chapter 8 outline - chemistry courses — penn state...

Chapter 8 outline

Ionic Bonding Lattice energy Exceptions to octet rule

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Covalent Bonding Bond Polarity

Electronegativity Lewis Structures

–  drawing Lewis structures –  deciding between alternate Lewis

structures: formal charge –  resonance structures –  exceptions to the octet rule

Bond properties Bond length, bond energy

Chemical Bonding

3 forms of bonding IONIC

electrons traded to form separate ions

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COVALENT electrons shared between a few nuclei distinct molecules

METALLIC electrons shared among all nuclei

2

For ionic and covalent bonding:

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1. Valence electrons are involved in bonding

2. Octet rule: elements tend to gain, lose or share electrons so as to gain an inert gas configuration. (Duet for H and He)

⇒

IONIC BONDING

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Ionic bond is due to electrostatic attraction.

Coulomb's Law:

Electrons are exchanged to form separate ions with complete octets.

IONIC COMPOUNDS

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Characteristics

Ionic vs. Molecular

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Ionic compounds: a compound that consists of positive and negative ions.

Metal + nonmetal (usually)

Molecular compounds: compounds consisting of individual molecules

All nonmetals or nonmetals and metalloids.

Molecular compounds usually have low melting points

Ionic compounds have high melting points and are brittle.

Naming Inorganic Compounds and Ions

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Name Formula potassium ion

CO32−

ammonium ion

sodium bicarbonate

H2SO4 " Combine Ca+2 and PO4

3−

Cu(II) and sulfate ion

See Section 2.8 Tables 2.4, 2.5

Strength of Ionic Bond

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Lattice energy is the change in energy when an ionic solid is separated into isolated ions in the gas phase.

Lattice energy cannot be determined experimentally

Born-Haber cycle

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Lattice Energy

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NaCl E = 788 kJ MgO E =3795 kJ Why the big difference???

dQQE 21∝

d

Salts and Ionic Lattices

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Which has the greatest lattice energy? (How do you know?)

NaF NaCl NaBr NaI

ION SIZES: Ion sizes are important in ionic bonds

lattice energy can’t be measured: what can we measure that is related to lattice energy?

Salts and Ionic Lattices

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Melting point increases as lattice energy increases.

mp NaF 993oC NaCl 801oC NaBr 747oC NaI 661oC

MgO 2800oC

TRANSITION METAL IONS (exception to octet rule)

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•  s electrons are part of valence electrons

•  transition metals can have variable charges.

When forming an ion: •  s electrons are lost first •  then maybe d electrons

COVALENT BONDING

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When neither atom is "willing" to give up electrons (completely) atoms share electrons: each atom has a noble gas configuration.

COVALENT BOND:

Bond strengths

Lewis Structures

Shared electron pairs ⇔ bonds

Lone pairs

Multiple Bonds

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N-N N=N N≡N

l 1.47Å 1.24Å 1.10 Å

E 163 418 941 kJ/mole

Only single, double, and triple bonds are allowed.

Bond Polarity

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Nonpolar covalent: electrons are shared equally (H2, Br2, Cl2)

Polar covalent: unequal sharing of electrons (HF, ICl)

Ionic: no sharing of electrons: (NaCl, LiF)

Bond polarity

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Electron sharing in covalent bonds depends on the electronegativity of atoms.

ELECTRONEGATIVITY:

not the same as electron affinity not directly measurable:

Most electronegative:

Least electronegative:

Electronegativity

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Periodic Trends:

nonmetals are most electronegative

metals are least electronegative

(especially active metals, Groups I and II)

Electronegativity and Bonding

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ionic: large difference in electronegativity

polar covalent: some difference in electronegativity

covalent: no difference in electronegativity.

RULES FOR WRITING LEWIS STRUCTURES

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1.  Count up the number of valence electrons.

3.  Write the atom symbols and connect bonded atoms with single bonds.

4.  Distribute electrons (in pairs) to complete octets of atoms.

5.  Not enough electrons? Make multiple bonds to complete octets if necessary.

6.  Extra electrons? Put them on the central atom.

Example

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Lewis structure of sulfite ion SO32-

valence electrons

draw structure

distribute electrons

Check to make sure all atoms have a complete octet!

Example: HCN

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1. valence electrons

2 connect atoms

3. distribute pairs

not enough electrons!

Example Carbonyl chloride: COCl2

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# of valence electrons =

Connect atoms

distribute electrons

Deciding between alternate Lewis Structures

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Use FORMAL CHARGE (FC):

FC = VE − LSE

VE =number of valence electrons in an isolated atom LSE = number of electrons on the atom in the Lewis structure

LSE = lone pair electrons + ½ shared electrons

The most stable structure is the one in which the atoms bear the smallest formal charge.

NITRATE ION: NO3−

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Valence electrons:

Connect atoms, distribute electron pairs

What does the Lewis structure indicate about bond lengths and strengths in NO3

−?

RESONANCE STRUCTURES

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Some molecules can not be represented by a single Lewis structure. NO3

-

N

O

O ON

O

O ON

O

O O

3 resonance structures

RESONANCE

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Different Lewis structures are equally good (usually).

Molecules with resonance exhibit extra stability

Properties (e.g. bond length, bond strength) are averaged over resonance structures

Molecule has RESONANCE when more than 1 Lewis structure can be drawn for a fixed nuclear

arrangement.

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Aromatic Hydrocarbons Contain alternating single

and double C-C Bonds Benzene is prototypical

molecule

Usually written:

Aromatic hydrocarbons are less reactive than alkenes:

Special kind of bonding Have “delocalized” π electrons: results in added stability

CCC

CCC

HH

HH

H

H CCCCCC

HH

HH

H

H

28

Review Lewis structures

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What is the Lewis structure for

C2H4

SiF4

XeF4

TeF4

EXCEPTIONS TO OCTET RULE

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1. Odd # of electrons (rare) E.g. NO, NO2, ClO2 # of valence e

BF3 reacts readily with electron pair donors (like NH3).

2. Incomplete octet (# e- < 8) Rare: Be, B

Example: BF3 B

F

F FB

F

F F

EXCEPTIONS TO OCTET RULE

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3. Molecule with atom having more than 8 electrons •  fairly common •  never occurs with period 2 atoms •  occurs with atoms in period 3 and below

WHY??

Examples NF5 PF5 PF5 PCl5 AsF5 PBr5 PI5

Conditions for expanded octet

Bond properties

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COVALENT BOND LENGTHS and ENERGIES

Bond length: distance between nuclei

BOND STRENGTH

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BOND DISSOCIATION ENERGY

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for diatomics, D is ΔH of one reaction: H⎯H(g) → 2H(g) DH-H= ΔHrxn = 436kJ/mol

for polyatomics, D is an averaged quantity H⎯O⎯H(g) → HO(g) + H(g) +494kJ/mol H⎯O(g) → H(g) + O(g) +424kJ/mol

DO⎯H = 463 kJ/mol * * value obtained from averaging over many molecules

bond (dissociation) energy: D enthalpy of bond breaking reaction in the gas phase.

D > 0 (ΔH > 0)

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λ Energy Interaction with matter???

gamma 10−12 m 1.20 × 108 kJ/mol

X-Ray 10−10 m 1.20 × 106 kJ/mol

ultraviolet 10−7 m =100nm

1.20 × 103 kJ/mol = 1200 kJ/mol

visible 400nm - 750nm

299 kJ/mol 160 kJ/mol

electronic transitions heat (translation)

infrared 10−6 m µm

120 kJ/mol vibration

microwave 10−2 m 12.0 J/mol rotation

radio 1m 0.12 J/mol flip nuclear spin 35 Fall 2010