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