Covalent Bonding

Chapter 9

What do the following have in common?

Oil and Vinegar

They are covalent compounds.

Review

Ionic bonds are formed by a transfer of e-

Metals + Nonmetals Ionic Compound

FU = Formula unit

smallest part of an ionic compound

Apply the octet rule to atoms that bond covalently

Octet Rule:Atoms will gain e-, lose e- or share e- in order to

get 8 valence e- to be stable

Covalent Bond

Sharing of e- to be stable

Usually occurs when elements are close on periodic table

NM + NM covalent compound

aka: molecular compoundhttp://school.discovery.com/clipart/category/anmt.html

molecule

2 or more atoms combine covalently

Ex: carbohydrates, proteins, DNA, fibers

Describe the formation of single, double and triple covalent bonds

• Single bond – formed because only 1 e- pair is shared

between 2 elements

• These elements need only one additional e- to be stable– Ex: hydrogen and the halogens (Group 7A)

H·+·H H– H H2

Double Bonds• Form because each element needs 2 e-

pairs

·:O:·+ ·:O:· ::O=O:: O2

Triple Bonds• Formed because 3 e- pairs are shared

between two elements

:N:· + :N:· :N Ξ N: N2

Diatomic molecule2 atoms that bond together

Featuring

“H and the Sensational 7”

• Covalent bonds are formed by diatomic molecules

• Different nonmetals can share e- to be stable and form covalent bonds

• By drawing e- dot structures of each atom, you can put them together so each has 8 valence e- making a

LEWIS STRUCTURE

Draw Lewis Structures

• PH3

• H2S

• CO2

Draw Lewis Structures

• PH3 ¨H—P—H

I

H

• H2S

• CO2

Draw Lewis Structures• PH3

•H2S ֵ

H—S—H

¨• CO2

Draw Lewis Structures

• PH3

• H2S

• CO2

:O=C=O:

¨ ¨

Relate the strength of covalent bonds to bond length

• The more bonds located between 2 atoms, the shorter the bonds are

• The shorter a bond is, the stronger it is

H – H single bond, not too strong

O=O double bonds, stronger

NΞN triple bonds, strongest

Review 9.1

• When 2 Nonmetals form a compound, they SHARE e-

• This is a covalent bond

• The compound formed is a molecular compound

• The smallest part of a molecular compound is a molecule

Bond Type Ionic Bond Covalent Bond

Elements M + NM

Polyatomic Ion

NM + NM

Atoms become stable by

Gain/Lose e- SHARE e-

Name of Compound

Ionic compound Molecular Compound

Smallest Particle

Formula Unit (FU)

Molecule

9.2 Naming molecules

Identify the names of binary molecular compounds from their formulas

Molecules are formed when nonmetallic atoms share e-

They can combine in different ratios, such as CO and CO2; therefore, PREFIXES are used in the name

PREFIXES

• KNOW the prefixes on table 9-1 (p. 248)

1 mono- 6 hexa-

2 di- 7 hepta-

3 tri- 8 octa-

4 tetra- 9 nona-

5 penta- 10 deca-

Using prefixes

• The prefix “mono” is NEVER used for the first element, but all prefixes are

• When the element begins with a vowel (O, I), you drop the a or o from the prefix

• The second element always has a prefix and ends in “-ide”

Examples• CCl4

Carbon tetrachloride

• As2S3

Diarsenic trisulfide

• CO

Carbon monoxide

• CO2

Carbon dioxide

Writing Formulas

• When writing formulas for molecular compounds, use prefixes to tell how many atoms are in the molecule

• Sulfur dioxide

SO2

• Diphosphorous pentoxide

P2O5

Name acidic solutions

• Acid formulas begin with an H

• H becomes hydro-

• The acid name comes from the second element or polyatomic ion name

Naming acid

2nd name ends in • -ide

HCl (chloride)

HF (fluoride)

• -ate

• -ite

Acid name• Hydro ____ic acid

Hydrochloric acid

Hydrofluoric acid

• ___ic acid

• ___ous acid

Naming acid

2nd name ends in • -ide

• -ate

H2SO4 (sulfate)

HNO3 (nitrate)

• -ite

Acid name• Hydro ____ic acid

• ___ic acid

sulfuric acid

nitric acid• ___ous acid

Naming acid

2nd name ends in • -ide

• -ate

• -ite

HClO3 (chlorite)

H3PO3 (phosphite)

Acid name• Hydro ____ic acid

• ___ic acid

• ___ous acid

Chlorous acid

Phosphorous acid

Naming acid

2nd name ends in • -ide

HCl (chloride)

HF (fluoride)

• -ate

H2SO4 (sulfate)

HNO3 (nitrate)

• -ite

HClO2 (chlorite)

H3PO3 (phosphite)

Acid name• Hydro ____ic acid

Hydrochloric acid

Hydrofluoric acid

• ___ic acid• sulfuric acid

nitric acid• ___ous acid

Chlorous acid

Phosphorous acid

Writing Acid Formulas• The first element in an acid is ALWAYS H+

• To find the second, use the same chart, work backwards:

-ide hydro__ic acid

-ate ___ic acid

-ite ____ous acid

Example: Phosphoric acid

___ ic acid means the second name ends in –ate

Phosphate is the second name

Hydrogen phosphate (now use charges and criss cross)

H+ PO4 3-

H3PO4

9.3 Molecular Structure

• Structural formulas = uses letter symbols and bonds to show relative position of atoms (same as Lewis Structure)

• Chem I:– Don’t need definition of resonance or

coordinate covalent bond– Only need step 1 of drawing Lewis structures– Don’t need obj. 2, 3, of 9.3

Rules

*1. Predict location of certain atoms

a) Hydrogen is always terminal (end) atom

b) Atom with least e- affinity (furthest to left on periodic table) is central atom

9.4 Molecular Shape

VSEPRValence Shell Electron Pair Repulsion Theory

States:

Repulsion of shared and unshared pairs (lone pairs) of e- around the central atom shapes the molecule

Table 9-3• Look at a correctly drawn structural

formula

• Count the number of shared pairs and lone pairs on the CENTRAL ATOM

• Compare to the table

• Examine table 9-3 examples on p. 260

Atoms in Molecule

Lone pairs on central atom

Molecule Shape

2 Linear

3 None Linear

3 Lone pair Bent

4 None Trigonal planar

4 Lone pair Trigonal pyramidal

5 tetrahedral

9.5 Electronegativity & Polarity

Electron Affinity

is the ability of an atom to accept an e-

• Excluding noble gases, e- affinity increases to the right of the periodic table

• Electron affinity increases going up in a group

Electronegativity

• Assigned values for elements that compare the ability of an atom to attract shared e- to itself to the ability of Fl to do the same

• Has the same trend as e- affinity

Bond Polarity

To determine if a bond is polar or not: (polar means having a negative and positive side)

– Compare the electronegativity of each atom connected by the bond

– The atom that is further to the right (or in the same group: further up) will hold the e- closer and be σ-

– The other atom will be σ+

Examples

H2O

CH3Cl

An atom may be + with respect to one bond and – with respect to another in the same molecule

More info

Electronegativity Difference

Bond Type

0 Nonpolar Covalent

0.1 – 1.7 Polar Covalent

Above 1.7 Ionic Bond

Molecule Polarity

To determine if the entire molecule is polar (has a negative and positive side) or not, you must look at 2 things

1. molecule shape

2. bond polarity

If the molecule contains only nonpolar bonds, then the molecule is also NONPOLAR

• If the molecule has polar bonds, it may be a NONPOLAR or a POLAR molecule, depending on the shape

• Nonpolar molecules are usually linear (with same charge on all sides); tetrahedral (with same charges on all sides); planar (with same charge on all sides)

• Polar molecules are bent or pyramidal (with opposite charges on sides)

• Polar molecules can be linear, tetrahedral, or planar (with opposite charges on sides)

Properties of Molecular Compounds

• REVIEW: Ionic compounds were crystalline solids at room temp with high melting points and high boiling points

• Molecular compounds may be gases, liquids or solids at room temp

• Molecular compounds have LOW melting points and LOW boiling points

Comparison

Ionic Molecular

State at room temp

Crystalline solid Gas, solid, liquid

Boiling Point High Low

Melting Point High Low