Procedure for Drawing Lewis Dot Structure

Steps for Drawing Lewis Dot Structures
comments by Dr. MJ Patterson

Here are the steps for drawing a Lewis dot structure for a molecule or ion.  These steps are also listed under the reference in section 24.6 of the Archipelago multimedia lesson, but I have added a few comments below.

1.  Arrange the atoms for the Lewis structure.  To find the central atom, pick the one with lowest electronegativity (module 25), which is typically the atom with the smallest subscript, and typically the atom listed first in the formula (but not always).  Then, arrange the other atoms around the central atom.

2.  Count the number of valence electrons in the compound.  Add up the group numbers of the individual elements, and adjust this total for ions by adding or subtracting the charge of ions.  See the Additional Study "Counting Valence Electrons" for more help.  This is the total number of electrons that will be shown in the Lewis dot structure.  They will be arranged either in bonding pairs represented with a line, or as lone pairs represented with a pair of dots.

3.  Make the initial indication of single bonds between the central atom and each surrounding atom.  Place one pair of electrons, represented by a line, between each pair of bonded atoms.

4.  Assign the remaining electrons as lone pairs to terminal (non-central) atoms.  Terminal atoms must have 8 electrons around them.  If pairs remain, assign them to the central atom.

5.  Make sure each atom is surrounded by 8 electrons - an octet.  If an atom is not, form double or triple bonds as needed to make an octet around each atom.  Normally, only the following elements form multiple bonds:  C, N, O, P and S.