Download Lewis Structures Lewis structures are used to represent the

Lewis Structures
Lewis structures are used to represent the distribution of valence electrons about atoms
in a molecule. The arrangement of valence electrons is determined by the "octet" rule.
Lewis structures are used to represent the basic structure of a molecule in two
dimensions. They indicate which atoms are bound together, and what type of bond is
present between the atoms. Lewis structures are also used to determine formal charge
and bond polarity, as well as indicate the presence of resonance.
Objectives:
1.
2.
3.
4.
Write the Lewis Structures for Molecular Compounds (covalent bonds).
Write resonance structures.
Calculate formal charges.
Determine the most stable resonance structure.
Equipment:
Model kits to build structures
Periodic table
Prior to lab read sections of our textbook that address writing Lewis Structures,
resonance, and formal charge calculations.
Print the spread sheet for this exercise. Follow the steps given below to write the Lewis
Structure for each substance listed on the spreadsheet. A periodic table will be needed.
Download Lewis Structures Spreadsheets
Directions for Writing Lewis Structures:
1. Determine the total number of valence electrons that will be used to make the Lewis
Structure. Enter this number on the data sheet under "number of Valence electrons
available". Use the periodic table to determine the number of valence electrons for each
atom.
For an anion, add an electron for each negative charge; for a cation, subtract an
electron for each positive charge.
2. Determine the center atom and draw the skeleton using symbols for the atoms to
show which atoms are attached to which.
Chemical formulas are often written in the order in which the atoms are connected
together in the molecule. When a central atom has a group of other atoms bonded to it,
the central atom is usually written first (far left) in the formula.
3. Draw a single bond between the center atom and each outside atom.
4. Complete the octets of the atoms bonded to the central atom.
Remember, hydrogen can have only two electrons.
Don't try to satisfy the octet for the central atom at this time; later, you will adjust the
bonding for this purpose.
5. Place any leftover electrons on the central atom, even if doing so results in more than
an octet of electrons on the central atom.
You will have to subtract the number of electrons used in the previous steps from the
total of valence electrons available.
Having more than an octet of electrons around the central atom is allowable only
for elements located in the 3rd row (third energy level) or greater. This is called
an expanded octet.
Other exceptions to the octet rule are odd number of electrons and less than an octet of
electrons; please refer to the textbook for examples of exceptions.
6. If there are not enough electrons left over to give the central atom an octet, try
changing 1 or more bonds from single bonds to double or triple bonds.
Use one or more unshared pairs of electrons from an outside atom to make a bond
between with the center atom.
7. If there is more than one possible way of making a Lewis Structure for the molecule,
draw the first possibility under "Lewis Structure" and draw any other possibilities under
"Resonance Structures".
Note, that the arrangement of the atoms (the skeleton) must be the same for each
structure drawn; only the placement of the electrons may differ in a resonance structure.
Calculation of Formal Charge
Each atom within a Lewis Structure has a formal charge F.C.
Formal Charge =
valence electrons for the atom minus nonbonding electrons minus ½ bonding electrons