Today’s ClassToday’s Class

-Exceptions to the octet rule-Exceptions to the octet rule

-Bond Length and Strength-Bond Length and Strength

-Pi and Sigma bonds-Pi and Sigma bonds

-Linking activity-Linking activity

Exceptions to the Octet ruleExceptions to the Octet rule

The rules that we have used for Lewis structures The rules that we have used for Lewis structures apply to most moleculesapply to most molecules

However, there are cases where the importance However, there are cases where the importance of an octet of electrons is called into questionof an octet of electrons is called into question

There are three types of exceptions to the rule:There are three types of exceptions to the rule:

1) Incomplete octet1) Incomplete octet

2) Expanded octet2) Expanded octet

3) Odd-electron molecules3) Odd-electron molecules

Incomplete octetIncomplete octet

Example Boron-group 3A row 2Example Boron-group 3A row 2 Tends to form compounds where it has less than 8 Tends to form compounds where it has less than 8

electrons surrounding it. (incomplete octet)electrons surrounding it. (incomplete octet) BFBF33-24 electrons-24 electrons

In this structure, Boron only has 6 electrons around it. In this structure, Boron only has 6 electrons around it. The octet rule can be satisfied be drawing a structure The octet rule can be satisfied be drawing a structure with a double bondwith a double bond

Incomplete octetIncomplete octet

Since fluorine is so much more electronegative Since fluorine is so much more electronegative than boron, the structure seems doubtful.than boron, the structure seems doubtful.

So it is more characteristic for boron to form So it is more characteristic for boron to form molecules where it lacks 8 valence electrons.molecules where it lacks 8 valence electrons.

On the other hand the elements that are in the On the other hand the elements that are in the same row as boron obey the octet rule(carbon, same row as boron obey the octet rule(carbon, nitrogen, oxygen, and fluorine)nitrogen, oxygen, and fluorine)

Other examples of elements who do not obey Other examples of elements who do not obey octet, Beryllium and hydrogenoctet, Beryllium and hydrogen

Expanded OctetExpanded Octet

Some atoms appear to exceed the octet Some atoms appear to exceed the octet rulerule

Behavior observed for elements in period Behavior observed for elements in period 3 of the periodic table and beyond3 of the periodic table and beyond

Consider the compound SFConsider the compound SF6 6

Has 48 valence electronsHas 48 valence electrons

Example continuedExample continued We used 12 electrons to form the S-F bonds, which leaves 36 We used 12 electrons to form the S-F bonds, which leaves 36

electronselectrons Since fluorine always follows the octet rule, we completed octets Since fluorine always follows the octet rule, we completed octets

to give the structure.to give the structure. Sulfur has 12 electrons around it, therefore it exceeds the octet Sulfur has 12 electrons around it, therefore it exceeds the octet

rule. How can this happen?rule. How can this happen? Involves using the empty 3d orbitals on the third-period elementsInvolves using the empty 3d orbitals on the third-period elements 33rdrd row elements have 3s, 3p, and 3d orbitals. row elements have 3s, 3p, and 3d orbitals. The 3s and 3p fill with electrons but the 3d orbitals remain emptyThe 3s and 3p fill with electrons but the 3d orbitals remain empty So those 3d orbitals in sulful can be used to accommodate extra So those 3d orbitals in sulful can be used to accommodate extra

electrons used the 3s and 3p to hold 8 electrons, with the 4 extra electrons used the 3s and 3p to hold 8 electrons, with the 4 extra in the formerly empty 3d orbitalsin the formerly empty 3d orbitals

3s 3p 3d- once empty

Summary Summary The second-row elements C, N, O, and F should be The second-row elements C, N, O, and F should be

assumed to obey octet ruleassumed to obey octet rule The second-row elements BE, and B often have fewer The second-row elements BE, and B often have fewer

than 8 electrons around them in their compoundsthan 8 electrons around them in their compounds The second-row elements never exceed the octet rule The second-row elements never exceed the octet rule

(valence orbitals can only accommodate only 8 electrons)(valence orbitals can only accommodate only 8 electrons) Third row and heavier elements often satisfy octet rule, but Third row and heavier elements often satisfy octet rule, but

can exceed rule by using their empty valence d orbitalscan exceed rule by using their empty valence d orbitals When writing Lewis structures for a molecule first draw When writing Lewis structures for a molecule first draw

single bonds between all bonded atoms, and then satisfy single bonds between all bonded atoms, and then satisfy the octet rule for all the atoms. If electrons remain, place the octet rule for all the atoms. If electrons remain, place them on the elements having available d orbitals(3them on the elements having available d orbitals(3rdrd period period or beyondor beyond

Odd-Electron MoleculesOdd-Electron Molecules Relatively few molecules contain an odd number Relatively few molecules contain an odd number

of electronsof electrons Because we need an even number of electrons Because we need an even number of electrons

for complete pairing the octet rule clearly can’t for complete pairing the octet rule clearly can’t be satisfied. (if there is an odd number)be satisfied. (if there is an odd number)

Example NO-nitric oxide or NOExample NO-nitric oxide or NO22-nitrogen dioxide-nitrogen dioxideN has five valence electronsN has five valence electronsO has six valence electronsO has six valence electrons

Lewis structures can be drawn for these molecules Lewis structures can be drawn for these molecules but to treat them accurately, a different model is but to treat them accurately, a different model is needed (won’t be discussed)needed (won’t be discussed)

PracticePractice

Explain the bonding in:Explain the bonding in:

IClICl44--

BeFBeF22

PClPCl55

Bond LengthBond Length Bond length is the distance between two bonded Bond length is the distance between two bonded

atoms in a molecule atoms in a molecule Bond length is inversely related to bond Bond length is inversely related to bond

order(number of bonds between atoms)order(number of bonds between atoms) When more electrons participate in bond When more electrons participate in bond

formation the bond will get shorter.formation the bond will get shorter.Example: Example: C-C bond length: 154pmC-C bond length: 154pm

C=C bond length:134pmC=C bond length:134pmC=C bond length:120pmC=C bond length:120pm

Bond length is also inversely related to bond Bond length is also inversely related to bond strength, a stronger bond is a shorter bond strength, a stronger bond is a shorter bond

Example bonding and resonanceExample bonding and resonance

Ozone-OOzone-O3 3 Lewis StructureLewis Structure As we’ve seen before, choosingAs we’ve seen before, choosing

which O to take the extra electronswhich O to take the extra electrons

is arbitrary. Therefore Ozone is a resonance structure:is arbitrary. Therefore Ozone is a resonance structure:

We know from experimental data that the bonds between the Os are We know from experimental data that the bonds between the Os are the same length, therefore neither of these structures are correct the same length, therefore neither of these structures are correct because we know a double bond is shorter than a singlebecause we know a double bond is shorter than a single

Instead when drawing the structure of ozone it is better like this:Instead when drawing the structure of ozone it is better like this:

Now each bond can be displayed correctlyNow each bond can be displayed correctly

Being the same lengthBeing the same length

Bond StrengthBond Strength

Bond energy is a measure of the strength Bond energy is a measure of the strength of a chemical bond. The larger the bond of a chemical bond. The larger the bond energy, the stronger the bond.energy, the stronger the bond.

The strength of the bond is determined by The strength of the bond is determined by the amount of energy required to break the the amount of energy required to break the bond.bond.

Sigma and Pi BondsSigma and Pi Bonds

Pi bonds- a covalent bond in which parallel p Pi bonds- a covalent bond in which parallel p orbitals share an electron pair occupying orbitals share an electron pair occupying the space above and below the line joining the space above and below the line joining the atomsthe atoms

Sigma bonds- a covalent bond in which the Sigma bonds- a covalent bond in which the electron pair is shared in an area centered electron pair is shared in an area centered on a line running between the atomson a line running between the atoms

Example of Pi and Sigma BondsExample of Pi and Sigma Bonds

Take NTake N22 for example, has a linear structure for example, has a linear structure

that looks like this:that looks like this:

N NN NSo the middle bond will be a sigma bond, and the two So the middle bond will be a sigma bond, and the two

outer bonds will be pi bondsouter bonds will be pi bonds

ActivityActivity

Use the words given to create links between Use the words given to create links between words and describe the correlation words and describe the correlation between them.between them.