configuration
• Regardless of the type of bond, the Octet Rule still must be obeyed (8 valence electrons)•All elements need 8 except Hydrogen, which needs 2.
Single Covalent Bond
• A Single Covalent Bond consists of two atoms held together by sharing 1 pair of electrons (2 e-)
Electron Dot Structure
Shared versus Unshared Electrons
• A Shared Pair is a pair of valence electrons that is shared between atoms
• An Unshared Pair is a pair of valence electrons that is not shared between atoms
Practice Lewis Dot Structures
Chemical Formula
# of Valence
Electrons
Single Line Bond
Structure
# of Remaining Electrons
Lewis Dot Structure
Octet Check All Atoms=8Hydrogen=2
F2
H2O
NH3
CH4
14 F-F 12 F, F = 8
8 H-O-H 4O = 8H, H = 2
8 2 N = 83 H = 2
8 0 C = 84 H = 2
Double Covalent Bonds
• Double Bond - bond that involves 2 shared pairs of electrons (4 e-)
• Sometimes atoms need to share 2 or 3 pairs of electrons• Hydrogen will NEVER form
double or triple bonds.
Triple Covalent Bond
• Triple Bond - bond that involves 3 shared pairs of electrons (6 e-)
Covalent Bonds
Practice Lewis Dot Structure
Chemical Formula
# of Valence
Electrons
Single Line Bond
Structure
# of Remaining Electrons
Lewis Dot Structure
Octet Check All Atoms=8Hydrogen=2
O2
CO2
N2
HCN
12 O-O 10 2 O = 8
16 O-C-O 12 C = 82 O = 8
10 N-N 82 N = 8
10 H-C-N 6C = 8N = 8H = 2
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Drawing Lewis Structures of Molecules
If the compound contains more than 2 atoms:
• how are the atoms bonded and,
• if there are nonbonding electron, where are they?
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Molecules with a central atom : NH3, PCl3, CHCl3
Central atom is generally the first in the molecular formula and the
most electronegative one.
NH H
HC
H
Cl Cl
Cl
PCl
Cl
Cl
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
…unless the first element is Hydrogen :
H2OO
H H
HCN C NH(same order as in formula)
How to find the # of bonds in a lewis structure
**Doesn’t work for molecule over an octet
1. Find the total # of valence electrons.2. Use the formula to find the number of
bonds. # of val e- needed (all have 8 or 2 e-) - # of val e- available = ____/2 to find the # of bonds
(# val e- – # val e- available)
2# of bonds=
1. Find the total # of valence electrons.2. Use the formula to find the number of
bonds. # of val e- needed (all have 8 or 2 e-) - # of val e- available
= ____/2 to find the # of bonds Ex: Find the number of bonds for each molecule or
compound and write the lewis dot structure # of Bonds LDS
a.) CO
b.) C2F4
c.) C2H6
**Doesn’t work for molecule over an octet
3
67
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
(1) Sum valence electrons from all atoms: these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
NH38
NH H
H
2
n/a
n/a
Rules for Drawing Lewis Structures
NH H
H
2 leftovers
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
CO(1) Sum valence electrons from all atoms: these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
10
C-O8
(treat C as central)
C-O
C-O
2 left
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
SF2(1) Sum valence electrons from all atoms:
these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
20
F-S-F 16
4 left
ChemicalBonding
Lewis Structure for Ions• If a molecule has a positive charge,
subtract that many electrons from the total valence electrons available.
• If a molecule has a negative charge, add that many electrons to the total valence electrons available.
• has 8 electrons available.
• has 24 electrons available.
• has 32 electrons available.
ChemicalBonding
For ions, the charge is generally indicated by
square brackets and the sign
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
IonsNH4
+(1) Sum valence electrons from all atoms:
these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
8
0 left
n/a
n/a
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Ions ClO2-
(1) Sum valence electrons from all atoms: these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
20
O-Cl-O 16 left
4 left
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule
On occasion, an atom in a molecule does not have an octet of valence electrons:
• If the molecule has an odd number of valence electrons
• an atom may have less than an octet [mainly Be, B]
• an atom may have more than an octet [periods 3-7]
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: odd number of electrons
NO2(1) Sum valence electrons from all atoms:
these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
17
O-N-O 17
1 left
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: less than an octet (B or Be)
BF3
(1) Sum valence electrons from all atoms: these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
24
18 left
0 left
ChemicalBonding
More Than Eight Electrons
• The only way PCl5 can exist is if phosphorus has 10 electrons around it.
• Periods 3-7 can expand its orbitals.
ChemicalBonding
More Than Eight Electrons
Even though we can draw a Lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygens.
ChemicalBonding
More Than Eight Electrons
• This eliminates the charge on the phosphorus and the charge on one of the oxygens.
• The lesson is: When the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.
ChemicalBonding
Chapter 8: Basic Concepts of Chemical Bonding
Exceptions to the Octet rule: more than an octet
BrF5(1) Sum valence electrons from all atoms:
these are the ones that need to be distributed
(3) Complete "octets" of atoms around central atom
(4) Place any leftover electrons on the central atom.Check that central atom has octet
(2) Connect atoms by covalent bonds: count electrons left
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
42
32
2 left
ChemicalBonding
Bond Dissociation Energy• Bond Dissociation Energy - energy required to
break a bond between two atoms
A large bond dissociation energy corresponds to a strong bond which makes it unreactive
Carbon has strong bonds, which makes carbon compounds stable and unreactive
Chapter 8.3 - Bonding Theories
• Determining shape through bonds
VSEPR Theory
• VSEPR Theory predicts the 3D shape of molecules
According to VSEPR, the shape of the molecule adjusts so that the electrons are far apart
A Few VSEPR Shapes
Nine possible molecular shapes
VSEPR Theory
Each bond (single, double, or triple) and unshared pair is considered a steric number
VSEPR can only be used with the central atom
Unshared pairs of electrons are important in predicting the shapes of molecules
Use the steric number to predict the molecular geometry
Molecule Lewis Dot Structure
Steric # Shape
H2O
CO2
XeF4
4Bent
2 Linear
6Square Planar
Hybrid Orbitals
• VSEPR is good at describing the molecular shapes, but not the types of bonds formed
In hybridization, several atomic orbitals mix to form hybrid orbitals
• Orbital hybridization provides information about both molecular bonding and molecular shape
Bond Hybridization
• Hybridization Involving Single Bonds – sp3 orbital Ethane (C2H6)
• Hybridization Involving Double Bonds – sp2 orbital Ethene (C2H4)
• Hybridization Involving Triple Bonds – sp orbital Ethyne (C2H2)
Gets another orbital added for each atom and lone pair around atom of interest.
Start with sp…
…then sp2…(what is the orbital hybridization of C?)
…then sp3
Chapter 8.4 – Polar Bonds and Molecules
• There are two types of covalent bonds Nonpolar Bonds (share equally) Polar Bonds (share unequally)
Polar Covalent
• Polar Bond - unequal sharing of electrons between two atoms (ex: HCl)
• In a polar bond, one atom typically has a negative charge, and the other atom has a positive charge
Nonpolar Covalent Bond
• Nonpolar Bond - equal sharing of electrons between two atoms (Cl2, N2, O2)
Classification of Bonds
You can determine the type of bond between two atoms by calculating the difference in electronegativity values between the elements
Type of Bond Electronegativity Difference
Nonpolar Covalent 0 0.4
Polar Covalent 0.5 1.9
Ionic 2.0 4.0
Practice
What type of bond is HCl? (H = 2.1, Cl = 3.1)
Your Turn To Practice N(3.0) and H(2.1)
H(2.1) and H(2.1)
Ca(1.0) and Cl(3.0)
Mg(1.2) and O(3.5)
H(2.1) and F(4.0)
Difference = 3.1 – 2.1 = 1.0Therefore it is polar covalent bond.
Polar molecules – • All molecules
with lone pairs (unless it’s linear)
• All molecules surrounded by different atoms.
Nonpolar molecule – • Linear molecules
with the same atoms• All molecules
surrounded by the same atoms.
Dipole
When there is unequal sharing of electrons, a dipole exists
Dipole - a molecule with two poles with opposite charges
Represented by an arrow pointing towards the more negative end
Practice Drawing Dipoles
P- BrP = 2.1Br = 2.8
P –Br + -
Practice H(2.1) – Cl(3.0) C(2.5) - F(4.0) H(2.1)– F(4.0)
Attractions Between Molecules
• Intermolecular attractions are weaker than ionic, covalent, and metallic bonds
• There are also attractions between molecules
• There are 2 main types of attractions between molecules: Van der Waals and Hydrogen
Van der Waals Forces
• Van der Waals forces consists of the two weak attractions between molecules
1. dipole interactions – polar molecules attracted to one another
2. dispersion forces – caused by the motion of electrons (weakest of all forces)
Hydrogen Bond
Hydrogen Bonds - forces where a hydrogen atom is weakly attracted to an unshared electron pair of another atom
Strongest of all intermolecular forces