10.1: Lewis Dot Structures

Last updated
Save as PDF

Page ID: 96590

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

Introduction: Lewis Dot Structure Review

In order to predict molecular properties you need to be able to draw a Lewis Dot Structure. There are multiple techniques for doing this, and if you have your general chemistry 1 notes and feel confident, review those. Chapter 8 of the UALR Gen Chem 1 LibreTexts covers this material.

Specifically:
8.2: Covalent Bonding and Lewis Structures
8.3 Resonance
8.5 Drawing Lewis Dot Structures (note: This is Handout 1.1 below, which is just 2 pages, and I suggest you print it, and follow it while working out Lewis dot structures)

Worksheets

Handout \(\PageIndex{1}\): Lewis Dot Structure Procedures
http://chemwiki.ucdavis.edu/@api/deki/files/60817/vsepr-1-Review.pdf

The above handout is what I give my students in General Chemistry 1, and is covered in section 8.5. By following this handout you will identify resonance structures, and exceptions to the octet rule. The two videos below show how you use this handout. If you are good with Lewis dot structures and have learned another technique, that is fine, and you do not need to use this technique. (There are many paths to the top of the mountain, and once there, the view is the same.) Simply speaking, this is the path I teach. You should print this and use it while working out the problems in the following worksheet (Handout \(\PageIndex{2}\))

Handout \(\PageIndex{2}\): Lewis Dot Structure Worksheet
http://chemwiki.ucdavis.edu/@api/deki/files/61154/vsepr1_WorkSheet.pdf

The above worksheet is designed to be used with handout \(\PageIndex{1}\). Once completed, the same set of molecules will be used in the VESPR worksheet R.2.2 (next section).

Video Tutorials on Lewis Dot Structures

Example \(\PageIndex{1}\):

1. Draw Lewis dot structure for Sulfur Trioxide (SO₃). (You should look at Handout \(\PageIndex{1}\) while watching.)

Video 1.1: Please look at Handout 1: Lewis dot structure technique while watching this video. Note steps 1 and 2 are switched between the video and the handout, but all the rest are the same.

Question: Why does sulfur trioxide require resonance structures? In your answer, think about how a Lewis dot structure represents a bond, and the nature of PI bonds.

Answer: Lewis dot structures treat a bond as an electron pair shared between 2 nuclei, which is drawn as a line between two atoms. In sulfur trioxide, there is a pi bond between 4 atoms, not 3. Thus you can not draw a Lewis dot structure of sulfur trioxide, but need to draw three resonance structures, with the average of them being a representation of the molecule. Note, the sulfur is sp² hybridized giving it a trigonal planar geometry, and the unhybridized p orbital forms the pi bond with with overlap of the p orbitals of each oxygen, with electron density above and below the plan of the molecule.

Example \(\PageIndex{2}\):

Lewis dot structure for Sulfite (SO₃^-2). You should look at Handout \(\PageIndex{1}\) while watching.

Video 8.5.2: Please look at Handout 1: Lewis dot structure technique while watching this video. Note steps 1 and 2 are switched but all the rest are the same.

Question: What is the formal charge of each atom in the Lewis dot structure of sulfite?

Answer: The sulfur has a [+1] formal charge as it donates 6 electrons to the structure, but has 5 around it. Each of the oxygens has a [-1] as they also donate 6 electrons to the molecule, but have 7 around them. Note the sum of the formal charges over all atoms is the charge of the ion. See section 8.4 Formal Charges if you need to review these.

Class Activity

ADAPT \(\PageIndex{1}\)