Covalent BondingCovalent Bonding

Chapter 6, Section 2Chapter 6, Section 2

How does a covalent bond form?How does a covalent bond form?

There is a balance between the attraction of There is a balance between the attraction of oppositely charged particles & the repulsion of oppositely charged particles & the repulsion of like charged particleslike charged particles– FYI – bond length is the distance btwn atomsFYI – bond length is the distance btwn atoms

How does a covalent bond form?How does a covalent bond form?

A bond is formed when . . .A bond is formed when . . .

a distance between the nuclei is reached a distance between the nuclei is reached in whichin which– repulsion and attraction forces are equalrepulsion and attraction forces are equal– potential energy is at the lowest point possiblepotential energy is at the lowest point possible– at the bottom of the curve on PE graphat the bottom of the curve on PE graph

Energy in BondsEnergy in Bonds

When atoms form bonds, they become When atoms form bonds, they become more stable and release energymore stable and release energy

That same amount of energy must be That same amount of energy must be absorbed to break the bond (ie. bond absorbed to break the bond (ie. bond energy)energy)

The higher the bond energy, the stronger The higher the bond energy, the stronger the bondthe bond

Bond EnergiesBond Energies

Do you see a trend between bond length Do you see a trend between bond length and bond energy?and bond energy?

Octet RuleOctet Rule

The ultimate goal of the atoms in a The ultimate goal of the atoms in a compound is to look like a noble gascompound is to look like a noble gas

Each atom will gain, lose, or share Each atom will gain, lose, or share electrons so that they have 8 electrons in electrons so that they have 8 electrons in their outer energy leveltheir outer energy level

Check out examples on p. 169Check out examples on p. 169

Exceptions to Octet RuleExceptions to Octet Rule

Hydrogen is happy with 2 electronsHydrogen is happy with 2 electrons

Boron can be happy with 6 electronsBoron can be happy with 6 electrons

Sulfur & phosphorus can actually get Sulfur & phosphorus can actually get MORE than 8 electrons (more on that MORE than 8 electrons (more on that later)later)

e- dot diagramse- dot diagrams

Shows the number of Shows the number of valence electrons on valence electrons on an s- or p-block an s- or p-block elementelement

Draw dots around Draw dots around element symbol to element symbol to match number of match number of valence electronsvalence electrons

Lewis StructuresLewis Structures

e- dot diagrams for e- dot diagrams for compoundscompounds– Dots show unshared Dots show unshared

electronselectrons– Dashes show shared Dashes show shared

electronselectrons

So how do you draw a Lewis So how do you draw a Lewis structure?structure?

1)1) Determine the type & # of atoms in a moleculeDetermine the type & # of atoms in a molecule

2)2) Determine the number of valence e- on each atomDetermine the number of valence e- on each atom

3)3) Add up total # of valence e-Add up total # of valence e-

4)4) Arrange the atoms with the first element in the middle Arrange the atoms with the first element in the middle and other elements radiating off. If carbon is present and other elements radiating off. If carbon is present it automatically goes in the middleit automatically goes in the middle

5)5) Connect the elements with dashesConnect the elements with dashes

6)6) Subtract shared electrons from total electronsSubtract shared electrons from total electrons

7)7) Distribute remaining electrons as unshared pairs first Distribute remaining electrons as unshared pairs first to outer elements (follow the octet rule) and then to to outer elements (follow the octet rule) and then to center element.center element.

WOW!!!WOW!!!

That last slide had a LOT of That last slide had a LOT of words!!!words!!!

Let’s draw a Lewis Structure!Let’s draw a Lewis Structure!Draw the Lewis Structure for CHDraw the Lewis Structure for CH33ClCl

duet

That’s all for now . . . That’s all for now . . .

We will finish up the rest of section 2 next We will finish up the rest of section 2 next class! class!

Homework: p. 197 #38 & 39Homework: p. 197 #38 & 39

November 1, 2010November 1, 2010

Get out your homework and show it to me.Get out your homework and show it to me.

We will grade it as soon as the bell rings.We will grade it as soon as the bell rings.

We will finish section 2 notes after grading.We will finish section 2 notes after grading.

There are a few different types of There are a few different types of covalent bonds . . . covalent bonds . . .

Single covalent bond = 2 e- are sharedSingle covalent bond = 2 e- are shared– Weakest & longestWeakest & longest– Ex. HEx. H22

Double covalent bond = 4 e- are sharedDouble covalent bond = 4 e- are shared– Stronger & shorterStronger & shorter– Ex. OEx. O22, CO, CO22

Triple covalent bond = 6 e- are sharedTriple covalent bond = 6 e- are shared– Strongest & shortestStrongest & shortest– Ex. NEx. N22, C, C22HH22

Multiple Bonds ExampleMultiple Bonds Example

What is the Lewis structure for silicon What is the Lewis structure for silicon dioxide?dioxide?

Multiple Bonds Example 2Multiple Bonds Example 2

What is the Lewis structure for What is the Lewis structure for hydrocyanic acid (HCN)?hydrocyanic acid (HCN)?

Polyatomic Ion Lewis StructuresPolyatomic Ion Lewis Structures

Follow the same steps for drawing a Follow the same steps for drawing a normal Lewis structure with the following normal Lewis structure with the following changes:changes:– If the polyatomic ion is negative, add the If the polyatomic ion is negative, add the

charge to the total number of valence e-charge to the total number of valence e-– If the polyatomic ion is positive, subtract the If the polyatomic ion is positive, subtract the

charge from the total number of valence e-charge from the total number of valence e-– Draw brackets around the entire diagram with Draw brackets around the entire diagram with

the charge written as a superscriptthe charge written as a superscript

Polyatomic IonsPolyatomic IonsExample: CNExample: CN--

NHNH44+ + : ammonium ion: ammonium ion

Polyatomic Ion ExamplePolyatomic Ion Example

What is the Lewis structure for sulfate (SOWhat is the Lewis structure for sulfate (SO442-2-)?)?

Polyatomic Ion ExamplePolyatomic Ion Example

What is the Lewis structure for hydroxide (OHWhat is the Lewis structure for hydroxide (OH--)?)?

Another exampleAnother example

Draw the Lewis structure for ozone (ODraw the Lewis structure for ozone (O33).).

ooooooo . . .resonance structuresooooooo . . .resonance structuresSometimes, molecular compounds can’t Sometimes, molecular compounds can’t make up their minds how they want to bemake up their minds how they want to be– One Lewis structure just is not enoughOne Lewis structure just is not enough

In these situations, 2+ Lewis structures In these situations, 2+ Lewis structures are drawn and the molecule actually is an are drawn and the molecule actually is an average of all of themaverage of all of them

Ex. Ozone (OEx. Ozone (O33))

Covalent Network BondingCovalent Network Bondinga different type of covalent bonding that is a different type of covalent bonding that is more like ionic bondingmore like ionic bondinglots of nonmetal atoms covalently bonded lots of nonmetal atoms covalently bonded together in a network that looks like a together in a network that looks like a crystalcrystalexample:example:– diamonddiamond– silicon dioxidesilicon dioxide– graphitegraphite