chem lecture 9

chem lecture 9 - What are the rules of nomenclature for...

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Chemistry 6A 1 What are the rules of nomenclature for ionic and covalent compounds? What is the electrostatic basis of ionic and covalent bonding? What general information do we need in order to draw Lewis structures of neutral molecules and polyatomic ions? What is bond order?
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Chemistry 6A 2
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Chemistry 6A 3 Arranging Atoms in Polyatomic Ions Atoms within these ions are covalently bonded. For each ion, use the same guidelines as for other molecular compounds. Next, enclose cation and anion structures in brackets with superscript indicating net charge; e.g. [ ] + , [ ] 2–
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Chemistry 6A 4
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Chemistry 6A 5 How is the concept of formal charge useful in selecting a skeletal structure and assessing the plausibility of a Lewis structure? What is resonance? When does the octet rule break down? How do we correct the covalent and ionic models? How are bond energies and bond lengths measured and related?
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Chemistry 6A 6 1. Count the total number of valence electrons in the molecule or ion. 2. Draw the basic structure, placing an electron pair between each pair of bonded atoms. In the absence of other information , put the least electronegative atom (lowest IE 1 , for the moment) at the center . Except in very unusual cases, H atoms and F atoms must be terminal. 3. Complete the octet on each terminal atom by adding lone pairs to the terminal atoms as needed. Remember that H can only accept 2 electrons. If lacking a sufficient number of electrons, form multiple bonds. 4. Add any remaining electrons on the central atom in the form of lone pairs. 5. Calculate the FORMAL CHARGE on each atom. If there is a positive charge next to a negative charge, transfer a lone pair from the negative atom to make another bond to the positive atom, UNLESS this will exceed the octet of a second period atom. 6. Repeat steps 5 and 6 until you minimize the number of non-zero formal charges.
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Chemistry 6A 7 A complete Lewis structure depicts formal charges, if any. A lowest energy Lewis structure has formal charges which have been minimized. We can use formal charges to assess: The contribution of a individual Lewis structures to resonance hybrids The plausibility of isomers
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Chemistry 6A 8 Formal charge is the charge calculated for an atom in a Lewis structure on the assumption that there is an equal sharing of bonded electron pairs. Example : Nitric Acid, HNO 3 Let’s work out the formal charges on the chalkboard …
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Chemistry 6A 9 A complete Lewis structure depicts formal charges (if any) As verification, the sum of the formal charges should equal the overall charge of the molecule or ion (which is zero for a neutral molecule). When two possible Lewis diagrams can be drawn for a molecule, the structure that assigns the lowest formal charge to the atoms is generally the best description of the bonding.
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This note was uploaded on 08/07/2009 for the course CHEM 6A taught by Professor Pomeroy during the Spring '08 term at UCSD.

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chem lecture 9 - What are the rules of nomenclature for...

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