Communication pubs.acs.org/jchemeduc
Teaching Beginning Chemistry Students Simple Lewis Dot Structures Peter Nassiff and Wendy A. Czerwinski* Burlington High School, Burlington, Massachusetts 01803, United States ABSTRACT: Students beginning their initial study of chemistry often have a difficult time mastering simple Lewis dot structures. Textbooks show students how to manipulate Lewis structures by moving valence electron dots around the chemical structure so each atom has an octet or duet. However, an easier method of teaching Lewis structures for simple molecules, based on the octet rule (the fundamental concept of Lewis structures), enables most students to master drawing Lewis structures in one class session.
KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, Curriculum, Communication/Writing, Lewis Structures
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INTRODUCTION Lewis dot structures are an integral part of introductory chemistry, leading to an understanding of simple bonding and molecular structure (VSEPR) and finally to important concepts such as polarity, intermolecular forces, and solubility. However, students often have a difficult time mastering simple Lewis dot structures.1−11 Students learn early on that the driving force in the stability of ions in groups 1A−7A corresponds to having the same number of valence electrons (VEs) as the nearest noble gas, leading to the octet rule or the duet rule. Logically, it follows that bonding in molecules should also follow the octet and duet rules. Students apply this concept to obtain an initial Lewis structure that has a full octet for all atoms, and then modify their structure until they obtain the required number of VEs, resulting in a correct Lewis structure. Methods given in the literature,1,2 or textbooks,4−6 start drawing Lewis structures by giving full octets only to terminal atoms. However, students often end up stopping with an incomplete Lewis structure because they “ran out” of electrons, or form too many bonds. These errors are prevented by initially applying the octet rule to each atom (except for hydrogen) at the start. Consequently, students do not run out of electrons or form too many bonds. In order to ensure that not too many valence electrons are used, students then compare the number of VEs in this structure with the VEs required and remove VEs until the number in their structure matches the number required, thereby completing the Lewis structure. Using this method helps students quickly and easily master simple Lewis dot structures.
not have enough background to determine the correct molecular arrangement, and an incorrect skeletal structure would distract from the targeted learning objective. Step 1
Using a periodic table, count the number of VEs for the atoms in the structure. For polyatomic ions add or subtract VEs as required. Step 2
Using the skeletal structure provided, give each atom (except hydrogen) eight valence electrons by using a line to represent a VE pair. Li, Be, and B are not examined because they are exceptions to the octet rule.3 Although not a necessity, using a line to represent electron pairs makes the structure neater and helps in counting VEs. A mistake some students initially make is to put VEs in one at a time. It is important to stress to them that, conceptually, valence electrons come in pairs either as bonds or as lone pairs. Step 3
Now compare the total number of electrons in the Lewis structure drawn with the number of required VEs from step 1. Table 1 gives the total number of valence electrons for molecules that have a full octet with no multiple bonds. Most often, there are too many valence electrons drawn in the initial structure, and so a pair or pairs of VEs must be removed. Determining how many VEs have to be removed and how this is done is the key to this method. Step 4
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Count the number of VEs drawn in the structure from step 2 or select the number of VEs from Table 1 corresponding to the
PROCEDURE Students are always given the correct skeletal structure because, as others have commented,2 beginning students generally do © XXXX American Chemical Society and Division of Chemical Education, Inc.
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DOI: 10.1021/ed5007162 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Communication
Table 1. Summary of Lewis Dot Structures Molecule
Number of Atoms
Examples
VEs in Complete Octeta
VEs Required
ΔVEs
A2
2
Cl2, HCl O2 N2, CN−, CO Cl2O SO2 CO2 NF3, ClO3− SO3, NO3−, CO32− SO42−, CH4
14
14 12 10 20 18 16 26 24 32
0 2 4 0 2 4 0 2 0
AB2
a
3
AB3
4
AB4
5
20
26 32
VEs Removed and Multiple Bonds Formed Correct Lewis structure Remove 2 pairs of VEs; Remove 4 pairs of VEs; Correct Lewis structure Remove 2 pairs of VEs; Remove 4 pairs of VEs; Correct Lewis structure Remove 2 pairs of VEs; Correct Lewis structure
form 1 multiple bond. form 2 multiple bonds. form 1 multiple bond. form 2 multiple bonds. form 1 multiple bond.
For each H, use the duet rule and subtract 6 VEs. For example for HCl, 14 VEs − 6 VEs = 8 required VEs.
number of atoms in the molecule. Then subtract the number of VEs from step 1 from number.
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EXAMPLES
Cl2
For Cl2 there are 14 VEs in the initial Lewis structure and 14 VEs from step 1, so the initial Lewis structure, Figure 1, is correct. Figure 3. Lewis Structure for N2.
CO2
For CO2 there are 16 VEs from step 1, yet 20 VEs (Table 1) are drawn in step 2. Since 20 VEs − 16 VEs = 4, 4 VEs (two pairs of VEs) need to be removed, and two extra bonds formed (Figure 4). This is done by removing a pair of electrons from Figure 1. Lewis Structure for Cl2.
O2
O2 has 12 VEs from step 1, yet 14 VEs (Table 1) are drawn in step 2. Since there are more VEs drawn than available (14 − 12 = 2 VEs), a pair of VEs must be removed. This is done by removing a pair of VEs from two adjacent atoms and forming a multiple bond as shown in Figure 2. Although it seems that too Figure 4. Lewis Structure for CO2.
each atom and adding a multiple bond between them. Most students have a Lewis structure with two double bonds, but when resonance structures are introduced, from the procedure outlined, after one multiple bond is formed between the two atoms, the idea of removing a pair of electrons from any two adjacent atoms shows that a combination of a single and a triple bond is possible.
Figure 2. Lewis Structure for O2.
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many VEs are being removed from the initial structure (4 VEs), there are 2 VEs added in the multiple bond formation. As is always the case, each pair of VEs that have to be removed from a Lewis structure requires the formation of one extra bond. For O2, removing a pair of VEs results in a double bond (Figure 2).
CONCLUSION The method of drawing Lewis dot structures, summarized in the text and in Table 1, is a fast, easy way to teach students how to draw Lewis structures because it builds on their understanding that atoms bond to have complete outer shells. Using this method, students learn that not all Lewis structures are acceptable, and that the structures must contain the proper number of VEs while ensuring that each atom has 8 electrons (exceptions not included). Most students master drawing simple Lewis dot structures in one class period using this method.
N2
N2 has 10 VEs from step 1, yet 14 VEs (Table 1) are drawn in step 2, so 4 VEs (or 2 pairs of VEs) must removed, and two extra bonds are needed. Two pairs of VEs must be removed from each atom, and replaced by a triple bond between the nitrogen atoms as shown in Figure 3. B
DOI: 10.1021/ed5007162 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Caricature of G. N. Lewis is used with permission of William B. Jensen. REFERENCES
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DOI: 10.1021/ed5007162 J. Chem. Educ. XXXX, XXX, XXX−XXX
