Concepts of Concepts of Chemical BondingChemical BondingChapter 12 – Basic ConceptsChapter 12 – Basic Concepts

Chapter 13 – Molecular Chapter 13 – Molecular StructureStructure

Chapter 14 – Polarity Chapter 14 – Polarity

Salt vs. SugarSalt vs. Sugar

Salt vs. Sugar Salt vs. Sugar Night and Night and DayDay

► SaltSalt Ionic compoundIonic compound NaNa++ and Cl and Cl-- Held together by Held together by

opposite charges opposite charges (ionic bond)(ionic bond)

An electrolyte (will An electrolyte (will conduct electrical conduct electrical current when current when dissolved in solution)dissolved in solution)

► Sugar (sucrose)Sugar (sucrose) Covalent compoundCovalent compound CC1212HH2222OO1111 Atoms share electrons Atoms share electrons

and are held together and are held together by covalent bondsby covalent bonds

A non-electrolyte (will A non-electrolyte (will NOT conduct an NOT conduct an electrical current electrical current when dissolved in when dissolved in solution)solution)

Two Things You Two Things You Should Always Remember:Should Always Remember:

►Atoms bond with other atoms to Atoms bond with other atoms to complete their outer energy levels complete their outer energy levels become stable. become stable.

►The properties of substances are The properties of substances are determined in large part by the chemical determined in large part by the chemical bonds that hold atoms together. bonds that hold atoms together.

Types of BondingTypes of Bonding► Chemical bond – forms when atoms or Chemical bond – forms when atoms or

ions are strongly attracted to one ions are strongly attracted to one another. another.

There are three basic types: There are three basic types: 1)1) Ionic bondIonic bond2)2) Covalent bondCovalent bond3)3) Metallic bondMetallic bond

Valence electrons are the outer shell electrons of an atom. The valence electrons are the electrons thatparticipate in chemical bonding.

1A 1ns1

2A 2ns2

3A 3ns2np1

4A 4ns2np2

5A 5ns2np3

6A 6ns2np4

7A 7ns2np5

Group # of valence e-e- configuration

Lewis Dot StructuresLewis Dot Structures► Includes chemical symbol of element, Includes chemical symbol of element,

plus dots for each valence electron. plus dots for each valence electron. TryTry

1) Li1) Li

2) O2) O

3) Ar3) Ar

The Octet RuleThe Octet Rule►We last stated: atoms that have eight We last stated: atoms that have eight

valence electrons are chemically stable. valence electrons are chemically stable. ►Related to bonding: atoms will gain, lose, Related to bonding: atoms will gain, lose,

or share valence electrons until they are or share valence electrons until they are surrounded by eight valence electrons. surrounded by eight valence electrons.

►For the exceptions on table: For the exceptions on table: atoms will atoms will gain, lose or share valence electrons to gain, lose or share valence electrons to achieve the same number of electrons as achieve the same number of electrons as the noble gas closest to them on the the noble gas closest to them on the periodic table.periodic table.

Li + F Li+ F -

The Ionic Bond

1s22s1 1s22s22p5 [He] [Ne]

Li Li+ + e-

e- + F F -

F -Li+ + Li+ F -

The actual bond attraction of opposite charges.

Ionic BondingIonic Bonding►Use Lewis dot symbols to show the Use Lewis dot symbols to show the

formation of NaCl.formation of NaCl.

Ionic BondingIonic Bonding►Use Lewis dot symbols to show the Use Lewis dot symbols to show the

formation of aluminum oxide. formation of aluminum oxide.

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-

F F

F F

Structure of F2

lone pairslone pairs

lone pairslone pairs

single covalent bond

single covalent bond

9.4

Covalent BondingCovalent Bonding►HH22, HCl, H, HCl, H22O, NHO, NH33, and CH, and CH44

►Lewis StructuresLewis Structures – shared electron – shared electron pairs are shown as lines; unshared pairs are shown as lines; unshared electron pairs are shown as dots. electron pairs are shown as dots.

8e-

H HO+ + OH H O HHor

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 N8e-8e-

N N

triple bondtriple bond

or

9.4

Bond Bond TypeType

Bond Bond LengthLength(pm)(pm)

CC--CC 154154CCCC 133133CCCC 120120CC--NN 143143CCNN 138138CCNN 116116

Lengths of Covalent Bonds

Bond Lengths

Triple bond < Double Bond < Single Bond 9.4

Ionic vs. Covalent Ionic vs. Covalent Compounds Compounds

► IonicIonic High melting pointsHigh melting points Conduct electricity in Conduct electricity in

molten state or when molten state or when dissolved in solutiondissolved in solution

Tend to be soluble in Tend to be soluble in waterwater

Usually crystallize as Usually crystallize as sharply defined sharply defined particles.particles.

Usually solid at room Usually solid at room temp.temp.

More Ionic . . .More Ionic . . .

Covalent PropertiesCovalent Properties►CovalentCovalent

Low melting pointsLow melting points Do not conduct electricityDo not conduct electricity Not very soluble in waterNot very soluble in water Usually liquid or gas at room temperature. Usually liquid or gas at room temperature.

Some are low-melting solids. Some are low-melting solids.

► Covalent BondsCovalent Bonds► Bond AxisBond Axis

► Bond AngleBond Angle

► Bond lengthBond length NOT FIXED NOT FIXED

Covalent Bond Length Covalent Bond Length ►Bond length measurements are Bond length measurements are

average measurements because the average measurements because the molecules undergo a variety of molecules undergo a variety of motions. (they have energy)motions. (they have energy)

►Symmetric stretching (same direction)Symmetric stretching (same direction)►Asymmetric stretching (opp. direction)Asymmetric stretching (opp. direction)►Bending (opp. Direction)Bending (opp. Direction)►Rotating (wagging) {same direction}Rotating (wagging) {same direction}

Get up and do the Get up and do the MOLECULE DANCE!!MOLECULE DANCE!!

http://www.gps.caltech.edu/~http://www.gps.caltech.edu/~edwin/MoleculeHTML/H2O_htedwin/MoleculeHTML/H2O_html/H2O_page.htmlml/H2O_page.html

How do you know How do you know

what type of what type of bond bond

will form???will form???

ElectronegativityElectronegativity►The ability of an atom The ability of an atom in a moleculein a molecule to to

attract electrons to itself. attract electrons to itself.

A tug of war forthose shared electrons!!

►A highly electronegative atom will A highly electronegative atom will really attract electrons toward itself. really attract electrons toward itself. High electron affinityHigh electron affinity High ionization energyHigh ionization energy

►Yes, there is a trend!!!!Yes, there is a trend!!!!

F – F F – F

N – ON – O

Rb – F Rb – F

Which is Which?Which is Which?

Classify the following bonds as ionic, polar covalent, or covalent: The bond in CsCl; the bond in H2S; andthe NN bond in H2NNH2.

Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic

H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent

N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent

9.5

Most Ionic; Most Covalent? Most Ionic; Most Covalent? Which of the following bonds is the most Which of the following bonds is the most

ionic?ionic?Li-O, Na-O, K-O, Rb-O, Cs-OLi-O, Na-O, K-O, Rb-O, Cs-O

Which of the following bonds is the most Which of the following bonds is the most covalent?covalent?

Li-O, Na-O, K-O, Rb-O, Cs-OLi-O, Na-O, K-O, Rb-O, Cs-O

Metallic BondingMetallic Bonding►Form between metal atomsForm between metal atoms

►Metals hold on to their valence Metals hold on to their valence electrons weakly.electrons weakly.

►Think of them as positive ions floating Think of them as positive ions floating in a sea of electrons.in a sea of electrons.

►The negative ‘cloud’ covers the The negative ‘cloud’ covers the positive charges forming a ‘glue’ that positive charges forming a ‘glue’ that hold the metal together. hold the metal together.

Metallic BondingMetallic Bonding

► A. Outermost electrons A. Outermost electrons wander freely through metal. wander freely through metal. Metal consists of cations held Metal consists of cations held together by negatively-together by negatively-charged electron "glue."charged electron "glue."

► B. Free electrons can move B. Free electrons can move rapidly in response to electric rapidly in response to electric fields, hence metals are a fields, hence metals are a good conductor of electricity.good conductor of electricity.

► C. Free electrons can C. Free electrons can transmit kinetic energy transmit kinetic energy rapidly, hence metals are rapidly, hence metals are good conductors of heat.good conductors of heat.

► D. The layers of atoms in D. The layers of atoms in metal are hard to pull apart metal are hard to pull apart because of the electrons because of the electrons holding them together, hence holding them together, hence metals are tough. But metals are tough. But individual atoms are not held individual atoms are not held to any other specific atoms, to any other specific atoms, hence atoms slip easily past hence atoms slip easily past one another. Thus metals are one another. Thus metals are ductile. ductile. Metallic Bonding is Metallic Bonding is the basis of our industrial the basis of our industrial civilization.civilization.