Lecture Notes Week 09

Lecture Notes Week 09 - Week 9 LEWIS STRUCTURES (Refer to...

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1 – Week 9 – LEWIS STRUCTURES (Refer to page 330 and Section 9.6) Notes & Hints: 1. Chemical properties mostly determined by number of electrons in outermost shell of atom. 2. Lack of reactivity of inert gases is associated with filled outer s and p shells . 3. Lewis Convention : chemical symbol stands for nucleus and inner electrons; outermost electrons are represented by dots (or a line signifies an electron pair), e.g., Na 4. Lewis diagrams only useful when only have s and p electrons (not great for d electrons). 5. The first period is 6. If know Lewis diagram for one atom in a group , we know them all ! 7. To write a Lewis diagram, need to know either electronic configuration or position in periodic table. 8. Atoms tend to achieve a complete outer shell (octet status) either by: a) losing or gaining e ’s (ionic bonding) e.g., Na Na + , Cl Cl b) sharing (covalent bonding). Simple rules for drawing covalent Lewis Diagrams 1. Determine the total number of valence electrons, V . 2. Draw skeleton structure using single bonds. 3. Subtract number of electrons used thus far from V and then distribute these extra electrons so each atom achieves octet status (access to 8 electrons). [Note: H can only get 2 electrons.] 4. If too few electrons to satisfy all, use double or triple bonds until each [non-H] atom has 8 electrons. 5. If too many electrons , add extras to central atom as lone pairs (expanded octet allowed when central atom has n 3 (row 3 or greater). Note : We are just "electron accounting" at present and NOT implying geometry (that comes later).
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2 EXAMPLES : NF 3 C 2 H 4 N 2 O (N-N-O) CO 3 2– CH 3 NH 3 + FORMAL CHARGES : To deduce the formal charge on an atom: a. Assign 1 electron from each SHARED electron pair (i.e., one per single bond, two for double, etc.) and then ADD to this the total number of UNSHARED pairs of electrons. b. If atom has ____________ outer electrons than it would as a neutral atom, it has a POSITIVE charge . c. If atom has MORE outer electrons than it would as a neutral atom, it has a ______________ charge . The SUM of all the formal charges from all the atoms: i) within a MOLECULE must be _____________ ii) within an ION must be the _______________ on the ion
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EXAMPLES : NH 4 + ClO (CH 3 ) 3 NO RESONANCE : Sometimes it’s not possible to write a single Lewis diagram which correctly reflects the known chemistry of the molecule, e.g., SO 2 S S O O O O It’s important to recognize what we mean and what we DO NOT mean by RESONANCE: ¾ true state of bonding is ________________ correctly represented by ANY ONE Lewis diagram ¾ instead it exists somewhere ______________________ these extremes! Thus the reason we link the so-called resonance structures by a double headed arrow.
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This note was uploaded on 04/17/2011 for the course CHEM 1040 taught by Professor Prokiacpak during the Spring '08 term at University of Guelph.

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Lecture Notes Week 09 - Week 9 LEWIS STRUCTURES (Refer to...

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