AP Notes, Outlines, Study Guides, Vocabulary, Practice Exams and more!

Lewis Structures

drawing Lewis structures -

  • valence electrons - electrons involved in chemical bonding
  • dots placed on 4 sides of chemical symbol
  • number of valence electrons in representative elements same as group number
  • add all valence electrons
    • same # as group # for representative elements
    • add electron for each negative charge in anions
    • subtract electron for each positive charge in cations
    • only keep track of total number of electrons (no need to associate electrons to certain atoms)
  • write symbols for atoms
    • central atom usually the least electronegative
    • single bond (2 shared electrons) represented by dash between atomic symbols
  • complete octets of atoms
    • arrange dots so all atoms have 8 electrons
    • hydrogen only needs 2 in outer shell
  • place extra electrons around central atom
    • even if it adds up to more than 8
  • use multiple bonds if not enough electrons
    • use unshared electron pairs to make double/triple bonds

formal charge - charge of an atom if all atoms had the same electronegativity

  • tells how electrons are distributed in a molecule
  • # of valence electrons - # of electrons assigned to the atom
    • # of assigned electrons = all nonbonding electrons + 1/2 of bonding electrons
  • sum of formal charges = overall charge of molecule

resonance structures - placement of electrons changes, but atom placement doesn’t

  • double arrow used to show resonance structures
  • cannot be described by a single Lewis structure
  • actual structure an average of the possible Lewis resonance structures

exceptions to octet rule -

  • odd # of electrons - not possible for octet to form around all atoms
    • not possible for complete pairing of electrons
  • less than an octet - weird exception for boron/beryllium
    • usually due to halogen’s reluctance to form double bonds
  • more than an octet
    • extra electrons placed around central atom
    • expanded electron shells only possible for elements in period 3 and beyond
    • d subshell can be used for bonding
    • central atom usually bonded to smallest/most electronegative atoms

Draw Lewis Structures for PCl3, PO33-, XeFl4, BCl3 

  • PCl3
  • PO33-
  • XeFl4
    • more than an octet
  • BCl3
    • less than an octet
Subject: 
Chemistry
Subject X2: 
Chemistry

Need Help?

We hope your visit has been a productive one. If you're having any problems, or would like to give some feedback, we'd love to hear from you.

For general help, questions, and suggestions, try our dedicated support forums.

If you need to contact the Course-Notes.Org web experience team, please use our contact form.

Need Notes?

While we strive to provide the most comprehensive notes for as many high school textbooks as possible, there are certainly going to be some that we miss. Drop us a note and let us know which textbooks you need. Be sure to include which edition of the textbook you are using! If we see enough demand, we'll do whatever we can to get those notes up on the site for you!