To find truth you have to try and you have to persist in trying. Sometimes it’s fun. Sometimes it’s hard or boring. But it’s always worth it. . . .

The Creator of the universe has implanted a message in every created thing. Geology, astronomy, physics—all science is really nothing more than an effort to

read those messages.~ Henry Eyring

•• How do nonmetal atoms bond to each How do nonmetal atoms bond to each other?other?

•• What are the properties of these What are the properties of these bonds? bonds?

•• How does this affect properties of How does this affect properties of the material?the material?

•• What forces exist between molecules? What forces exist between molecules? •• How do these forces influence How do these forces influence

physical properties such as melting & physical properties such as melting & boiling points, hardness, electrical boiling points, hardness, electrical conductivity, etc.?conductivity, etc.?

Have you read chapter 23?

a) Yes, b) No, I’m conserving

brain storage.

Review of Bonding Motifs

Metallic bonding: many atoms collectively share orbitals and electronsIonic bonding: metals easily give up electrons to nonmetals, which “want” themToday add a 3rd type: covalent bonding

Review of Bonding

Empty orbitals

Full shells

Empty orbitals

Full shells

Band gap

Empty orbitals

Full shells

Molecular Orbitals & Bonding

When atoms get close, MO’s can formIf the electrons are lower in energy in the resulting MO’s than they were in the atomic orbitals, bonds form

+ ↑↑

↓↑

Energy

H•

H–H or H2

H•

Sharing Electrons to Form Covalent Bonds

A few molecular orbitals are lower in energy than atomic orbitalsTypically correspond to filled shells for each atom in bondResult: molecules!

Orbitals are at specific energies between specific atoms.

Example: H + H → H2

Demonstration: covalent bond formation in nylon

“the code” for covalent bonds

In covalent bonds, electrons are shared so that each atom has access to enough electrons to fill its shell.Electrons are always shared in pairs. (An orbital holds how many electrons?)Each pair is represented by a single line in a diagram.Two atoms can share up to 3 pairs of electrons.

A few questions

If you have more electrons at low energy levels, what happens to the ‘strength’ of the bond?

A. It increasesB. It decreasesIf you have more electrons packed close together what happens to the electric force?

A. You get a larger attractive forceB. You get a smaller attractive forceC. You get a larger repulsive forceD. You get a smaller repulsive force

Sharing of More Than One Electron Pair: Multiple Bonds

Multiple bonds are strongerHigh electron density between multiply-bonded atoms makes them relatively reactive.

Basic units

metals: individual + ions stacked in negative overlapping electron cloudionic compounds: oppositely charged ions stackedcovalent compounds: individual molecules (even in solid)

What does the energy well look like?

Just a few low energy molecular orbitals that are full, and lots of individual empty atomic an molecular orbitalsthat are sometimes stacked pretty close together.

Empty orbitals

Full shells

Characteristics of materials that have covalent bonds.

Would they be transparent or opaque?Will they conduct?

Can you explain why?

Would they have high or low melting and boiling points? What determines the melting and boiling points?

Forces Between molecules

Things like phase changes and brittleness are determined by the forces between molecules.What types of fundamental interactions can you have at a microscopic level?Can you have electric forces when the total charge is zero?

Electron Sharing is Not Equal for Unlike Atoms

Different elements have different ability to attract electrons (“electronegativity”)Electronegativity increases toward the upper right of periodic table (ignoring noble gases)

ElectronegativityIncreases

ElectronegativityIncreases

Polar Molecules

Charge map for H2OFor a molecule to be polar,

Bonds must have unequal sharing: “dipoles”Bond dipoles must not “cancel”

Water is polar (demo)

δδ++ δδ++

2δ2δ--

No net dipole!No net dipole!

Which of the fundamental interactions is important for forces between molecules?

1. The weak nuclear interaction2. The strong nuclear interaction3. The electromagnetic interaction4. The gravitational interaction

Which of the following are important for determining the strength of electromagnetic forces?

A. The distance between the chargesB. The magnitude of the chargesC. The mass of the particlesD. Both A & B

Comparing the distances between two bound atoms in a molecule and the distances between molecules, which is greater (on average)?

A. The distance between bound atoms

B. The distance between moleculesC. Neither

Which forces do you expect to be greatest?

1. Bonding forces between atoms within a molecule

2. Attractive forces between different molecules

3. There is no way to know

Intermolecular Forces

“Between” different moleculesCaused by permanent or temporary charges or asymmetric charge distributionsMuch weaker than covalent bonding interactionsWide range of strengths explains wide range of boiling, melting points of covalent materials

Types of forces in chemistry

Bonds:MetalicIonicCovalent

Intermolecular forces

Hydrogen bondingDipole-dipole bondingDispersion forces

Only pertinent for what type of matter?

Hydrogen Bonding

The ultimate polar bond.H bound to N, O, or FAmong strongest intermolecular interactionsH is small and has only 1 electron. If the atom it is bound to is quite electronegative, H is essentially a “bare” protonThe most important H-bond: water

δδ++ δδ++

2δ2δ--

δδ++ δδ++

2δ2δ--

δδ++ δδ++

2δ2δ--

Even Nonpolar Molecules Are “Sticky”

Electron distribution not staticFormation of temporary dipoles+/- combination is favoredVery weakName: “dispersion” or van Der Waals interactions

Relative Strengths of Intermolecular Interactions

Nitrogen

PropertiesChemically unreactiveBoiling point 77 K (-196 °C; -321 °F)Colorless

Why?Triple bondWhat are strongest intermolecularforces between N2 molecules?No low-lying orbitals interconnecting the N2.

NN≡≡NN

What causes nitrogen (N2) molecules to liquefy?

1. Covalent bonds between the molecules2. Hydrogen bonds between the molecules3. Dispersion forces between the molecules4. Attraction of the permanent dipole in one

molecule to the dipole in another

WaterProperties

Fairly reactive, “universal” solventBoils at 373 K (100 °C; 212 °F)More dense as liquid than solidCan react with itself: 2H2O = H3O+ + OH-

Water conducts electricity (but not well)This reaction goes only to a very small extent (1 molecule in 10 million!)

Why?Highly polar, so dissolves ionic materialsExtensive hydrogen bonding interactions dissolve nonionic materials like sugars, lead to high boiling point, unusual crystal structure

What is the best explanation for the high freezing point of water?

1. Covalent bonds between the molecules2. Hydrogen bonds between the molecules3. Dispersion forces between the molecules4. Attraction of the permanent dipole in one

molecule to the dipole in another

What happens when you mix two polar materials?

They will mix together (dissolve)Why?What about a non-polar material mixed with a polar material?

What force holds non-polar atoms together?What determines the size of this force?Compare the size of the force holding the polar molecules to non-polar ones to the forces that holds the same type of molecule.

Glucose (a Kind of Sugar)

PropertiesCrystalline, molecular solidMelts, then decomposes on heatingSticky

Why?Many hydrogen bonds hold molecules together in crystalMany H-bonds add up, so melting point is relatively high (compared to water, for example)H-bonds make molecules “sticky”Alternating + & - is like what other type of bond?

Diamond (Pure Carbon)

PropertiesVery hardCrystallineColorlessDecomposes rather than melting

Why?Covalent network material; like a giant molecule all held together by strong bonds

Makes diamond hardPrevents melting

No low-lying unoccupied molecular orbitals (unless impurities are present)

Graphite (Another form of Pure Carbon)

PropertiesSoftCrystallineDark color

Why?Planes of strongly bound carbon atomsPlanes held to each other by very weak intermolecular forcesMany low-energy unoccupied molecular orbitals

Buckminsterfullerene (a Third Form of Pure Carbon)

Discoverers won Nobel Prize in Chemistry

Molecular IonsCovalent bonding within the ionIf the number of electrons available doesn’t match the number of electrons the atoms want, the molecule will tend to behave like a non-metal (or occasionally a metal) and steal (or give away) electrons.These charged molecules assemble together in crystal lattice like ionic materialsExamples: nitrate, silicate