Unit 2 Chp 8 Notes B and L.pptx

UNIT 2

Chemical Bonding

Types of Chemical

Bonds

•

Ionic (metal and nonmetal)

•

Covalent (nonmetals only)

• Non-polar • Polar

Types of Bonds Formed?

• _{Type of bond that forms}

results from properties of elements joining

together and

ELECTRONEGATIVITY

differences of those elements forming bonds.

• _{Trend of EN increases L}

Electronegativity and Bond Classification

• _{Non-polar: equal sharing of electrons}

• _{Polar: unequal sharing of electrons, stronger}

Intermolecular forces (IMFs)

• _{Ionic: no sharing of electrons, transfer cation to anion}

EN Difference Bond Classification

0.0 - 0.3 Non-polar covalent (NPC) 0.4 - 1.7 Polar covalent (PC)

Bonds, in general:

• _{Valence e- transferred or shared to create bonds to}

achieve full octet

• _{The difference in electronegativity is not the only factor in}

determining if a bond should be designated as ionic or covalent.

• _Generally:

– _{bonds between a metal and nonmetal are ionic} – _{bonds between two nonmetals are covalent}

Ionic Compounds Structure & Prop.

nonmetal (anion, -)

• _{Transfer an electron}

• _{Form crystal lattice structures} • _{Solids at room temperature} • _Brittle

• _{High melting point}

• _{Strong electrostatic force holds ions}

together (Coulomb’s Law)

• _{Electrolytes (conduct as liquids or}

Ionic Solids Brittle?

• _{Can be brittle due to}

improper stacking or shifting of anions and cations

• _{The cations and anions in an}

ionic crystal are arranged in a systematic, periodic 3-D array that maximizes the attractive forces among cations and

Lattice Energy (Electrostatic force)

Lattice Energy: is the energy required to completely

separate one mole of a solid compound into its

gaseous ions.

NaCl

(s)



Na

(g)

+ Cl

(g)

ΔH

lattice

= +788 kJ mol

Predict Energy of bond?

• _{Use Coulomb’s Law to predict strength}

of ionic bond:

• _{interaction strength is proportional to}

the charge on each ion, larger charges lead to stronger interactions.

• _{interaction strength increases as the}

distance between the centers of the ions (nuclei) decreases, smaller ions lead to stronger interactions.

*Can also be used to predict properties of two different ionic compounds,

melting point

-9.3

Lattice energy (E) increases as their charges increase

and/or

as radii decreases.

compd lattice energy MgF₂ MgO LiF LiCl 2957 3938 1036 853 Q= +2,-1 Q= +2,-2

r F < r Cl Lattice Energy

E = kQ+_rQ

-Q₊ is the charge on the cation

Q_- is the charge on the anion

r is the distance between the ions NaCl(s)  Na+(g) + Cl-(g)

Example:

Arrange the following substances in order of

increasing lattice E?

NaF, CsI, CaO

Answer:

CsI < NaF < CaO

AP Practice Question

The lattice energy of a salt is related to the energy

required to separate the ions. For which of the

following pairs of ions is the energy that is required

to separate the ions largest? (Assume that the

distance between the ions in each pair is equal to the

sum of the ionic radii.)

A. Na

(

g

) and Cl

(

g

)

B. Cs

(

g

) and Br

(

g

)

C. Mg

(

g

) and O

(

g

)

AP Practice Question

The energy required to dissociate an ionic

solid into gaseous ions (lattice energy) for

the compounds NaF and MgF

 is shown in

the table above. On the basis of Coulomb’s

law, which of the following best helps to

explain the large difference between the

lattice energies of NaF and MgF

 ?

A. Solubility Difference

C. Mass

Difference

B.

D. Charge

AP Practice Question

Two pure elements react to form a compound. One

element is an alkali metal, X, and the other element

is a halogen, Z. Which of the following is the most

valid scientific claim that can be made about the

compound?

A. It has the formula XZ

.

B. It does not dissolve in water.

C. It contains ionic bonds.

Transition Metals and Ions

Because of their location, transition metals will

not usually form ions that have a noble-gas

configuration.

*Recall that transition metals will lose valence

shell (s sublevel) electrons first, then as many d

sublevel electrons as required to form the ion.

Example: Tin, Sn

Atom: [Kr] 5s

4d

5p

Sn

ion: [Kr] 4d

Ionic Compd Formula Writing Review:

Write the formulas for the following ionic compounds:

1. Iron (III) oxide

Fe

O

2. Magnesium bromide

MgBr

3. Tin (IV) sulfide

SnS

4. Aluminum phosphide

AP Practice Question

Which of the following correctly indicates whether the solid represented by the particulate model shown above conducts electricity and explains why or why not?

A. It conducts electricity because it is made of positive and negative ions.

B. It conducts electricity because it is made of particles of different sizes.

C. It does not conduct electricity because its ions cannot move freely within the solid.

AP Practice Question

Of the following compounds, which is the most

ionic?

Bonding with Transition Metals --

Metallic Bonding

• _{Electrons easily move between}

orbitals creating a sea of electrons

• _{Good conductors of electricity}

and heat

• _{Malleable because electrons}

can slide past each other without breaking

• _Many

More Properties of Metals

• _{In a metallic solid, the}

valence electrons from the metal atoms are considered to be delocalized and not associated with any

individual atom.

• _{Usually pure substances,}

Properties of metal alloys:

1. Understood in terms of size of component

atoms:

A. Interstitial alloys B. Substitutional alloys

2. Typically remain a sea of mobile e- and remain conducting.

Metal Alloys

 Substitutional Alloy: some

metal atoms replaced by others of similar size.

 _Properties:

 Density lies between

component atoms

 Alloy remains malleable

and ductile

Metal Alloys

(continued)

 Interstitial Alloy: Interstices

(holes) in closest packed

metal structure are occupied by small atoms.

 Properties:

 Make lattice more rigid,

decreasing malleability and ductility

AP Practice Question

AP Practice Question

Consider two alloys, one of Au and Cu and one of Au and Ag, each with the same mole fraction of Au. If the Au/Cu alloy is harder than the Au/Ag alloy, then which of the following is the best explanation based on the information in the table above? A.Cu has two common oxidation states, but Ag has only one.

B.Cu has a higher melting point than Au has, but Ag has a lower melting point than Au has.

C.Cu atoms are smaller than Ag atoms, thus they interfere more with the displacement of atoms in the alloy.

Covalent Bonding

• _{Nonmetals only}

• _{Share electrons to achieve}

octet

• _{Gases, liquids, or solids at}

room temperature

• _{Low melting points}

• _{Pliable or malleable materials} • _{Use Lewis Structures to}

Electrons involved in bonding: Valence

Lewis electron-dot symbols or Lewis symbols:

• _{Shows the element symbol surrounded by its}

number of valence electrons.

• _{Can be used to represent metals or nonmetals} • _Examples:

Octet Rule:

Octet Rule: Atoms tend to gain, lose, or share

electrons until they are surrounded by eight

valence electrons.

A covalent bond is a chemical bond in which two or more electrons are shared by two atoms.

Why should two atoms share electrons?

F + F

7e- 7e

-F -F

8e- 8e

Lewis Structures

•

Lewis structures or Lewis electron-dot

structures show how covalent molecules share

a pair or more of e

between them.

To draw a molecule:

1. List both atoms next to each other. 2. Figure out total # valence electrons 3. Place dots around each atom

4. Place unpaired electrons facing each other 5. Replace electrons with a single bond

8e

-H + O + H H O H or H O H

2e- _2e

-Lewis structure of water

Double bond – two atoms share two pairs of electrons

single covalent bonds

O C O or O C O

8e- _8e- _8e

-double bonds double bonds

Triple bond – two atoms share three pairs of electrons

N N

8e-_8e

-N N

triple bond triple bond

or

Trend: Remember 8 is magic #

Group 14 Group 15 Group 16 Group 17

Needs 4 bonds

Needs 3

More Dots Practice

CH₄

NH₃

C₂H₄

CN

AP Practice Question

The electron-dot structure (Lewis structure) for which of the following molecules would have two unshared pairs of electrons on the central atom?

What about Expanded Octets?

*Only possible in Period 3 elements or larger

Atom arrangement?

•

PCl

– 5 ligands

Three Exceptions to the Octet Rule

1. The Incomplete Octet

BF₃ B – 3e

-3F – 3x7e

-24e

-F B F

F

3 single bonds (3x2) = 6 9 lone pairs (9x2) = 18

Exceptions to the Octet Rule

2. Odd-Electron Molecules

N – 5e

-O – 6e

-11e

-NO N O

3. The Expanded Octet (central atom with principal quantum number n > 2)

SF₆ S – 6e

-6F – 42e

-48e- S

F F F F F F

6 single bonds (6x2) = 12 18 lone pairs (18x2) = 36

Formal Charge

Formal charge can be calculated to help determine the most favorable (lowest formal charges) Lewis

Structure if multiple structures exist. Rules:

1. All unshared electrons are assigned to the atom on which they are found.

2. For any bond – single, double or triple – half the bonding electrons are assigned to each atom in a bond.

3. Formal Charge = VE - # of e- assigned

Draw the possible structures: Ex: SCN- _{(Draw 3 possibilities)}

*Favored structure is where formal charge is the

least.

*

Favored structure is also where the charge

exists on the more electronegative element

.

Use formal charge to determine which is

the best structure to depict the molecule

AP Practice Question

Which of the following Lewis diagrams best

represents the bonding in the N

O molecule,

considering formal charges?

A resonance structure is one of two or more Lewis structures for a single molecule that cannot be

represented accurately by only one Lewis structure, mostly due to delocalized electrons (e- not stationary on ea atom).

O O+ O - - O +O O

Resonance in an acetate ion: CH3COO

Resonance

  The actual structure is an average of the resonance

structures.

Benzene, C₆H₆

  The bond lengths in the ring are identical, and

AP Practice Question

For which of the following molecules are reso nance structures necessary to describe the bond ing satisfac torily?

AP Practice Question

The questions below refer to the following species.

(A) H₂O (B) NH₃ (C) BH₃ (D) CH₄ (E) SiH₄

1. Has a central atom with less than an octet of electrons?

C. BH₃

2. Has two lone pairs of electrons?

What are isomers?

Isomer

: Each of two or more

compounds/molecules with the same formula

but different arrangement of atoms in the

molecule and different properties

Covalent Bonding Forces?

What

needs to be considered in order

for covalent bonds to form?

 Electron – electron

repulsive forces

 Proton – proton

repulsive forces

 Electron – proton

Internuclear Distance Graph:

Bond Length

Bond Length Diagram

Bond Energy vs. Distance Diagram

Internuclear Distance

•

Bond forms at

lowest

potential energy (PE)

state possible (most exothermic)

– _{This spot maximizes the attractive forces}

(Coulomb’s Law) between protons in one nucleus to electrons in neighboring cloud

– _{This spot minimizes the repulsive forces}

Bond Type

Bond Length

(pm)

C

-

C



C



C

-

C



C



Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond

Bond Length and Strength Summary:

•

As number of bonds between atoms

increases, distance between bonded atoms

decreases.

•

In general:

– _{Single bonds have longest bond length} – _{Triple bonds have shortest bond length}

•

What about strength?

Resonance Bond Length and Bond Energy

 Resonance bonds are shorter and stronger

than single bonds.

  Resonance bonds are longer and weaker than double

bonds.

Bond Order

•

Bond order is the number of chemical bonds between

a pair of atoms and indicates the stability of a bond.

•

Bond order and length are

inversely proportional

to

each other: when bond order is increased, bond

length is decreased.

•

Bond Orders for Covalent Molecules:

– _{0: No bond} – _{1: Single bond} – _{2: double bond} – _{3: triple bond}

AP Practice Question

Which of the following molecules has the

shortest bond length?

H F H F

Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms

electron rich region electron poor

region _e- _{poor e}- _rich

Electronegativity Classification

• _{Non-polar: equal sharing of electrons} • Polar: unequal sharing of electrons

• Ionic: no sharing of electrons

*The greater the difference in electronegativity, the more polar the bond!

Subtraction Value Bond Classification

0.0 - 0.3 Non-polar covalent (NPC)

0.4 – 1.7 Polar covalent (PC)

Can measure Polarity using Dipole

Moment, μ

μ = Q r

μ is Dipole moment, measured in Debyes, D

Q = charge

r = bond distance

Sample Question