Chem 161-2011 Lecture 10, Chapter 6

Chem 161-2011 Lecture 10, Chapter 6 - CHEMISTRY 161-2011...

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Chem 161-2011 Lecture 10 1 CHEMISTRY 161-2011 LECTURE 10 CHAPTER 6 REPRESENTING MOLECULES ANNOUNCEMENTS E-MAIL EXAMS
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Chem 161-2011 Lecture 10 2 PLAN FOR TODAY The octet rule Lewis structures Multiple bonds Drawing Lewis structures Lewis structures and formal charge Resonance Exceptions to the octet rule Incomplete octets Odd numbers of electrons Expanded octets Electronegativity and polarity Electronegativity Dipole moment, partial charges, and percent ionic character
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Chem 161-2011 Lecture 10 3 COVALENT BONDS Lewis structures ET: Define covalent bond: A bond formed by a pair of electrons shared between atoms RULES FOR WRITING LEWIS STRUCTURES FOR COVALENT BONDED MOLECULES Sketch a symmetrical molecule (usually). Put H and most electronegative atoms at terminals, if possible; another way of saying this is put the most electropositive atom in the middle. ¾ For oxo-acids, bond the H to an oxygen. Total the valence electrons of all of the atoms (i.e., add the group numbers) + the charge. Use electrons to make skeletal structure. Distribute any additional electrons (1) initially around outside of terminal atoms (not H) to form octets, then any extra electrons around outside of center atom (octet of center atom may be expanded for periods 3). (2) move electrons from outside of atoms to between atoms, if necessary, to satisfy the octet rule of the center atom (but without worsening formal charge).
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Chem 161-2011 Lecture 10 4 H&P 33 (mod.). Write Lewis structures for the following covalent molecules, assuming that the octet rule (and duet rule for hydrogen) is followed in each case. (a) SiH 4 H | H Si H | H (b) NCl 3 . . : Cl : . . | . . : C l N Cl : . . . . . . HClO 3 . . : O : . . | . . H O Cl O : . . . . . . NH 4 + H + | H N H | H
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Chem 161-2011 Lecture 10 5 H&P 35. Write plausible Lewis structures for the following covalent molecules. (c) C 2 H 4 (e) H 2 SiO 3 (f) HCN double bond (c) C 2 H 4 H C = C H | | H H (e) H 2 SiO 3 H bonding electron pair (i.e., single bond) | : O : “lone pair” of electrons, or . . | . . “non-bonding pair” of electrons : O = S i O H double bond . . (f) HCN H C N: triple bond
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Chem 161-2011 Lecture 10 6 LEWIS STRUCTURES THAT VIOLATE THE OCTET RULE H&P 53 (mod). Write the simplest Lewis structure for each of the following. Comment on any unusual features of the structures. (a) NO (c) BF 3 (a) . . . . . N = O : Free radical ET: Molecules with an unpaired electron are called radicals. Lewis structure has odd no. of electrons. .. .. .. .. Although it is rare, molecules can have two unpaired electrons, and are called biradicals, e.g., O 2 . •O-O• not O=O (very low priority for students) Radicals tend to be unstable, i.e., very reactive. They can react with compounds in your body, resulting in an abnormal growth of cells (cancer).
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Chem 161-2011 Lecture 10, Chapter 6 - CHEMISTRY 161-2011...

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