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


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


…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


(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


CO(1) Sum valence electrons from all atoms: these are the ones that need to be distributed





10

C-O8

(treat C as central)

C-O

C-O

2 left

ChemicalBonding


SF2(1) Sum valence electrons from all atoms:

these are the ones that need to be distributed





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.



ChemicalBonding

For ions, the charge is generally indicated by

square brackets and the sign

ChemicalBonding


IonsNH4

+(1) Sum valence electrons from all atoms:






8

0 left

n/a

n/a

ChemicalBonding


Ions ClO2-






20

O-Cl-O 16 left

4 left

ChemicalBonding


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


Exceptions to the Octet rule: odd number of electrons

NO2(1) Sum valence electrons from all atoms:






17

O-N-O 17

1 left

ChemicalBonding


Exceptions to the Octet rule: less than an octet (B or Be)

BF3






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


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


• 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


Exceptions to the Octet rule: more than an octet

BrF5(1) Sum valence electrons from all atoms:






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

http://en.wikipedia.org/wiki/Image:VSEPR_geometries.PNG

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

chapter 8 – covalent bonding the unspoken hero: “covalent bond”

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