Chapters 8 & 9

Chemical Bonding

Sections we are skipping & why… 8.4 – covered in Ch. 7 (good to rvw) 8.8 – covered in Ch. 6 (good to rvw) Formal charges (pp. 386-389) – don’t

need to know anymore! 9.2, 9.3, 9.4 – beyond the scope of

this course 9.6 – covered in Ch. 7 (good to rvw)

Dogs teaching Chemistry

Ionic bonding Electrostatic attraction

b/w oppositely charged ions

Ions form crystal lattice orderly pattern Maximizes attraction b/w

oppositely charged ions Minimizes repulsions

among like-charged ions.

What’s an ionic bond?

So… Technically, there are NO true ionic

bonds… wait, WHAT?!?! See graph…

However… chemists avoid this problem by using an operational definition: any compound that conducts an electric current when melted is ionic.

Coulomb’s law Coulomb’s Law:

used to calc nrg of interaction b/w a pair of ionsQ1 and Q2 = charges

on ionsR = distance b/w ion

centers (in nm)k = 2.31 x 10-19 J•nm

If E < 0, what does that imply?

Lattice energy Modified form of

Coulomb’s law Only difference is k

(value depends on ions in crystal lattice)

Measures stability of crystal lattice

Lattice nrg ↑ as charge of ions and radius .

Practice Problem #1 Arrange the following ionic

compounds in order of increasing lattice energy (least exothermic to most exothermic): NaF, CsI, and CaO

Practice Problem #2 Which substance would you expect

to have the greatest (most exothermic) lattice energy: AgCl, CuO, or CrN? Why?

Covalent bonding E’s shared between nuclei

Shared equally: Shared unequally:

Formed b/w....

Bonding of H2

Network vs. Molecular Solids Network are VERY strong bonds – high melting

points, solids at room temp Some common network solids:

Molecules weaker than ionic bonds – low melting points, mostly liquids and gases at room temp

Some common molecular solids:

Properties are a reflection of their structure Examples: Graphite vs. Diamond

Graphite vs. Diamond

Silicon – a semiconductor Structure like diamond – why? Low conductivity so called a

“semi-conductor” Conductivity as temp

increases – why? Can change conductivity by doping

with other elements

Doped silicon

How doping works Let’s watch a video!

Bonding models Chemical bonds result from a tendency

of a system to seek its lowest possible energy.

Therefore, bonding is a model proposed to explain molecular stability – an attempt to explain how atoms (something microscopic) operate based on experience in the macroscopic world.

Fundamental properties of models Human inventions – based on

incomplete understanding of how nature works.

Often wrong – always speculations & oversimplifications

Difficult to make predictions from – many exceptions (ex: Aufbau principle)

Still very useful

Sooo... What does this mean?

Models are still important and useful… just realize there are limitations. Bonding model assumes electrons are shared

and located between two nuclei Makes sense – atoms share electrons to

achieve stability But… electrons move freely and are

delocalized. This does NOT mean we throw out the

model!!!!

Localized Electron (LE) Model Used to describe covalent bonds in

molecules Assumes:

Has three parts: (1) Drawing Lewis Structures, (2) VSEPR, (3) Description of orbitals used (Hybridization)

Lewis Structures Review notes from your previous

Chemistry class if you don’t remember how to do these!

Do not worry about keeping track of which electrons come from which atoms. It’s the total number of electrons that’s important!!!!

A few reminders… Make your octets around each atom and

THEN count. Adjust as necessary…

For Lewis structures of anions and cations:

Determining central atom:

Octet rules & exceptions:

Practice Problem #3Draw the Lewis structures for

C2H2

ICl4-

NO2

Review: Resonance Sometimes, more than one valid

Lewis structure can be drawn. When this happens, we say that there is resonance.

Resonant structures for NO3

-

Which is the correct description of the bonding in NO3

-? Why?

Practice Problem #4 How many resonance structures

are possible for NO2-?

Practice Problem #5 Regarding the structure of NO2

- drawn in PP #4, which of the following statement(s) is/are true?

a) There is one single bond and one double bond in NO2

-

b) There are two equivalent N-O bonds, each one intermediate b/w a single and double bond

c) NO2- flips from one resonance structure to the

other.d) The actual structure is an average of the two

resonance structures.

Practice Problem #6 Which is predicted to have shorter

sulfur-oxygen bonds, SO3 or SO32-?

VSEPR Used to predict geometry

Molecular geometry (shape/structure) Electron-pair geometry

Based on the premise that lone e- pairs require room than bonding e- pairs and tend to bond angles.

Explain how to use handout Need to know bond angles!

Molecular geometry vs. Electron pair geometry

Molecular geometry a.k.a. molecular structure a.k.a. shape = shape formed by

Electron-pair geometry (aka

electron-domain geometry) = shape formed by

See example

E- pair geometry vs. Molecular geometry: Octahedral

Practice Problem #7 Determine (a) electron-pair geometry and (b)

molecular geometry (VSEPR) for each of the following:

1) SnCl3-

2) O3

Practice Problem #7 (cont)Determine (a) electron-pair geometry and (b)

molecular geometry (VSEPR) for each of the following:

3) SF4

4) IF5

What about CH3OH?

How do you describe the shape when there is more than one central atom?

What CH3OH looks like…

Bent around O

Tetrahedral around C

Hybridization Hybridization = mixing of native

atomic orbitals (s and p, for example) to form special orbitals (sp2, for example)

Hybrid orbitals are Hybrid orbitals arrange themselves in

space such that we can determine hybridization from electron pair geometry – easiest way!

5 main types of hybridization: sp, sp2, sp3, sp3d, sp3d2

Degenerate orbitals

sp hybridization

sp2 hybridization

sp3 hybridization

sp3d and sp3d2 hybrids

Practice Problem #8 Give the expected hybridization of the

central atom in the following molecules:

a) SF4

b) XeF2

c) PF6-

d) CO

Sigma (σ) bonding Centered on line

running b/w 2 atoms sharing a pair of e’s

Shared e pr occupies space b/w the two atoms

σ bond = bond Can form from

overlap of….

Pi () bonding Represents shared e- pr

above and below the σ bond.

Formed using the p orbitals perpendicular to σ bond axis

Parallel p orbitals share an e- pr, forming a bond

Forms from sideways overlap of orbitals. Can also form from overlap of d orbitals, but we won’t consider these.

Sigma & pi bonds in C2H4

What about triple bonds?

Determining sigma and pi bonds Easy as 1-2-3! Single bonds = bond Double bonds = bonds Triple bonds = bonds

Practice problem #9Determine # of sigma & pi bonds in the

following molecules:

1. C2H2

2. C3H4

3. CH3COCOCH3

Practice Problem #10 Consider the acetonitrile molecule:

H3C-CΞN:

a) Predict the bond angles around each carbon

b) Give the hybridizations on each carbon

c) Determine the total number of sigma and pi bonds in the molecule.

Predicting molecular polarity What shapes will ALWAYS be

polar? Why?

What shape is an exception to this rule? Why?

Review: Dipole moments Caused by difference in

electronegativity of the bonded atoms

Note: “covalent bond” a.k.a. “nonpolar covalent bond”

More on dipole moments

Ways to show polarity

Practice problem #11Classify following as polar or nonpolar:

a) BF3: c) O2:

b) CS2: d) IF3