Tuesday, May 17, 2011

Electron Dot and Lewis Structures

Drawing Electron Dot and Lewis Structures

  • the nucleus is represented by the Atomic symbol
  • for individual elements you have to determine number of valence electrons
  • electrons are represented by dots :      .   
  • 4 orbitals (max. of 2 per each orbital)
  • each orbital has 1 electron before they pair up
  • the dots are placed into 4 groups of one/ 2 electrons, with 8 electrons representing a closed shell (noble gas configuration)
  • the dots are placed on the 4 sides in pairs
Ex:       Li .

Each bond in the structural formula represents 2 electrons. All valence electrons must be used. Each element must have a full valence shell (8 electrons), except hydrogen which only needs 2 electrons.

Ex:      C2H6       Carbon- 4e  Hydrogen- 1e
A structural diagram is one that shows each bond as a line. Each line represents 2 dots. Lone pairs aren't shown in structural diagrams:        H       H     
                                                                            H-C    -  C -H
                                                                                H        H    <----(structural lines connecting bottom and top H's with the C's.)

Lewis Structures

  • structural diagrams show the number of bonds in a compound
  • electron dot diagrams show the electrons involved in the bond
  • if you know the structure of a molecule it means you know which atoms are bonded to which
Octet Rules

  • H only needs 2 electrons
  • CNOF (carbon, nitrogen, etc) always follow the octet rule
Rules for Writing Lewis Structures

For ex: CCl4
  1. The central atom (H and F are never in the center)
  • if a metal is present, place it in the center
  • if a molecule has only 1 atom of a particular element and several atoms of another, the single one is the one in the center
  • place the atom that needs the most electrons in the center
    2.   Place one pair of the electrons into each bond

    3.   Can't up the total number of valence electrons in a molecule. Adjust the number by subtracting one
          electron, for every positive charge. Adding one for negative charges.

    4.   Find the number of valence electrons remaining.

    5.   Use the number of valence electrons remaining to complete the octets of the atoms bonded to the central atoms.

   6.   Then place any remaining electrons on the central atoms in pairs.

   7.   If the central has less than an octet, make multiple bonds.

   8.   Replace each pair of electrons engaged in a bond with a dash "  - "

Check out this link for extra help!


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