Section 12.3 Lewis Structures 1.To predict the electron arrangement in molecules 2.To learn to write simple Lewis structures 3.To learn to write Lewis structures for molecules with multiple bonds Objectives

Section 12.3 Lewis Structures 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)

Section 12.3 Lewis Structures A. Writing Lewis Structures Bonding pairs are shared between 2 atoms Unshared pairs (lone pairs) are not shared and not involved in bonding

Section 12.3 Lewis Structures A. Writing Lewis Structures

Section 12.3 Lewis Structures 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, C 2 H 6 Double bond – covalent bond in which 2 pairs of electrons are shared by 2 atoms e.g. Ethylene, C 2 H 4 Triple bond – covalent bond in which 3 pairs of electrons are shared by 2 atoms e.g. Acetylene, C 2 H 2

Section 12.3 Lewis Structures 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 SO 2 and C 6 H 6

Section 12.3 Lewis Structures 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?

Section 12.3 Lewis Structures 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

Section 12.3 Lewis Structures 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 NO 2