Today’s Class -Exceptions to the octet rule -Bond Length and Strength -Pi and Sigma bonds -Linking activity

Exceptions to the Octet rule The rules that we have used for Lewis structures apply to most molecules The rules that we have used for Lewis structures apply to most molecules However, there are cases where the importance of an octet of electrons is called into question However, there are cases where the importance of 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 octet 2) Expanded octet 3) Odd-electron molecules

Incomplete octet Example Boron-group 3A row 2 Example Boron-group 3A row 2 Tends to form compounds where it has less than 8 electrons surrounding it. (incomplete octet) Tends to form compounds where it has less than 8 electrons surrounding it. (incomplete octet) BF electrons BF electrons In this structure, Boron only has 6 electrons around it. The octet rule can be satisfied be drawing a structure with a double bond In this structure, Boron only has 6 electrons around it. The octet rule can be satisfied be drawing a structure with a double bond

Incomplete octet Since fluorine is so much more electronegative than boron, the structure seems doubtful. Since fluorine is so much more electronegative than boron, the structure seems doubtful. So it is more characteristic for boron to form molecules where it lacks 8 valence electrons. So it is more characteristic for boron to form molecules where it lacks 8 valence electrons. On the other hand the elements that are in the same row as boron obey the octet rule(carbon, nitrogen, oxygen, and fluorine) On the other hand the elements that are in the same row as boron obey the octet rule(carbon, nitrogen, oxygen, and fluorine) Other examples of elements who do not obey octet, Beryllium and hydrogen Other examples of elements who do not obey octet, Beryllium and hydrogen

Expanded Octet Some atoms appear to exceed the octet rule Some atoms appear to exceed the octet rule Behavior observed for elements in period 3 of the periodic table and beyond Behavior observed for elements in period 3 of the periodic table and beyond Consider the compound SF 6 Consider the compound SF 6 Has 48 valence electrons Has 48 valence electrons

Example continued We used 12 electrons to form the S-F bonds, which leaves 36 electrons We used 12 electrons to form the S-F bonds, which leaves 36 electrons Since fluorine always follows the octet rule, we completed octets to give the structure. Since fluorine always follows the octet rule, we completed octets to give the structure. Sulfur has 12 electrons around it, therefore it exceeds the octet rule. How can this happen? Sulfur has 12 electrons around it, therefore it exceeds the octet rule. How can this happen? Involves using the empty 3d orbitals on the third-period elements Involves using the empty 3d orbitals on the third-period elements 3 rd row elements have 3s, 3p, and 3d orbitals. 3 rd row elements have 3s, 3p, and 3d orbitals. The 3s and 3p fill with electrons but the 3d orbitals remain empty The 3s and 3p fill with electrons but the 3d orbitals remain empty 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 in the formerly empty 3d orbitals 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 in the formerly empty 3d orbitals 3s3p3d- once empty

Summary The second-row elements C, N, O, and F should be assumed to obey octet rule The second-row elements C, N, O, and F should be assumed to obey octet rule The second-row elements BE, and B often have fewer than 8 electrons around them in their compounds The second-row elements BE, and B often have fewer than 8 electrons around them in their compounds The second-row elements never exceed the octet rule (valence orbitals can only accommodate only 8 electrons) The second-row elements never exceed the octet rule (valence orbitals can only accommodate only 8 electrons) Third row and heavier elements often satisfy octet rule, but can exceed rule by using their empty valence d orbitals Third row and heavier elements often satisfy octet rule, but can exceed rule by using their empty valence d orbitals When writing Lewis structures for a molecule first draw single bonds between all bonded atoms, and then satisfy the octet rule for all the atoms. If electrons remain, place them on the elements having available d orbitals(3 rd period or beyond When writing Lewis structures for a molecule first draw single bonds between all bonded atoms, and then satisfy the octet rule for all the atoms. If electrons remain, place them on the elements having available d orbitals(3 rd period or beyond

Odd-Electron Molecules Relatively few molecules contain an odd number of electrons Relatively few molecules contain an odd number of electrons Because we need an even number of electrons for complete pairing the octet rule clearly can’t be satisfied. (if there is an odd number) Because we need an even number of electrons for complete pairing the octet rule clearly can’t be satisfied. (if there is an odd number) Example NO-nitric oxide or NO 2 -nitrogen dioxide Example NO-nitric oxide or NO 2 -nitrogen dioxide N has five valence electrons O has six valence electrons Lewis structures can be drawn for these molecules but to treat them accurately, a different model is needed (won’t be discussed)

Practice Explain the bonding in: Explain the bonding in: ICl 4 - BeF 2 PCl 5

Bond Length Bond length is the distance between two bonded atoms in a molecule Bond length is the distance between two bonded atoms in a molecule Bond length is inversely related to bond order(number of bonds between atoms) Bond length is inversely related to bond order(number of bonds between atoms) When more electrons participate in bond formation the bond will get shorter. When more electrons participate in bond formation the bond will get shorter. Example: C-C bond length: 154pm C=C bond length:134pm C=C bond length:120pm Bond length is also inversely related to bond strength, a stronger bond is a shorter bond

Example bonding and resonance Ozone-O 3 Lewis Structure Ozone-O 3 Lewis Structure As we’ve seen before, choosing As we’ve seen before, choosing which O to take the extra electrons which 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 the same length, therefore neither of these structures are correct because we know a double bond is shorter than a single Instead when drawing the structure of ozone it is better like this: Now each bond can be displayed correctly Being the same length

Bond Strength Bond energy is a measure of the strength of a chemical bond. The larger the bond energy, the stronger the bond. Bond energy is a measure of the strength of a chemical bond. The larger the bond energy, the stronger the bond. The strength of the bond is determined by the amount of energy required to break the bond. The strength of the bond is determined by the amount of energy required to break the bond.

Sigma and Pi Bonds Pi bonds- a covalent bond in which parallel p orbitals share an electron pair occupying the space above and below the line joining the atoms Sigma bonds- a covalent bond in which the electron pair is shared in an area centered on a line running between the atoms

Example of Pi and Sigma Bonds Take N 2 for example, has a linear structure that looks like this: Take N 2 for example, has a linear structure that looks like this: N N So the middle bond will be a sigma bond, and the two outer bonds will be pi bonds

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