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CHM120H5 (20)
Chapter 10

Chm-Chapter 10.docx

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Judith C Poe

Chapter 10: The Shapes of Molecules 10. 1: Depicting Molecules and Ions With Lewis Structures  Shows electron dot symbols for the atoms, and bonding pairs as line sand the lone pairs that fill each atom’s outer level (valence shell as pairs of dots) Applying the Octet Rule to Write Lewis Structures B. Resonance: Delocalized Electron-Pair Bonding Electron Delocalization In a single, double or triple bond each electron pair is localized b/w the bonded atoms. In a resonance hybrid, two of the electron pairs are delocalized: their density is spread over a few adjacent atoms. Fractional Bond Orders: Partial bonding, as in resonance structures, often leads to fractional bond orders. C. Formal Charge: Selecting the More Important Resonance Structure  One way to select the more important resonance form is by determining each atom’s formal charge, the charge it would have if the bonding electrons were shared equally  Smaller formal charges(positive or negative) are preferable to large ones  The same nonzero formal charges on adjacent atoms are not preferred  A more negative formal charge should reside on a more electronegative atom Formal Charge vs. Oxidation Number  Formal charge: bonding electrons are shared equally by the atoms (as if the bonding were nonpolar covalent), so each atom has ½ of them o Formal Charge = valence e - (lone pair e + ½ bonding e ) -  Oxidation Numbers: bonding electrons are transferred completely to the more electronegative atom o Oxidation # = valence e - (lone pair e + bonding e ) -  Oxidation numbers do not change from one resonance structure to another b/c the electronegativities don’t change  Formal charges do change b/c the numbers of bonding and lone pairs change. D. Lewis Structures for Exceptions to the Octet Rule  The octet rule apples to most molecules and ions with Period 2 central atoms but not to every one, and not to many with central atoms from period 3 and higher  Three important exceptions occur in molecules w.: o electron-deficient atoms o odd-electron atoms o atoms w. expanded valence shells 1. Electron Deficient  gaseous molecules containing either Be or B as the central atom are often electron deficient: they have fewer than 8 e around the central atom o Be has four electrons around it o B has six electrons around it o very reactive 2. Molecules with Odd-Electron Atoms  a few molecules have a central atom w. odd number of electrons so they can’t have all their electrons in pairs  these are called free radicals: species that contain a lone (unpaired) electron, which makes the paramagnetic and extremely reactive
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