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CHEMISTRY The Molecular Science Chapter eight. Slides prepared by S. Michael Condren Department of Chemistry Christian Brothers University. to Accompany CHEMISTRY The Molecular Science by John W. Moore, Conrad Stanitski, & Peter C. Jurs. Chapter 8. Covalent Bonding. Covalent Bonding.

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chemistry the molecular science chapter eight

CHEMISTRYThe Molecular ScienceChapter eight

Slides prepared by

S. Michael Condren

Department of Chemistry

Christian Brothers University

to Accompany

CHEMISTRY The Molecular Science

by John W. Moore, Conrad Stanitski, & Peter C. Jurs

chapter 8

Chapter 8

Covalent Bonding

covalent bonding
Covalent Bonding

sharing of electrons

H:H

H–H

shell model of the atom
Shell Model of the Atom

G.N.Lewis

1875-1946

AmericanChemist

Lewis Symbol

valence shell electrons = Group A

lewis symbol
Lewis Symbol

Sulfur  S

Group VIA

6 valence electron

.

: S :

.

lewis structures
Lewis Structures
  • combinations of Lewis symbols which represent possible bonding

Octet Rule

In compound formation an atom gains or

loses electrons, or shares pairs of electrons,

until its valence shell has eight electrons.

lewis structures1
Lewis Structures

H

CH4 methane H C H

H

bonding pair

of electrons

lewis structures2
Lewis Structures

C2H6 ethane

H H

××××

H : C : C : H

××××

H H

selected lewis structures
Selected Lewis Structures

Number of electrons

Number shared to complete an

Group of valence octet (8-group

number electrons number) Example

4A 4 4 C in CH4

5A 5 3 N in NF3

6A 6 2 O in H2O

7A 7 1 F in HF

guidelines for writing lewis structures
Guidelines for Writing Lewis Structures

1. Add up the number of valence electrons in the molecule or ion, adding electrons for each negative charge on ion and removing electrons for each positive charge.

2. Create a skeleton structure using the chemical symbols for the atoms and a line for bonding pairs of electrons.

guidelines for writing lewis structures1
Guidelines for Writing Lewis Structures

3. Satisfy the Octet Rule for each atom, starting with the central atom.

4. All remaining electrons should be placed around central atom

5. Add multiple bonds to central atom if less than eight electrons are around central atom.

single covalent bonds in hydrocarbons
Single Covalent Bondsin Hydrocarbons

Saturated hydrocarbons

- C - C - C - C - C - C - C - C - C - C - C -

or

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3

saturated hydrocarbons
Saturated hydrocarbons

butane

2-methylpropane

pentanes
Pentanes

H H H H H

| | | | |

H - C - C - C - C - C - H

| | | | |

H H H H H

pentane

H H H H

| | | |

H - C - C - C - C - H

| | | |

H CH3 H H

2-methylbutane

H CH3 H

| | |

H - C - C - C - H

| | |

H CH3 H

2,2-dimethylpropane

multiple covalent bonds
Multiple Covalent Bonds

double bond  2 pairs shared

triple bond  3 pairs shared

normally occurs between:

C atoms; N atoms; O atoms;

a C atom and a N, O or S atom

a N atom and a O or S atom

a S atom and an O atom

lewis structures3
Lewis Structures

CO2 carbon dioxide

××××

: O :: C :: O :

double bonds

lewis structures4
Lewis Structures

CO carbon monoxide

: C ::: O :

triple bond

properties of cis and trans 1 2 dichloroethene
Properties of cis- and trans-1,2-dichloroethene

Physical Property cis-1,2-dichloroethene trans-1,2-dichloroethene

Melting Point -80.5 °C -50.0 °C

Boiling Point (at 1 atm) 60.3 °C 47.5 °C

Density (at 20 °C) 1.284 g/ml 1.265 g/ml

triglycerides fats and oils
Triglycerides, Fats and Oils

triglyceride - 1 mole of glycerol plus three moles of fatty acids

fat - solid triglyceride at room temperature

oil - liquid triglyceride at room temperature

3 fatty acids + glycerol  triglyceride + 3H2O

the 3 fatty acids can be the same or different

slide27
Fats

saturated fats – fats containing only C–C single bonds

unsaturated fats – fats containing one or more C=C double bonds

monounsaturated fats – fats containing one C=C double bond

polyunsaturated fats – fats containing two or more C=C double bonds

fatty acids
Fatty Acids

animal fat – primarily saturated fatty acids

vegetable fat – primarily unsaturated fatty acids

more unsaturation, lower melting point, more liquid in character

most natural unsaturated fatty acids are cis- configuration, producing lower melting points than corresponding trans- isomers

slide29
Fats

animal fat

  • more likely solid, less unsaturation

vegetable fat

  • more likely liquids, more unsaturation
  • converted to semisolids by hydrogentation (addition of gaseous hydrogen to some of the double bonds)
example using bond energy data calculate the dh o for
EXAMPLE: Using bond energy data, calculate the DHo for:

CH4(g) + 2 O2(g)CO2(g) + 2 H2O(g)

DHo = ([ 4( 414 ) + 2 ( 498 )]broken

- [2 ( 715 ) + 4 ( 464 )]formed)kJ/mol

= ([ 1656 + 996 ] - [ 1430 + 1856 ])kJ/mol

= ([ 2652 ] - [ 3286 ])kJ/mol

= - 634 kJ/mol

from DHfo data: DHo = - 802.3 kJ/mol

linus pauling
Linus Pauling

1901–94

American chemist

One of the few recipients of two Nobel prizes. His paper “The Nature of the Chemical Bond and the Structure of Molecules and Crystals” won him the 1954 Nobel Prize in chemistry. Won the peace prize in 1962.

electronegativity
Electronegativity

Pauling Scale

  • relative attraction of an atom for electrons, its own and those of other atoms
  • same trends as ionization energy, increases from lower left corner to the upper right corner
  • fluorine: E.N. = 4.0
covalent bond properties
Covalent BondProperties

electronegativity

nonpolar bonds DEN = 0

polar bonds DEN > 0

ionic bonds DEN > 1.5

formal charge
Formal Charge

Formal Charge is the charge on an atom if all of the shared electrons were equally shared

  • lone pair electrons are assigned to atom about which they are found
  • half of the bonding electrons are assigned to each atom of the bonded pair
  • the sume of formal charges on a molecule is zero and the magnitude of the charge on ions
formal charge calculations
Formal Charge Calculations

Formal charge = (# valence electrons on an atom) - [(number of lone pair electrons)

+ (1/2number of shared electrons)]

N C O

Valence electrons 5 4 6

Lone pair electrons 2 0 6

1/2 shared electrons 3 4 1

Formal charge 0 0 –1

resonance
Resonance

Resonance is present when two or more electron dot structures that differ only in the arrangement of the electrons. The resonant hybrid, an average structure, best represents the structure

ozone
Ozone

O = O - O O - O = O

resonance in so 3
Resonance in SO3

O

S

O O

O

S

O O

O

S

O O

exceptions to octet rule
Exceptions to Octet Rule

NO nitric oxide

less than octet

××××

×N :: O:

exceptions to octet rule1
Exceptions to Octet Rule

expanded octet

PF5

F

F P F

F F

exceptions to octet rule2
Exceptions to Octet Rule

expanded octet

SF6

F

F F

S

F F

F

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