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To predict the electron arrangement in molecules To learn to write simple Lewis structures

Objectives. To predict the electron arrangement in molecules To learn to write simple Lewis structures To learn to write Lewis structures for molecules with multiple bonds. A. Writing Lewis Structures. In writing Lewis structures we include only the valence electrons

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To predict the electron arrangement in molecules To learn to write simple Lewis structures

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  1. Objectives • To predict the electron arrangement in molecules • To learn to write simple Lewis structures • To learn to write Lewis structures for molecules with multiple bonds

  2. A. Writing Lewis Structures • In writing Lewis structures we include only the valence electrons • Most important requirement • Atoms achieve noble gas electron configuration (octet rule, duet rule)

  3. A. Writing Lewis Structures • Bonding pairs are shared between 2 atoms • Unshared pairs (lone pairs) are not shared and not involved in bonding

  4. A. Writing Lewis Structures

  5. B. Lewis Structures of Molecules with Multiple Bonds • Single bond – covalent bond in which 1 pair of electrons is shared by 2 atoms e.g. Ethane, C2H6 • Double bond – covalent bond in which 2 pairs of electrons are shared by 2 atoms e.g. Ethylene, C2H4 • Triple bond – covalent bond in which 3 pairs of electrons are shared by 2 atoms e.g. Acetylene, C2H2

  6. B. Lewis Structures of Molecules with Multiple Bonds • A molecule shows resonance when more than one Lewis structure can be drawn for the molecule • When you can write resonance structures this generally indicates stability of the molecule • Draw resonance structures for SO2 and C6H6

  7. How Do Lewis Structures Explain Polyatomics? • A polyatomic ion is a “charged molecule”. It has valence electrons +/- “extra” electrons • Can Lewis structures explain the common occurrence of polyatomics?

  8. B. Lewis Structures of Polyatomic Ions • Consider the Lewis structure for the cyanide ion, CN- • Number of valence electrons from C and N is 4+5 = 9 • Plus one from the negative charge = 10 • Apply the octet rule to the various options :C≡N: -1 charge Draw the line structures, including lone pairs, for carbonate, sulfate, hydroxide, nitrate and ammonium ions. Show also the resonance structures where relevant

  9. B. Lewis Structures of Molecules with Multiple Bonds • Boron – incomplete octet • Some Exceptions to the Octet Rule • Molecules containing odd numbers of electrons – NO and NO2

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