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Lecture 24: Beyond the Octet

Lecture 24: Beyond the Octet. Reading: Zumdahl 13.9-13.12 Outline Sub-octet systems Valence-shell expansion Radicals: odd-electron systems. Beyond the Octet Rule. There are numerous exceptions to the octet rule. . We’ll deal with three classes of violation here:

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Lecture 24: Beyond the Octet

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  1. Lecture 24: Beyond the Octet • Reading: Zumdahl 13.9-13.12 • Outline • Sub-octet systems • Valence-shell expansion • Radicals: odd-electron systems

  2. Beyond the Octet Rule • There are numerous exceptions to the octet rule. • We’ll deal with three classes of violation here: • Sub-octet systems (less than 8 electrons) • Valence shell expansion (more than 8 electrons) • Radicals ( an odd number of electrons)

  3. Sub-Valent Systems • Some atoms (for example, Be, B, and Al) undergo bonding, but will form stable molecules that do not fulfill the octet rule. • Experiments demonstrate that the B-F bond strength is consistent with single bonds only.

  4. Sub-Valent Systems (cont.) • Sub-valent molecules will react with other molecules such that the octet rule is satisfied. • The octet rule is satisfied by the reacting partner providing an electron pair. A bond in which both electrons come from one of the atoms is called a coordinate covalent bond.

  5. Valence Shell Expansion • For third-row elements (“Period 3”), the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding. • When this occurs, more than 8 electrons can surround a third-row element. • Example: ClF3 (a 28 e- system) F obey octet rule Cl has 10e-

  6. Valence Shell Expansion (cont.) • Typical atoms that demonstrate valence-shell expansion are P, S, and larger halogens (Cl, Br, and I). • Example: PCl5 40 e-

  7. Valence Shell Expansion (cont.) • Lewis-dot structure & valence shell expansion: • Assign all electrons to bonds and electrons to give each atom an octet. • For halogen complexes (like PCl5), expand octet of period 3 using single bonds until halogens are bonded. • Finally, assign any remaining electrons to the period-3 element. 22 e- • Example: I3-

  8. Valence Shell Expansion (cont.) • Bonding in molecules containing nobel-gas atoms involves valence shell expansion • A famous example: XeO3 26 e- +3 +2 -1 -1 -1 -1 -1 0 0 +1 0 0 0 -1 0 0

  9. Radicals: Odd-Electron Systems (cont) • Finally, one can encounter odd electron systems where full pairs will not exist. • Example: Chlorine Dioxide. 19 e- Unpaired electron

  10. Radicals: Odd-Electron Systems (cont.) • Strategy: Generally attempt to put the odd electron on the central atom. However, the LDS rules were not intended to be applied to odd-electron systems. • Example: NO2. 17 e- • N is sub octet; therefore, predict that NO2 is highly reactive.

  11. Radicals: Odd-Electron Systems (cont.) • Since radicals are highly reactive (oxidative), excess concentrations can lead to health effects. • One biological mechanism to combat radicals are antioxidants, molecules that are easily oxidized, and remain benign in the oxidized form. • Example: Coenzyme Q: -

  12. Summary • Remember the following: • C, N, O, and F almost always obey the octet rule. • B, Be, and Al are often sub-octet • Second row (Period 2) elements never exceed the octet rule • Third Row (Period 3) elements and beyond can use valence shell expansion to exceed the octet rule. • In the end, you have to practice…..a lot!

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