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## Lecture Notes:

### Lewis Structures and Naming Molecular Compounds

• Lewis Structures
•  Lewis structures allow us to draw the shape of a molecule which helps us to understand its properties
• Some hints to guide you in drawing Lewis Structures:
1. Find the total number of valence electrons; add 1 electron per unit negative charge, subtract 1 electron per unit positive charge
2. Find the maximum number of electrons needed to give each atom a noble gas configuration (i.e., an octet): for example, O has 6 and needs 2 electrons so you would write 8, H has 1 and needs 1 so you would write 2, S has 6 and needs 2 so you would write 8, N has 5 and needs 3 so you would write 8, etc.; add all these figures up
3. Calculate the number of bonds by subtracting the valence electrons from the maximum found in step two; divide this result by 2 and you have the number of bonds
4. Calculate the number of non-bonding electrons by subtracting 2 times the number of bonds from the number of valence electrons
5. Choose your ‘central’ atom based on which one is most likely to have multiple bonds; draw the structure and include the lone pairs
6. Assign formal charge, if necessary; formal charge is calculated by using this formula:
FC = valence electrons – non-bonding electrons – ½ of the bonding electrrons
7. Check your formal charge assignments by drawing a circle around each atom; if it has more electrons in its circle than the number of valence electrons then it has a negative charge; if it has fewer electrons in its circle than the number of valence electrons then it has a positive charge; multiply the number greater/fewer by either –1/+1
• If the number of bonds you calculated is too few, then an atom in the structure has an expanded octet
• If the number of bonds you calculated is fractional (i.e., 1.5 or 3.5) then the structure is a radical (i.e., it has a single electron somewhere instead of a lone pair)
• About assigning polarity: if the structure is at all unsymmetrical, then it is probably polar
• Halogens never have double bonds!
• See the Group Activity for more information about Lewis Structures and how to draw them, plus plenty of practice (this is a PDF)
• Resonance
•
• A word or two about resonance structures:
• If there are no double bonds, there are no resonance forms
• Only electrons change positions
• Each resonance form is to be thought of as a ‘contributor’ to the overall ‘real’ structure
• They are not different molecules!
• Naming Molecular Compounds
•  Prefix Number mono- 1 di- 2 tri- 3 tetra- 4 penta- 5 hexa- 6 hepta- 7 octa- 8 nona- 9
•      There are two methods for naming molecular compounds:
• Prefix, Root, Suffix System
• A prefix tells you how many of that atom appear in the formula of the compound
• The root is the name of an element
• A suffix shows that the element is part of a compound
• Elements are listed in order of increasing electronegativity
• The prefix mono- is not used for the first element in a compound
• Other prefixes are used to show how many of each atom are in the compound (see table)
• Stock System
• Unequally shared electrons determine what is called an oxidation number
• The oxidation number is used in naming the compound as in the following examples:
CO – carbon (II) oxide | CO2 – carbon (IV) oxide | SO3 – sulfur (VI) oxide
• Oxidation Numbers
•  The complete rules for assigning oxidation numbers are not important at this stage and these simple rules should suffice:
• The halogens are nearly always -1
• Oxygen is nearly always -2
• Hydrogen is nearly always +1
• Elements in their neutral state have an oxidation number of 0