4.2 How 4.2 How Elements Form Elements Form

CompoundsCompounds

ObjectivesObjectives

Model two types of compound Model two types of compound formation: ionic and covalent at the formation: ionic and covalent at the atomic level.atomic level.

Demonstrate how and why atoms Demonstrate how and why atoms achieve chemical stability by bonding.achieve chemical stability by bonding.

Compare, using examples the effect of Compare, using examples the effect of covalent and ionic bonding on the covalent and ionic bonding on the physical properties of compounds.physical properties of compounds.

How Elements Form How Elements Form CompoundsCompounds

In the 19In the 19thth Century chemists tried to Century chemists tried to picture how atoms combine.picture how atoms combine.

Early models included atoms with Early models included atoms with “hooks” that allowed them to “hooks” that allowed them to attached to one another.attached to one another.

When Atoms CollideWhen Atoms Collide

Atoms collide when they reactAtoms collide when they react These collisions determine what kind These collisions determine what kind

of compounds are formed.of compounds are formed. When atoms collide it is the electron When atoms collide it is the electron

clouds that interact with one another. clouds that interact with one another. To be more precise it is the valence To be more precise it is the valence

electrons that interact.electrons that interact. Remember: valence electrons are Remember: valence electrons are

responsible for the chemical properties of responsible for the chemical properties of elementselements

Valence Electrons and Valence Electrons and BondsBonds

To understand how valence To understand how valence electrons affect the bonds of electrons affect the bonds of compounds let look at elements with compounds let look at elements with unusual properties….the noble gasesunusual properties….the noble gases

Noble Chemical Stability Noble Chemical Stability

Noble Gases are located in Group 18Noble Gases are located in Group 18 They are inert or unreactiveThey are inert or unreactive This lack of reactivity makes them This lack of reactivity makes them

useful as incandescent light bulbs useful as incandescent light bulbs (argon or krypton) and neon lights (argon or krypton) and neon lights (orange-neon, blue-argon, yellow-(orange-neon, blue-argon, yellow-helium)helium)

Occur naturally in the environmentOccur naturally in the environment

Noble GasesNoble Gases

Not a single compound of these elements Not a single compound of these elements has ever been found naturally in the has ever been found naturally in the environment.environment.

In 1960 chemists were able to react In 1960 chemists were able to react fluorine with krypton and xenon under fluorine with krypton and xenon under high temperature and pressure.high temperature and pressure.

Since then a few additional compounds of Since then a few additional compounds of xenon and krypton have been xenon and krypton have been synthesized, but none with helium, neon synthesized, but none with helium, neon or argon.or argon.

The Octet RuleThe Octet Rule

Lack of reactivity of the noble gases Lack of reactivity of the noble gases must mean that they are stable.must mean that they are stable.

Elements in the same group have Elements in the same group have similar valence arrangements.similar valence arrangements.

Noble gases have 8 valence electrons, Noble gases have 8 valence electrons, except for helium that has 2.except for helium that has 2.

Because the electron arrangement Because the electron arrangement determines chemical properties determines chemical properties unreactive nature of noble gasesunreactive nature of noble gases

Octet RuleOctet Rule

Atoms combine because they become Atoms combine because they become more stable by doing so.more stable by doing so.

Modern model of bonding is based on Modern model of bonding is based on the fact that the stability of noble the fact that the stability of noble gases is a result of their valence gases is a result of their valence electron arrangements.electron arrangements.

Octet RuleOctet Rule- atoms can become stable - atoms can become stable by having eight electrons in their outer by having eight electrons in their outer energy level (or two electrons in the energy level (or two electrons in the case of some of the smaller atoms).case of some of the smaller atoms).

Noble Gas ConfigurationNoble Gas Configuration

Atoms obtain stability by achieving Atoms obtain stability by achieving the same configuration of valence the same configuration of valence electron as found in noble gases; electron as found in noble gases; most stable configuration.most stable configuration.

Valence Electron ReviewValence Electron Review

How many valence electrons do you How many valence electrons do you find in Group 1, 2, 13, 15, 16 and17?find in Group 1, 2, 13, 15, 16 and17?

ReviewReview

What is the octet rule?What is the octet rule?

4.2 Continued4.2 Continued

Ways to Achieve a Stable Ways to Achieve a Stable Outer Energy LevelOuter Energy Level

Forming CompoundsForming Compounds

When atoms collide with enough When atoms collide with enough force their outer electrons may force their outer electrons may rearrange to achieve a stable octet rearrange to achieve a stable octet of valence electrons.of valence electrons.

When it occurs they achieve a noble When it occurs they achieve a noble gas configuration and the atoms gas configuration and the atoms form compoundsform compounds

How do the electrons How do the electrons rearrange?rearrange?

Electrons are transferred from one Electrons are transferred from one atom to anotheratom to another

Electrons are shared between atomsElectrons are shared between atoms

Electrons Can Be Electrons Can Be TransferredTransferred

Example Sodium (Na) and Chlorine (Cl)Example Sodium (Na) and Chlorine (Cl) Na has one valence electronNa has one valence electron Cl has seven valence electronsCl has seven valence electrons Na transfers its 1 to Cl 7 Na transfers its 1 to Cl 7 Na now has 8 Na now has 8

electrons in its outer shell and Cl now has electrons in its outer shell and Cl now has 8 in its outer shell8 in its outer shell

Na now has 1 less electron Na now has 1 less electron has +1 has +1 chargecharge

Cl now has 1 more electronCl now has 1 more electron has a -1 has a -1 chargecharge

IonsIons

Atom or group of atoms that has a Atom or group of atoms that has a charge b/c they have either lost or charge b/c they have either lost or gained electronsgained electrons

Ions form when valence electrons Ions form when valence electrons rearrange by transferrearrange by transfer

Compounds composed on ions are Compounds composed on ions are ionic compoundsionic compounds

Table 4.2 p. 134Table 4.2 p. 134

Ions Attract Each OtherIons Attract Each Other

Opposites attractOpposites attract Positive sodium is attracted to the Positive sodium is attracted to the

negative chloride ion.negative chloride ion. Positive ions are called cationsPositive ions are called cations

Names stays the sameNames stays the same Negative ions are called anionsNegative ions are called anions

Add Add –ide –ide to the endto the end Sulfur Sulfur Sulfide ion Sulfide ion

Ionic bondIonic bond

Strong attractive force between ions Strong attractive force between ions of opposite chargeof opposite charge

Hold ions together in an ionic Hold ions together in an ionic compoundcompound

Crystal is a regular, repeating Crystal is a regular, repeating arrangement of atoms, ions or arrangement of atoms, ions or moleculesmolecules

The Results of Ionic The Results of Ionic AttractionAttraction

b/c of the strong attractive forces b/c of the strong attractive forces and the degree of organization and the degree of organization solid at room tempsolid at room temp

Raising temp causes particles to Raising temp causes particles to move faster move faster particles overcome particles overcome attractive forces and the crystal attractive forces and the crystal organization breaks downorganization breaks down

Representing Compounds Representing Compounds with Formulaswith Formulas

Sodium ChlorideSodium Chloride Formula: NaClFormula: NaCl

1.1. Write cation first followed by the Write cation first followed by the anionanion

2.2. Always in the smallest whole Always in the smallest whole number rationumber ratio

ReviewReview

What is a negatively charged ion What is a negatively charged ion called?called?

What is a positively charged ion What is a positively charged ion called?called?

What is the noble gas configuration?What is the noble gas configuration?

Electrons Can Be SharedElectrons Can Be Shared

What happens when hydrogen and What happens when hydrogen and oxygen collide?oxygen collide? Oxygen has 6 valence electronsOxygen has 6 valence electrons Hydrogen has 1 valence electronHydrogen has 1 valence electron Can hydrogen lose its one valence Can hydrogen lose its one valence

electrons?electrons?

Colliding atomsColliding atoms

Transfer electrons only when one atom Transfer electrons only when one atom has a stronger attraction for valence has a stronger attraction for valence electronselectrons

In the case of sodium and chlorine, In the case of sodium and chlorine, chlorine attracts sodium’s valence chlorine attracts sodium’s valence electrons strongly and sodium is holding electrons strongly and sodium is holding its electrons weaklyits electrons weakly

In the case of hydrogen and oxygen In the case of hydrogen and oxygen neither atom attracts strongly enough to neither atom attracts strongly enough to take electrons from the other.take electrons from the other.

Hydrogen and OxygenHydrogen and Oxygen

They will share their electronsThey will share their electrons When atoms collide with enough When atoms collide with enough

energy to react, but neither atom energy to react, but neither atom attracts electrons strongly enough to attracts electrons strongly enough to take electrons from the other, the take electrons from the other, the atoms combine by sharing valence atoms combine by sharing valence electrons. electrons.

Let’s Look at WaterLet’s Look at Water

Formation of CompoundsFormation of Compounds

Like the formation of sodium Like the formation of sodium chloride all the components present chloride all the components present before the reaction are still present before the reaction are still present after the reactionsafter the reactions

The valence electrons no longer The valence electrons no longer reside in the same position.reside in the same position. They have rearrangedThey have rearranged

Electrons Sharing Produces Electrons Sharing Produces Molecules Molecules

Covalent bondCovalent bond- the attraction of two - the attraction of two atoms for a shared pair of electronsatoms for a shared pair of electrons Electrons are sharedElectrons are shared Neither atom has an ionic chargeNeither atom has an ionic charge

Covalent compoundCovalent compound- compound whose - compound whose atoms are held together by a covalent atoms are held together by a covalent bond, also called bond, also called molecular compoundsmolecular compounds Water is an exampleWater is an example MoleculesMolecules--uncharged group of two or more uncharged group of two or more

atoms held together by covalent bonds (water atoms held together by covalent bonds (water molecules)molecules)

More Than Two Electrons More Than Two Electrons Can Be SharedCan Be Shared

Let’s look at Carbon Dioxide Let’s look at Carbon Dioxide Carbon has 4 valence electronsCarbon has 4 valence electrons Oxygen has 6 valence electronsOxygen has 6 valence electrons

Covalent and Ionic bondsCovalent and Ionic bonds

Two nonmetallic elements usually Two nonmetallic elements usually form molecular or covalent form molecular or covalent compounds by sharing electronscompounds by sharing electrons

Nonmetallic and a metallic element Nonmetallic and a metallic element usually form ionic compoundsusually form ionic compounds

ReviewReview

What is a covalent bond?What is a covalent bond? What is the definition for a What is the definition for a

molecular compound?molecular compound? Give me an example of a molecular Give me an example of a molecular

compound?compound?

ReviewReview

What is a covalent bond?What is a covalent bond? What is the definition of a molecular What is the definition of a molecular

compounds?compounds? What are two examples of a What are two examples of a

molecular compound?molecular compound?

How do ionic and How do ionic and covalent covalent

compounds compounds compare?compare?

BondingBonding

When elements combine they either form When elements combine they either form ions or molecules…there are no other ions or molecules…there are no other possibilities.possibilities.

When particles change dramatically, When particles change dramatically, when they change from sodium to sodium when they change from sodium to sodium ions or from hydrogen and oxygen to ions or from hydrogen and oxygen to water molecules.water molecules.

These changes explain why compound These changes explain why compound have different properties from the have different properties from the elements they are composed of.elements they are composed of.

Explaining the Properties of Explaining the Properties of Ionic CompoundsIonic Compounds

Physical properties are a result of Physical properties are a result of the well-organized, tightly bound the well-organized, tightly bound ions.ions.

Ions forms strong, three-dimensional Ions forms strong, three-dimensional crystalscrystals

Properties of Ionic Properties of Ionic CompoundsCompounds

Generally ionic compounds are Generally ionic compounds are crystalline solids at room crystalline solids at room temperature.temperature.

Generally hard, rough and brittleGenerally hard, rough and brittle High melting points and boiling High melting points and boiling

pointspoints This is because of the strong attraction This is because of the strong attraction

between ionsbetween ions Usually dissolve in waterUsually dissolve in water

Properties of Ionic Properties of Ionic CompoundsCompounds

Solutions will conduct electricitySolutions will conduct electricity Liquid state (melted state) will Liquid state (melted state) will

conduct electricityconduct electricity They are They are electrolyteselectrolytes- any compound - any compound

that conducts electricity when melted that conducts electricity when melted or dissolved in wateror dissolved in water In order to conduct electricity the ions must be In order to conduct electricity the ions must be

free to move. Must take on or give up electronsfree to move. Must take on or give up electrons Solid state does not conduct electricity. Why?Solid state does not conduct electricity. Why?

Explaining the Properties of Explaining the Properties of Covalent CompoundsCovalent Compounds

Composed of moleculesComposed of molecules Held together by covalent bonds-Held together by covalent bonds-

make the molecule a stable unitmake the molecule a stable unit Molecules have no ionic chargeMolecules have no ionic charge Attractive forces between molecules Attractive forces between molecules

are usually weakare usually weak Interparticle forcesInterparticle forces- forces between - forces between

particles that make up a substanceparticles that make up a substance

Properties of Covalent Properties of Covalent CompoundsCompounds

Many are liquids or gases at room Many are liquids or gases at room temperaturetemperature

Low melting points and boiling pointsLow melting points and boiling points Do not conduct electricityDo not conduct electricity Some do not dissolve in waterSome do not dissolve in water

Vegetable oil and gasolineVegetable oil and gasoline In general they are less soluble in water In general they are less soluble in water

than ionic compoundsthan ionic compounds

Ionic Vs. CovalentIonic Vs. Covalent

It is the strength between interparticle forces It is the strength between interparticle forces in covalent compounds compared to the strong in covalent compounds compared to the strong attractive forces of ions in ionic compounds is attractive forces of ions in ionic compounds is what explains many of the differences in their what explains many of the differences in their physical propertiesphysical properties

Because covalent compounds do not have ions Because covalent compounds do not have ions you would expect that they don’t conduct you would expect that they don’t conduct electricityelectricity

Ionic compounds are more soluble in water Ionic compounds are more soluble in water because ions are attracted by the water because ions are attracted by the water molecules, while molecular compounds are notmolecules, while molecular compounds are not

Ionic Vs. Covalent Ionic Vs. Covalent CompoundsCompounds

High melting pointHigh melting point High boiling pointHigh boiling point Solid at room Solid at room

temperaturetemperature Soluble in waterSoluble in water Excellent Excellent

conductor of conductor of electricityelectricity

Many crystallineMany crystalline

Low melting pointLow melting point Low boiling pointLow boiling point Liquid or gas at Liquid or gas at

room temperatureroom temperature Less soluble in Less soluble in

waterwater Poor conductor of Poor conductor of

electricityelectricity