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Chapter 6Chapter 6Chemical Bonding and Chemical Bonding and Molecular CompoundsMolecular Compounds

Ionic bondIonic bond:

Electrons are given or taken, occurs between metal and nonmetal (NaCl)

Covalent bondCovalent bond:

Electron pairs are shared, occurs between two or more nonmetals (CO2)

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IonsIons IonIon

atom that has atom that has gained or lost one or gained or lost one or more electronsmore electrons..

Octet RuleOctet Rule all atoms want all atoms want zero or eight zero or eight

electrons in their outer shellelectrons in their outer shell.. MetalMetal and and nonmetalnonmetal bond: bond:

each atom each atom seeks to gain electrons seeks to gain electrons or lose themor lose them. For instance:. For instance:

ClCl wants to wants to gaingain one electron one electron NaNa wants to wants to loselose one electron one electron

Notice the sizes of the ionsNotice the sizes of the ions

Cl Cl-

Na Na+

Positive ions are called “cations”

Negative ions are called “anions”

Shielding - Ion Sizes.mov

+e-

-e-

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Ionic attractionIonic attraction Oppositely charged ions Oppositely charged ions

attractattract Like opposite poles of a Like opposite poles of a

magnet attractmagnet attract Positively charged Positively charged Sodium Sodium

IonIon and negatively charged and negatively charged Chloride IonChloride Ion are attracted. are attracted.

They remain ions, but stick They remain ions, but stick together in a together in a latticelattice (3D grid pattern) as (3D grid pattern) as other ionsother ions

join them.join them.

Cl- Na+

Cl- Na+

Cl-Na+

Na+

Cl-

Dissolution of NaCl in Water.mov Electrolytes.movDissolution of KMnO4.MOV

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Formula UnitsFormula Units NaNa++ and and ClCl-- combine to form combine to form NaClNaCl.. NaClNaCl is one is one formula unitformula unit::

the smallest the smallest unitunit that has the that has the correct formula for our compound.correct formula for our compound.

NaCl is also our compound’s NaCl is also our compound’s empirical formulaempirical formula:: the smallest the smallest ratioratio of atoms that of atoms that

make up our compound.make up our compound. Although there are several Although there are several NaNa++

ions and ions and ClCl-- ions in the lattice, the ions in the lattice, the formula unitformula unit is still just is still just NaClNaCl..

Cl-Na+

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MetallicMetallic Bonding Bonding In In metallic bondingmetallic bonding electrons are shared and flow between metal atoms electrons are shared and flow between metal atoms

in an “in an “electron seaelectron sea.”.” This is an This is an extremely strong bondextremely strong bond

This is why metals:This is why metals: Have Have lusterluster

(reflect light)(reflect light) Are Are malleablemalleable

(able to formed and hold that form)(able to formed and hold that form) Are Are ductileductile

(able to formed into wires)(able to formed into wires) Conduct electricityConduct electricity

(pass an electrical charge)(pass an electrical charge) Conduct heatConduct heat

(pass heat easily)(pass heat easily)

d-orbital electrons

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CovalentCovalent Bonding Bonding

CovalentCovalent bonds are bonds formed when bonds are bonds formed when atoms atoms shareshare electron pairselectron pairs to complete their octet. to complete their octet. This is a very strong bond.This is a very strong bond.

Atoms on the Atoms on the upper-right of the periodicupper-right of the periodic table table ((nonmetalsnonmetals) covalently bond with each other. ) covalently bond with each other. (except nobles)(except nobles)

Molecular compoundsMolecular compounds covalently bonded compounds.covalently bonded compounds.

Diatomic moleculesDiatomic molecules are covalently bonded. are covalently bonded. BrBr22,, I I22, , NN22,, ClCl22,, HH22,, OO22, , FF22

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Covalent Bond EnergyCovalent Bond Energy Bond lengthBond length and and bond energybond energy are inversely are inversely

related.related. That is, a That is, a smaller bondsmaller bond has a has a greater energygreater energy, and , and

will release more energy when broken.will release more energy when broken. Atoms will bond at a distance that is most stableAtoms will bond at a distance that is most stable

(lowest potential energy)(lowest potential energy) H2 Bond formation and distance-energy.mov

High potential energy = unstable

Low potential energy = more stable

BondLengthBondStrength.swf

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ElectronegativityElectronegativity Electronegativity:Electronegativity:

Atom’s pull for electrons Atom’s pull for electrons in a covalent bondin a covalent bond

Electron pairs are Electron pairs are sometimessometimes not shared not shared equallyequally among atoms. among atoms.

PPolar covalentolar covalent Electrons are shared, Electrons are shared, but one but one

atom has a higher atom has a higher ElectronegativityElectronegativity

Like in Like in COCO.. Nonpolar covalentNonpolar covalent

Electrons are shared evenlyElectrons are shared evenly Like in Like in OO22. . O O

OC

Cl-Na+

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Partial chargesPartial charges PolarPolar molecules molecules::

have have partial positivepartial positive δδ++ site(s) site(s) ((less electron densityless electron density)) and a and a partial negativepartial negative δδ-- site(s) site(s) ((more electron densitymore electron density)) like the poles of a magnet.like the poles of a magnet.

Dipole ForcesDipole Forces Forces between partial positive and partial negative Forces between partial positive and partial negative

sites on molecules.sites on molecules. The more polar a molecule, the greater the dipole The more polar a molecule, the greater the dipole

force.force. Dipole forces get weaker with distance between Dipole forces get weaker with distance between

molecules.molecules. Do larger molecules have larger or smaller Dipole force?Do larger molecules have larger or smaller Dipole force?

“Molecular dipole”

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Hydrogen bondingHydrogen bonding Hydrogen bondHydrogen bond

Attraction between the Attraction between the partial positive charge of the partial positive charge of the hydrogen atomshydrogen atoms in a covalent molecule and the in a covalent molecule and the partial negative charge on another moleculepartial negative charge on another molecule..

We call this “bond” We call this “bond” intermolecular attractionintermolecular attraction.. Hydrogen bonding accounts for why ice is less dense Hydrogen bonding accounts for why ice is less dense

than liquid water.than liquid water.Can you tell why?Can you tell why?

Hydrogen bonding was key to predicting the shape of DNA molecules.

Hydrogen Bonding.MOV

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State is determined by Intermolecular forcesState is determined by Intermolecular forces Recall Kinetic Molecular Theory (Particle Model)Recall Kinetic Molecular Theory (Particle Model)

Solids:Solids: Particles close-rotating/vibrating about a central pointParticles close-rotating/vibrating about a central point Very Strong like-molecule intermolecular forcesVery Strong like-molecule intermolecular forces

Liquids:Liquids: Particles close-flowing over each other but staying togetherParticles close-flowing over each other but staying together Strong like-molecule intermolecular forcesStrong like-molecule intermolecular forces

Gases:Gases: Particles far apart-freely flying very fastParticles far apart-freely flying very fast Weak intermolecular forcesWeak intermolecular forces

How does a substance change its state?

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Other Other intermolecularintermolecular forces forces Recall:Recall: “ “interinter” “” “molecularmolecular” means “” means “betweenbetween” ”

““moleculesmolecules”.”. These These bondsbonds are only attractions are only attractions..

London Forces (London Forces (AKA: Van der Vaals ForcesAKA: Van der Vaals Forces)) Forces between Forces between like moleculeslike molecules account for account for physical propertiesphysical properties like boiling point and like boiling point and

melting point.melting point. Stronger attraction Stronger attraction higher boiling point higher boiling point Stronger attraction Stronger attraction ?? melting point melting point Stronger attraction Stronger attraction ?? freezing point freezing point Stronger attraction Stronger attraction ?? density density

IntermolecularForces.swf

22 COCO

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Molecular notation - Molecular notation - Lewis DotLewis Dot Lewis DotLewis Dot structures structures

tool used to draw molecules in 2D with stick tool used to draw molecules in 2D with stick diagrams.diagrams.

Steps:Steps: Determine each atom’s number of valence Determine each atom’s number of valence

electronselectrons Number of Number of valence electronsvalence electrons = number of = number of dotsdots Molecules share dots until each atoms is Molecules share dots until each atoms is

surrounded by 8 dots, representing a completed surrounded by 8 dots, representing a completed octet.octet.

Hydrogen gets only 2 dots on one sideHydrogen gets only 2 dots on one side Pairs of dots between atoms are changed to sticks Pairs of dots between atoms are changed to sticks

– representing covalent bonds.– representing covalent bonds. Any pairs of dots that are not shared between Any pairs of dots that are not shared between

atoms are called atoms are called unshared pairsunshared pairs..

Xee-

e-

e-

e-e-

e-e-

e-

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Lewis Dot Lewis Dot Molecular Model Molecular Model Consider Consider MethaneMethane, , CHCH44

The atoms are C and HThe atoms are C and H Add each atom’s Add each atom’s

valence electrons as valence electrons as dots, clockwise.dots, clockwise.

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Lewis Dot Lewis Dot Molecular Model Molecular Model Consider Methane, CHConsider Methane, CH44

The atoms are C and HThe atoms are C and H Add each atom’s Add each atom’s

valence electrons as valence electrons as dots, clockwise.dots, clockwise.

Combine the dots to Combine the dots to create octets, carbon create octets, carbon belongs in the center.belongs in the center.

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Lewis Dot Lewis Dot Molecular Model Molecular Model Consider Methane, CHConsider Methane, CH44

The atoms are C and HThe atoms are C and H Add each atom’s Add each atom’s

valence electrons as valence electrons as dots, clockwise.dots, clockwise.

Combine the dots to Combine the dots to create octets, carbon create octets, carbon belongs in the center.belongs in the center.

Replace pairs of dots Replace pairs of dots with sticks, representing with sticks, representing bonds.bonds.

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Lewis-Dot Practice:Lewis-Dot Practice: Try the following on your own:Try the following on your own:

Determine each atom’s number of valence electrons Determine each atom’s number of valence electrons Arrange until all atoms except H have 8 valence electrons. Arrange until all atoms except H have 8 valence electrons.

(H has 2)(H has 2) Replace ( Replace ( :: ) with sticks ) with sticks (hint-carbon is always in the middle)(hint-carbon is always in the middle)

OH 2 3NHHCl 62HC OHHC 52

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Multiple bondsMultiple bonds If you have If you have more bonding sitesmore bonding sites than you have than you have

valence electrons, valence electrons, doubledouble or or tripletriple bonds must bonds must be used.be used.

Consider Consider Carbon Dioxide, COCarbon Dioxide, CO22: Can you draw : Can you draw this structure with a Lewis Dot-to-stick this structure with a Lewis Dot-to-stick diagram? Try it.diagram? Try it.

Next, try to draw Next, try to draw Acetylene, CAcetylene, C22HH22, a chemical , a chemical that releases a great deal of energy when that releases a great deal of energy when burned with excess oxygen.burned with excess oxygen.

:....

: OCO

HCCH

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Resonance StructuresResonance Structures

Resonance Hybrid StructureResonance Hybrid Structure Lewis molecular that can rapidly switch their Lewis molecular that can rapidly switch their

bonds between single and double bonds.bonds between single and double bonds. These tend to be exceptions to the octet ruleThese tend to be exceptions to the octet rule

NONO22 is a good example of a resonance hybrid is a good example of a resonance hybrid structure. Can you see why?structure. Can you see why?

Page 203

ResonanceStructures.swf

Looks like 1½ bonds

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Molecular GeometryMolecular Geometry Molecular GeometryMolecular Geometry

How a molecule “looks” in real space (3D).How a molecule “looks” in real space (3D). Based upon electron Based upon electron domainsdomains (where electrons are) (where electrons are)

VSEPR:VSEPR: VValence alence SShell hell EElectron lectron PPair air RRepulsion theoryepulsion theory

Bonds are domains (doubles/triples count as one)Bonds are domains (doubles/triples count as one) Unshared electron pairs are domainsUnshared electron pairs are domains Electron domains repel each otherElectron domains repel each other Unshared pairs repel more than bonding pairsUnshared pairs repel more than bonding pairs DomainsDomains repelrepel and move themselves as far away and move themselves as far away

from each other as possible.from each other as possible.

Vsepr.mov

(e-) (e-)

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Common GeometriesCommon Geometries Begin with the Lewis dot diagram … Begin with the Lewis dot diagram …

Count the domains, bonding and unsharedCount the domains, bonding and unshared There are five fundamental geometries:There are five fundamental geometries:

2 Domains

3 Domains

4 Domains

5 Domains 6 Domains

Each of these geometries (except linear) can be further separated;

Depending upon whether an electron domain is a bonding or unshared pair

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3 Domains3 Domains

Trigonal PlanarTrigonal Planar: 0 unshared domains: 0 unshared domains BentBent: 1 unshared domain: 1 unshared domain

Note: unshared (nonbonding) pairs exert a greater repulsion on domains than do bonding pairs

Also: multiple bonds exert a greater repulsive force than do single bonds, (act as 1 domain)

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4 Domains4 Domains

TetrahedralTetrahedral: 0 unshared domains (CH: 0 unshared domains (CH44)) Trigonal PyramidalTrigonal Pyramidal: 1 unshared domain (NH: 1 unshared domain (NH33)) BentBent: 2 unshared domains (H: 2 unshared domains (H22O)O)

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Modeling complex moleculesModeling complex molecules We can extend the concept of VSEPR to more We can extend the concept of VSEPR to more

complex molecules.complex molecules. With complex molecules:With complex molecules:

Domains exert repulsion about a central atomDomains exert repulsion about a central atom Domains in Domains in nearby geometriesnearby geometries exert repulsion exert repulsion

Computers allow us to see more complex Computers allow us to see more complex molecules as they exist, doing the math for us.molecules as they exist, doing the math for us.

Vitamin C

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Molecular DipoleMolecular Dipole = = PPolar Moleculeolar Molecule

If a molecule has a If a molecule has a molecular dipolemolecular dipole, it is , it is polarpolar to some extentto some extent

Polar moleculePolar molecule Electron density is Electron density is

not symmetricalnot symmetrical Electron density is Electron density is

“bunched up”“bunched up”

Nonpolar moleculeNonpolar molecule Has Has symmetrysymmetry Dipoles cancel outDipoles cancel out

“Molecular dipole”

“Like Dissolves Like”

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Are these molecules polar?Are these molecules polar?

3NHHCl 4CCl ClCH3 3BF1. Draw Lewis-

dot structures

2. Determine symmetry

Symmetrical

nonpolar

Nonsymmetrical polar

(tricky)

(Trigonal Planar)

End of chapter 6

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CCSD Syllabus ObjectivesCCSD Syllabus Objectives 7.1: Bond Types7.1: Bond Types 7.2: Lewis Formulas7.2: Lewis Formulas 7.3: Polarity/Geometry7.3: Polarity/Geometry 7.5: Resonance7.5: Resonance 12.4: Kinetic Molecular theory-solids and liquids12.4: Kinetic Molecular theory-solids and liquids 12.5: Intermolecular Forces12.5: Intermolecular Forces