Drawing Lewis Structures

Chemistry                                                                                                     Guided Instructional Activity 4-5
Unit 4: Chemical Compounds and Reactions                                                                          Reading: Chapter 8

Working Mode: Pairs. Follow the instructions below, answer the questions using the white board or paper.

Learning Objectives:  To draw Lewis structures of molecular compounds.

Steps for writing Lewis Structures:

Step 1: Add up the valence electrons of each element in the compound.  In the case of a polyatomic ion, add an electron to the total per negative charge and subtract an electron per positive charge.

Step 2: Select as the central atom from the chemical formula the first non-hydrogen element in the chemical formula.

Step 3: Attach the other atoms as ligands (terminal atoms) to the central atom with a single bond, representing a bonding pair of electrons.  (This bond is represented by a dash, “—“ between the two bonded elements.)

Step 4: Subtract two electrons per bond from the total valence electrons.

Step 5: Place the remaining electrons, as lone-pairs, around the atoms giving each atom an octet.  Start by giving each ligand an octet, then add any remaining electrons to the central atom.  (***Remember that hydrogen’s “octet” consists of only 2 electrons i.e. a single bond.***)

Step 6: Check for an octet around each atom.  If the central atom does not have an octet, take a lone pair from a ligand with an octet and make that lone pair a bonding pair with the central atom.  If necessary change another lone pair into a bonding pair until the central atom has an octet.

Example: CCl₄            Valence e^-s: C = 4, Cl = 7  à  4 + 4(7) = 32 e-s
                                    C is the central atom and Cl s are ligands.
                        Cl
                         |
                 Cl—C—Cl        32 e-s – 8 (bonding electrons)  = 24 electrons remaining for lone pairs
                          |
                        Cl

                         . .
                        :Cl :
                  . .      |     . .               3 lone pairs of e-s around each –Cl use all 24 electrons.
                 :Cl—C—Cl :  
                   “      |      “               Checking for octets we see that the Cl’s and the central
                        :Cl :                             carbon all have octets.  Therefore, we have
                          “                                  completed the Lewis structure of CCl₄.

For the following molecules or polyatomic ions draw the Lewis structure, following the steps given above. 

Lewis Structures

            (a) H₂S

            (b) CO₂

(c)    PO₄^3-

(d)    O₂

(e)  NCl₃

Lewis structures with Resonance Structures:  When a molecule or polyatomic ion has the same ligands and only one gets a double bond, resonance structures exist.  Resonance structures give a more complete picture of the Lewis structure of a molecule.

                                                            . .    . .    . .              . .    . .    . .
                    eg.  SO₂  à  Lewis Str.  :O—S==O:  ßà  :O==S—O:
                                                             “                                           “

Notice how there are two equally correct ways of drawing the Lewis structure of SO₂ with the only difference the location of the double bond.  The complete picture of these molecules is to draw all the resonance structures using the double arrow between them.

Draw the Lewis structures for the following molecules giving all resonance structures.

SO₃ (3 resonance structures)                                O₃ (two resonance structures)