Chapter 8 Basic Concepts of Chemical Bonding. 6.3 Describing Chemical Bonding Page: 288 - 330

Chapter 8Basic Concepts

of Chemical Bonding

6.3 Describing Chemical Bonding

Page: 288 - 330

Thermochemistry

CHEMICAL BONDSIONIC

Electrostatic attraction

between ions.

METALLIC

Metal atoms

bonded to several other

atoms.

COVALENT

Sharing of electrons.

Thermochemistry

The Ionic Bonding

Thermochemistry

Na(s) + ½Cl2(g) NaCl(s)

8.2 The Ionic Bonding

Thermochemistry

Na: loss of an electronCl: gain of an electron

HOW CAN YOU KNOW WHETHER THIS ELECTRON

TRANSFER OCCURS READILY?

The Ionic Bonding

Thermochemistry

HOW CAN YOU KNOW WHETHER THIS ELECTRON

TRANSFER OCCURS READILY?

The Ionic Bonding1. Ionization energy

2. Electron Affinity

3. Lattice Energy

4. Electronegativity Difference

Thermochemistry


1. IONIZATION ENERGY

One species must have very low ionization

energy (Na)

The Ionic Bonding

Thermochemistry


1. IONIZATION ENERGY2. ELECTRON AFFINITY

One species must have very high electron

affinity (Cl)

The Ionic Bonding

Thermochemistry

Na(s) + ½Cl2(g) NaCl(s) Δ Hf˚= -410.9 kJ

ELECTROSTATIC ATTRACTIONS

The Ionic Bonding

Thermochemistry

Attraction between oppositely charged ions release of energy

formation of lattice

LE is a measure of a

stability of ions arranged within

an ionic solid

3. LATTICE ENERGY

Thermochemistry

If the difference in electronegativity (ΔEN) between

the species is:ΔEN > 1.7

An Ionic Bond would form between these

species

4. ELECTRONEGATIVITY DIFFERENCE

Thermochemistry

The Ionic Bonding SUMMARY

Thermochemistry

Thermochemistry

Thermochemistry

Thermochemistry

6.3 THE COVALENT BONDING

Covalent Bonding

Covalent Bonding

Thermochemistry

ELECTRONEGATIVITYis the ability of atoms in a

molecule to attract electrons to themselves.

Thermochemistry

DIPOLE and ELECTRONEGATIVITY

DIPOLE: a partial separation of charge One end of the molecule has slightly positive

charge The other end of the molecule has slightly

negative charge

Slight excess of a negative

charge

Slight excess of

a positive charge

DIPOLE and ELECTRONEGATIVITY

Slight excess of

a positive charge

Thermochemistry

DIPOLES are caused by ELECTRONEGATIVITY

Chlorine has higher electronegativity

The electrons spend MORE time around

Chlorine

Hydrogen has lower

electronegativity

The electrons spend LESS time around Hydrogen

BOND POLARITYIn different

compounds, electrons are

not shared equally

Homo nuclear diatomic compounds: H2, Cl2, O2,N2 … share electrons equally =

NON POLAR COVALENT BOND

BOND POLARITY

Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons

equally

a. POLAR COVALENT BOND(the attraction of one of the atoms for the bonding electrons is LARGE)δ+ = slightly positive

δ- = slightly negative

BOND POLARITY


equally

a. POLAR COVALENT BOND(the attraction of one of the atoms for the bonding electrons is LARGE)

H – Fδ+ δ-

BOND POLARITY


equally

b. MOSTLY COVALENT BOND (the attraction of one of the atoms for the bonding electrons is

SLIGHTLY GREATER)

H – Fδ+ δ-

PeriodicProperties

of the Elements

© 2012 Pearson Education, Inc.

BOND POLARITY

Hetero nuclear diatomic compounds: HF, HCl, NO, NaCl, LiO… DO NOT share electrons equallya. POLAR COVALENT BOND(the

attraction of one of the atoms for the bonding electrons is LARGE)

b. IONIC BONDS (the attraction of one of the atoms for the bonding electrons is VERY LARGE)

HOW DO YOU KNOW IF THE GIVEN BOND IS

POLAR, NONPOLAR, OR IONIC?

ELECTRONEGATIVITY

The greater the difference in

electronegativity, the more polar the bond is

NON POLAR COVALENT BOND

ELECTRONEGATIVITY

MOSTLY COVALENT BOND

POLAR COVALENT BOND

IONIC BOND

ΔE = 0

0 < ΔE < 0.4

0.4 < ΔE < 1.7

ΔE > 1.7

H2, Cl2, O2,N2

MgO, NaCl, LiF

H2O, CO2, HF

H2O, CO2, HF

NON POLAR COVALENT

BONDH2, Cl2, O2,N2

POLAR COVALENT

BONDH2O, CO2, HF

IONIC BONDMgO, NaCl,

LiFelectronegativity

difference > 2.1

ELECTRONEGATIVITY

Thermochemistry

EXAMPLE In each case, which bond is more

polar: a) B – Cl or C – Cl,

b) P – F or P – Cl? Indicate in each case which atom has the partial

negative charge.B – Cl

C – Cl

1.0

0.5

δ+ δ-

δ+ δ-

F – P

P – Cl

1.9

0.9

δ+δ-

δ+δ-

IONIC vs COVALENT Compounds

Metal + nonmetalHigh melting point

Lattice(crystal) structures

Strong electrolytes

Nonmetal + nonmetal

Low melting pointLow boiling pointNon - electrolytes

IONIC Bonding

COVALENT Bonding

HOMEWORKPAGE: 324 - 323

PROBLEMS: all even

37

6.4 LEWIS STRUCTURE DIAGRAMS

LEWIS DIAGRAMS - REVIEWShow only an atom’s valence electrons

and the chemical symbol.

LEWIS DIAGRAMS

Rule # 1Dots representing valence

electrons are placed around the element symbols

Electron dots are placed singly until the fifth electron

is reached then they are paired

Rule # 2

Lewis Diagrams of IONS and IONIC BONDS

• For positive ions, one electron dot is removed from the valence shell for each positive charge.

• For negative ions, one electron dot is added to each valence shell for each negative charge.

• Square brackets are placed around each ion to indicate transfer of electrons.

LEWIS STRUCTURE DIAGRAMS for MOLECULES

Lewis structures are representations of molecules showing all valence

electrons:• bonding and • nonbonding

LEWIS STRUCTURE DIAGRAMS for MOLECULES

Lewis structures are representations of molecules showing all valence

electrons:• bonding and • nonbonding

a lone pair

Lewis Structures and Multiple Bonds

When two electron pairs are shared, two lines are drawn, i.e. double

bond

::O :: C :: O::

or ::O = C = O::

When three electron pairs are shared, three lines are drawn, i.e. triple

bond

:N ::: N: or

:N ≡ N:

Lewis Structures and Multiple Bonds

BOND LENGTHSThe length of the bond between two atoms decreases as the number of

shared electrons increases

N ≡ N

N = N

N – N > >

BOND STRENGTH?

STEPS TO FOLLOW…

1. Find the sum of valence electrons of all atoms in the polyatomic ion or molecule

a. If it is anion add e-b. If it is cation subtract e-

PCl35 + 21

valence electrons= 26 v.e.

2. Write the symbols for the atoms – Make one of the atoms a central atom (usually the least electronegative atom)

– connect it with the other atoms by single bonds

STEPS TO FOLLOW…

PCl35 + 21

valence electrons= 26 v.e.

26 – 6 valence

electrons= 20 v.e. left

STEPS TO FOLLOW…

2. Write the symbols for the atoms – Subtract those electrons from your total

number of valence electrons

3. Fill the octets of the outer atoms

20 valence electrons

STEPS TO FOLLOW…

3. Fill the octets of the outer atoms


STEPS TO FOLLOW…

Subtract the added electrons from your total number of valence electrons


STEPS TO FOLLOW…

Subtract the added electrons from your total number of valence electrons

STEPS TO FOLLOW…

20 – 18 valence

electrons= 2 v.e. left

4. Fill the octet of the central atom

2 valence electrons

STEPS TO FOLLOW…

4. Fill the octet of the central atom

0 valence electrons

STEPS TO FOLLOW…

5. If you run out of electrons before the central atom has an octet…

…form multiple bonds until it does

STEPS TO FOLLOW…

WORKSHEET EXAMPLE Draw Lewis Structures for

a) CH2Cl2

b) C2H4

c) BrO3-

WORKSHEET EXAMPLE Draw Lewis Structures for

a) NOb) BF3

c) PF5

EXCEPTIONTS TO THE OCTET RULE1. For molecules and polyatomic ions

containing an odd number of valence electrons

2. For molecules and polyatomic ions in which an atom has fewer than an octet of

valence electrons

EXCEPTIONTS TO THE OCTET RULE3. For molecules and polyatomic ions in

which an atom has more than an octet of valence electrons

1. For molecules and polyatomic ions containing an odd number of valence

electronsComplete pairing of valence electrons is impossible due to the odd number of

valence electrons

E.g.: ClO2, NO, NO2, O2-

2. For molecules and polyatomic ions in which an atom has fewer than an octet of

valence electrons• Not very common

• Mostly Boron or Beryllium compounds

3. For molecules and polyatomic ions in which an atom has more than an octet of

valence electrons• Very common• Such molecules/ions are called

HYPERVALENT• Only for atoms of 3rd period or higher

WHY?1. They have available and unfilled d

orbitals for bonding2. Their central atom (P, S, I, Xe…) is large enough to be bonded to even five different

atoms (Cl, F or O)

Thermochemistry

EXAMPLE Draw a Lewis Structure for ion:

ICl4-

Thermochemistry

EXAMPLE Draw a Lewis Structure for the

thiocyanate ion: NCS-

Thermochemistry

WHICH ONE IS CORRECT ?

WHICH ONE IS THE MOST IMPORTANT?

Thermochemistry

Calculate THE FORMAL CHARGE of each ion to find

out…

FORMAL CHARGEThe charge the atom would have if all the

atoms in the molecule had the same electronegativity

1. ALL unshared electrons are assigned to the atom on which

they are found

2. For a single/double/triple bond, half of the bonding electrons is assigned to to

each atom in the bond

FORMAL CHARGE OF AN ATOM =

# of VALENCE ELECTRON – # OF ELECTRONS ASSIGNED

1. ALL unshared electrons are assigned to the atom on which

they are found

2. For a single/double/triple bond, half of the bonding electrons is assigned to to

each atom in the bond

Thermochemistry

EXAMPLE What are the formal charges of C

and N in the cyanide ion: CN-?

[:C ≡ N:]-

1. ALL unshared electrons are

assigned to the atom on which they are found

2. For a single/double/triple bond, half of the bonding electrons is assigned to to each atom in the

bond# of VALENCE ELECTRON – # OF ELECTRONS

ASSIGNED

Thermochemistry

EXAMPLE What are the formal charges of C

and N in the cyanide ion: CN-?

[:C ≡ N:]-

1. ALL unshared electrons are

assigned to the atom on which they are found

2. For a single/double/triple bond, half of the bonding electrons is assigned to to each atom in the

bond# of VALENCE ELECTRON – # OF ELECTRONS

ASSIGNED

C NValence e- 4 5

Assigned e- 5 5

Formal Charge

-1 0

-1

0

Thermochemistry

WORKSHEET EXAMPLE What are the formal charges on

the thiocyanate ions?

WHICH STRUCTURE IS THE MOST IMPORTANT (DOMINANT) ONE?

1. The most important (most dominant) Lewis structure is the one which has its value closest to 0 (the one with the

fewest charges)

2. The most important (most dominant) Lewis structure is the one which has any negative charges reside on the

more electronegative atoms

Thermochemistry

WORKSHEET EXAMPLE What are the formal charges the

thiocyanate ions?

WHICH STRUCTURE IS THE MOST IMPORTANT (DOMINANT) ONE?

1. The most important (most dominant) Lewis structure is the one which has its value closest to 0 (the one with the

fewest charges)

2. The most important (most dominant) Lewis structure is the one which has any negative charges reside on the

more electronegative atoms

Thermochemistry

WORKSHEET EXAMPLE The cyanate ion, NCO-, has three

possible Lewis structures. a) Draw these three structures

assign formal charges in each.b) Which Lewis structure is

dominant?

8.6 RESONANCE STRUCTURESDraw a Lewis Structure of OZONE,

O3

RESONANCE STRUCTURES

OZONE: A mix of different

resonance structures

GREEN PAINT: A mix

of different colors

Not a single color

Not a single resonance structure


SAME BOND LENGTHS

Somewhere between single bond and double bond

RESONANCE STRUCTURES• Is the use of two or more Lewis structures to

represent a molecule• this is because that molecule can not be represented by only a single Lewis structure

Basic Concepts of Chemical

Bonding


• In truth, the electrons that form the second C—O bond in the double bonds below do not always sit between that C and that O, but rather can move among the two oxygen atoms and the carbon.

• They are not localized; they are delocalized.


Basic Concepts of Chemical

Bonding


RESONANCE STRUCTURES• The organic

compound benzene, C6H6, has two resonance structures.

• It is a hexagon with a circle inside to signify the delocalized electrons in the ring.

Thermochemistry

EXAMPLE What are the resonance

structures of nitrate ion: NO3-

Thermochemistry

WORKSHEET EXAMPLE Using Formal Charges and the

concept of Resonance, show why the correct Lewis structure of BF3

is the one in which B has an incomplete octet and only single

bonds are present.

8.8 STRENGTHS OF COVALENT BONDSLearn on your own

HOMEWORKPAGE: 325 - 326

PROBLEMS: 49, 51, 53, 55, 57, 61, 67, 69, 71, 73,

87

ANSWERS TO

WORKSHEET

EXAMPLES

Thermochemistry

WORKSHEET EXAMPLE Copy this table and fill out the

missing information.

Thermochemistry

WORKSHEET EXAMPLE Which substance would you expect to have the greatest

lattice energy, MgF2, CaF2, and ZrO2 ?

ZrO2

Thermochemistry

WORKSHEET EXAMPLE Ionizing an H2 molecule to H2

+

changes the strength of the bond. Based on the description of

covalent bonding given previously, do you expect the

bond H—H in H2+ to be weaker or

stronger than the bond in H2?Weaker. In both H2 and H2

+ the two H atoms are principally

held together by the electrostatic attractions between the nuclei and

the electron(s) concentrated between them. H2+ has only one

electron between the nuclei whereas H2 has two and this results

in the H—H bond in being stronger.

Thermochemistry

WORKSHEET EXAMPLE How does the

ELECTRONEGATIVITY of an element differ from its ELECTRON

AFFINITY?Electron affinity measures:a. the energy released when an isolated atom

gains an electron to form a 1- ion.

The electronegativity measures: b. the ability of the atom to hold on to its own

electrons c. the ability of the atom to attract electrons from

other atoms in compounds.

Thermochemistry

WORKSHEET EXAMPLE In each case, which bond is more

polar: a) B – Cl or C – Cl,

b) P – F or ? Indicate in each case which atom has the partial

negative charge.

Thermochemistry

WORKSHEET EXAMPLE The cyanate ion, NCO-, has three

possible Lewis structures. a) Draw these three structures

assign formal charges in each.b) Which lewis structure is

dominant?

Thermochemistry

WORKSHEET EXAMPLE Using Formal Charges and the concept

of Resonance, show why the correct Lewis structure of BF3 is the one in

which B has an incomplete octet and there are only single bonds present.

– Giving boron a filled octet places a negative charge on the boron and a positive charge on fluorine.

– This would not be an accurate picture of the distribution of electrons in BF3.

– Therefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electrons.

Documents

Chapter 8 Basic Concepts of Chemical Bonding. 6.3 Describing Chemical Bonding Page: 288 - 330