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Today’s Class

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

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Today’s Class

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

  2. Exceptions to the Octet rule • 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 • There are three types of exceptions to the rule: 1) Incomplete octet 2) Expanded octet 3) Odd-electron molecules

  3. Incomplete octet • Example Boron-group 3A row 2 • Tends to form compounds where it has less than 8 electrons surrounding it. (incomplete octet) • BF3-24 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

  4. Incomplete octet • 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. • 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

  5. Expanded Octet • Some atoms appear to exceed the octet rule • Behavior observed for elements in period 3 of the periodic table and beyond • Consider the compound SF6 • Has 48 valence electrons

  6. Example continued • 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. • 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 • 3rd row elements have 3s, 3p, and 3d orbitals. • 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 3s 3p 3d- once empty

  7. Summary • 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 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 • 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(3rd period or beyond

  8. Odd-Electron Molecules • 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) • Example NO-nitric oxide or NO2-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)

  9. Practice • Explain the bonding in: ICl4- BeF2 PCl5

  10. Bond Length • 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) • 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

  11. Example bonding and resonance • Ozone-O3 Lewis Structure • As we’ve seen before, choosing which O to take the extra electrons 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

  12. Bond Strength • 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.

  13. 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

  14. Example of Pi and Sigma Bonds • Take N2 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

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

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