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Catalog No. AP4580

Publication No. 4580

Lewis Electron Dot Models Student Laboratory Kit

Introduction

Lewis electron dot structures are a useful and popular way in chemistry classes to visualize the bonding in molecules. Practice making Lewis electron dot structures using bingo chips and cut-out element symbols for the first 18 elements and then for a series of molecules.

Concepts

• Molecular bonding • Lewis electron dot structures

Background

A chemical bond is a strong attractive force between atoms or ions in a compound. Electrons in the outermost shells or orbit­als are often the electrons involved in chemical bonding. One type of chemical bond between two atoms is called a covalent bond. The covalent bond consists of a pair of shared electrons from the outermost orbitals. The electrons in this outermost shell, or high­est occupied energy level, have a special importance and are referred to as valence electrons. The electrons in the inner orbitals of an atom are referred to as core electrons. Since the maximum number of electrons in an orbital is 2 and since there is one s orbital and three p orbitals for any given energy shell, the maximum number of valence electrons is 8. Note: This holds for elements in the first three rows of the periodic table. Beyond that, exceptions may apply.

How are the number of valence electrons in an atom determined? Scientists discovered that there is a relationship between the organization of the periodic table and the electron configurations of the elements. All elements in a particular group of the peri­odic table have similar properties and thus the same number of valence electrons. In general then, the number of valence electrons is equal to the group number for each group of representative elements. The representative elements are designated by different headings on different periodic tables. Some tables use the American system IA to VIIIA group heading designations while other tables have the IUPAC recommendation with the 1- 18 group headings. For those who prefer the IUPAC method, the number of valence electrons in columns 14-18 is the group number minus ten. Thus, for example, potassium has 1 valence electron-its group number is IA or 1. Bromine has 7 valence electrons-its group number is VIIA or 17.

Molecules are formed when atoms come together through chemical bonding. Many different molecular models have been devised as a way to visualize the arrangement of atoms in molecules. In 1916, G. N. Lewis, an American chemist, developed a sys­tem of arranging dots, representing valence electrons, around the symbols of the elements to form electron dot symbols, or Lewis electron dot symbols. Using this Lewis representation for the atom, the element symbol represents the nucleus and the inner core electrons of the atom while the dots represent the outer valence electrons involved in the bonding. This model of using electron dot structures, while not all-encompassing due to the variety of molecules in the world, provides a simple representation of many mol­ecules. It is a useful tool and can lead to an increase in understanding the properties of many different molecules. The procedures for writing Lewis electron dot structures for individual atoms as well as for a variety of molecules are described below.

Writing Lewis Electron Dot Structures for Atoms

A Lewis electron dot structure for an atom includes the element symbol and the "electron" dots. The number of dots is equal to the number of valence electrons which, as explained previously, is equal to the group number of the periodic table where the element is located. The maximum number of "electron" dots on any atom is 8. Thus the Lewis electron dot structure for potas­sium, which is located in Group IA or 1 on the periodic table, contains only one dot (see below) while the Lewis electron dot structure for bromine, which is located in Group VII or 17 on the periodic table, contains seven dots:

CHEM-FAlC~ . . makes science teaching easier. IN4580 011413

.. K· :Br:

Potassium Bromine

Note: The exact placement of the dots is arbitrary, so the single dot around K can be on any of the four sides.

Writing Lewis Electron Dot Structures for Molecules

A Lewis electron dot structure for a molecule includes each atom in the molecule connected by bonds. Each bond may be drawn with two electron dots or, more commonly, with a single line. In order to predict the bonding arrangement that occurs between atoms in a molecule, the stable nature of noble gases must be recognized. Noble gases, group VIII of the periodic table, are unreactive and form very few compounds. This stability is due to the outermost s and p orbitals being completely filled and containing two s electrons and six p electrons. Helium has a filled valence orbital with only two s electrons while all of the other noble gases have eight electrons in their valence orbitals, two in the s orbital and six in the p orbital. From these observations of the noble gases, a rule called the octet rule was developed. The octet rule assumes that atoms form bonds to achieve stability and thus a noble gas configuration, in which each atom has eight electrons (an octet) in its valence orbitals. Thus, through electron sharing, each atom in a molecule can attain the electron configuration of a noble gas.

To write a Lewis dot structure for a molecule, a simple process may be followed. Determine the number of valence electrons supplied by each atom involved. (See step-by-step procedure below.) Visualize each atom donating its valence electrons to a large electron "pool." When the atoms come together to form a molecule, picture the electrons from the "pool" being distributed to the molecule so that each atom in the molecule attains a noble gas structure. Some electrons may need to be shared to form bonds while others may be left unshared.

In General

1. H atoms want to be surrounded by two valence electrons, to attain the stability of He.

2. All other nonmetal atoms want to be surrounded by eight valence electrons, and thus should follow the "octet rule." Note: There are many exceptions to the "octet rule" which are beyond the scope of this introductory activity.

Step-by-Step Procedure

1. Count the total number of valence electrons available. To do this, determine the number of valence electrons supplied by each atom in the compound and add them up. Remember that:

• Group IA or 1 atoms (H, Li, ... )have 1 valence electron.

• Group IIA or 2 atoms (Be, Mg, ... ) have 2 valence electrons.

• Group IIIA or 13 atoms (B, Al, ... ) have 3 valence electrons.

• Group IVA or 14 atoms (C, Si, ... ) have 4 valence electrons.

• Group VA or 15 atoms (N, P, ... )have 5 valence electrons.

• Group VIA or 16 atoms (0, S, ... )have 6 valence electrons.

• Group VIIA or 17 atoms (F, Cl, ... )have 7 valence electrons.

• Group VIIIA or 18 atoms (Ne, Ar, ... )have 8 valence electrons except He which has only 2 valence electrons.

• Polyatomic anions-add one electron for each unit of negative charge.

• Cations-subtract one electron for each unit of positive charge.

Example: CC14-C is in group IVA or 14, so it has 4 valence electrons. Cl is in group VIIA or 17, so each Cl has 7 valence electrons x 4 chlorine atoms equals 28 electrons. Total valence electrons for CCl4 = 32.

2. Draw a "skeleton structure" for the molecule, joining atoms by single bonds.

The skeleton structure for the molecule, X2, is:

X-X

The skeleton structure for the molecule, XY, is: X-Y

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For more complex molecules, more than one skeleton structure is possible. In general, if there is a single atom of one ele­ment and several atoms of another element, the single atom is usually the central atom. Thus, the skeleton structure for the molecule, XY 2, is most likely

Y-X-Y

Tips

• Nature tends toward symmetry, so it is best to make your skeleton structure symmetrical when possible.

• Hydrogen is never the central atom; it is always a surrounding atom.

• Group VII halogens tend to be surrounding atoms rather than central atoms.

• Carbon tends to be a central atom.

Example: CC14-Carbon tends to be a central atom and halogens tend to be surrounding atoms. Since nature tends toward symmetry, the following structure is proposed:

Cl I

Cl-C-CI I Cl

3. From the total number of valence electrons, subtract two for each single bond in the skeleton structure to determine how many valence electrons are left to distribute.

Example: CC14 has 32 total valence electrons. Subtract 2 electrons for each single bond x 4 single bonds. This leaves 32 - 8 or 24 remaining valence electrons.

4. Distribute the remaining valence electrons as unshared pairs around the atoms in the molecule. Follow the octet rule, distributing electrons to give each atom a total of 8 electrons (except H which should have 2).

Example: Carbon has 4 bonds , or 8 electrons already surrounding it. Thus, the 24 remaining valence electrons are dis­tributed around the 4 chlorine atoms to give each chlorine 6 paired electrons and one bond, or 8 total electrons .

. . ® -.-.-----Chlorine is surrounded by 8 electrons :c1-c-c1:

1--------Each single bond : Cl : represents 2 electrons ••

5. If this point is reached and there are too few valence electrons to give each atom an octet, a multiple bond is most likely necessary. While a single bond involves the sharing of one pair (or 2) electrons, multiple bonds involve the shar­ing of more than one pair of electrons. A double bond involves the sharing of two pairs (or 4) electrons and a triple bond involves the sharing of three pairs (or 6) electrons. Thus the electrons in multiple bonds are counted in the octets of both of the bonded atoms.

Example: HCN

His in group IA or 1 with 1 valence electron; C is in group IVA or 14 with 4 valence electrons; and N is in group VA or 15 with 5 valence electrons. Total valence electrons= 10.

The proposed skeleton structure can be drawn as follows, since C tends to be a central atom and H is always a surround­ing atom:

H-C-N

Proposed Skeleton Structure

From the total of 10 valence electrons, subtract 2 for each single bond, leaving 6 extra valence electrons to be distrib­uted. Try distributing the electrons around the nitrogen atom:

H-•::::-N: (WRONG!) -- .

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This structure is incorrect because there are too few electrons to give each atom an octet. Remember that each bond counts as 2 electrons. Hydrogen is stable with 2 electrons and nitrogen is stable with 8 electrons surrounding it; how­ever, carbon only has 4 electrons rather than the octet it needs to be stable. Therefore, a multiple bond is most likely necessary. Try a double bond: ..

H-·=~-=N (WRONG!)

The structure is still incorrect because, although nitrogen has 8 electrons surrounding it, carbon only has 6 electrons. This is not enough to complete its octet, so try a triple bond:

H-C==N: (RIGHT!)

This structure is now complete and correct. Hydrogen is stable with 2 electrons (1 bond), carbon is stable with 8 elec­trons (4 bonds) and nitrogen is stable with 8 electrons (3 bonds plus 2 unpaired electrons).

Materials

Bingo Chips

Element sheets, 4

Pre-Lab

Envelope

Scissors

Working individually or in teams, cut out the 32 element squares from the four sheets of elements.

Procedure

Part 1. Elements

1. Locate Data Table 1.

2. Record the family or group number for each element listed in Data Table 1. Refer to a periodic table to do this step.

3. List the number of valence electrons for each element in the next column of the Data Table 1.

4. Locate the element square for the first element in the table and set the square on a flat surface. Use bingo chips to place the correct number of valence electrons around the element. Follow the rules for writing Lewis Dot Structures outlined in the background section of this lab handout. This is the Lewis Electron Dot Structure for that element.

5. Draw the Lewis Electron Dot Structure in the last column of Data Table 1.

6. Repeat steps 4 and 5 for each element listed in Data Table 1.

7. Answer questions 1-4 in the Questions section.

Part 2. Covalent Compounds

8. Locate Data Table 2.

9. Determine the number of valence electrons for each individual atom in the first compound. Write each separate number in Data Table 2. Refer to Example 1 in Data Table 2.

10. Add up the valence electrons for each atom and record the total number of valence electrons for the compound in the next column of the Data Table 2.

11. Locate element squares for each atom in the compound and set the squares on a flat surface. Gather enough colored bingo chips to equal the total number of valence electrons. Use a different color for each element in the compound.

12. Use the bingo chips to place the correct number of valence electrons in the compound. Follow the rules for atom place­ment and electron placement described in the background section of this lab handout.

13. Once the structure is made and all rules have been followed, draw the Lewis Electron Dot Structure in the last column of the data table.

14. Repeat steps 8-12 for each compound listed in Data Table 2.

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Name:

Data Tables Data Table 1. Elements

Element Family or Group # Number of Valence Electrons Lewis Electron Dot Structure

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

-5- IN4580

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Name: ________________ _

Data Table 2. Covalent Compounds

Valence Electrons Total Number Lewis Electron Dot Structure Formula

for Each Atom of Valence Electrons

1. H2

2. HCI

3.Hp

4. NH3

5. CH4

6. CH2Cl2

7. Cl2

8. HOC!

9. 0 2

10.0H-

11.H30+

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Name:

Data Table 2. Covalent Compounds, continued

Formula Valence Electrons Total Number Lewis Electron Dot Structure

for Each Atom of Valence Electrons

12. NH/

13. so/-

14. N2

15. CHC13

16. C2H6

17. co2

18. CHpH

19. HP2

20. C2H4

21. C2H2

22. so2

-7- IN4580

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Name:

Questions

1. Use the completed Data Table 1 to write a general rule relating the placement of an element on the periodic table and the number of valence electrons.

2. Are there exceptions to the general rule proposed in #1? Why does this exception occur?

3. Why can any atom have no more than eight valence electrons?

4. Write the Lewis electron dot structures for the following elements:

A. Barium, Ba d. Gallium, Ga

b. Tin, Sn e. Potassium, K

c. Radon, Rn f. Iodine, I

-8- IN4580

© 2013 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Lewis Electron Dot Models, Catalog No. AP4580, from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in any form or by any means, electronic or mechanical, including, but not limited LO photocopy, recording, or any information storage and retrieval system, without permission in writing from Flinn Scientific, Inc.

Teacher's Notes Lewis Electron Dot Models

Materials Included in Kit

Bingo chips, red, 250

Bingo chips, blue, 250

Bingo chips, yellow, 250

Bingo chips, green, 250

Additional Materials Needed (for each group)

Cup (to carry chips)

Envelope

Additional Materials Needed (for overhead demonstration)

Overhead projector

Connecting to the National Standards

Element symbol masters, 4 sheets

Magnetic wand

Reproducible Data Tables

Wire-rimmed counting chips, 100

Scissors

Overhead transparency sheets, 4

This laboratory activity relates to the following National Science Education Standards (1996):

Unifying Concepts and Processes: Grades K-12 Systems, order, and organization

Content Standards: Grades 5-8 Content Standard B: Physical Science, properties and changes of properties in matter

Content Standards: Grades 9-12 Content Standard B: Physical Science, structure of atoms, structure and properties of matter

Tips

• Teacher Demonstration: To introduce and explain the activity, it is suggested that a teacher overhead demonstration be performed. The element symbol master sheets can be copied onto overhead transparency sheets (colored, if available) and the elements cut out into squares. Examples of Lewis electron dot models can then be demonstrated on the overhead projector, using wire-rimmed counting chips as electrons and using the magnetic wand to easily swipe up the chips.

• Student Activity: For the student activity part, it is suggested that the element symbol master sheets be copied onto cardstock paper. Each student group will need one set of the 4 element sheets. The colored bingo chips should be distributed to the students to be used as electrons.

• There are more than enough bingo chips to be used by a class of 30 working individually or in pairs.

• Before performing this activity, students should have ample background in the periodic table, writing electron configura­tions, and counting valence electrons. The information provided in the background section is provided as brief supplemen­tary information and is not meant to accomplish the encompassing job of the chemistry textbook.

• Extensions: This activity has the possibility for many extensions. After making and drawing Lewis electron dot structures on paper, students can then build three-dimensional models of each molecule using a molecular model set. From here could follow a discussion of shapes and polarities of molecules, electronegativity, electronic and molecular geometry, and bonded and non-bonded electrons.

• There are differing viewpoints on how to arrange the electron dots around an element symbol. Some textbooks show elec­trons being placed separately on four sides before pairing electrons. Other textbooks first pair two electrons, like in the s-orbital, then place separate electrons on the three remaining sides like in the p-orbitals. Both methods accomplish the goal of counting valence electrons.

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© 2012 Flinn Scientific, Inc. All Rights Reserved. IN4580

Teacher's Notes continued

Data Table 1. Elements

Element Family or Group # Number of Valence Electrons Lewis Electron Dot Structure

l.H 1 1 H·

. 2.He 8or18 2 He·

3. Li 1 1 Li·

4.Be 2 2 Be·

5. B 3or13 3 ·Be·

. 6. C 4or14 4 ·<;·

. 7. N 5or15 5 ·N: .

8. 0 6or16 6 ·O: . ..

9.F 7or17 7 :F: . . ..

10. Ne 8or18 8 :Ne:

11. Na 1 1 Na·

. 12. Mg 2 2 Mg·

. 13. Al 3or13 3 ·AI·

. 14. Si 4or14 4 ·Si·

. 15. p 5or15 5 . P: .

16.S 6or16 6 ·S:

.. 17. Cl 7or17 7 :Cl: .

.. 18. Ar 8or18 8 :Ar: ..

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Teacher's Notes continued

Data Table 2. Covalent Compounds

Formula

2. HCl

3. Hp

8. HOCl

10. OH-

Valence Electrons for Each Atom

1+1

1+ 7

1+1+ 6

5 +1+1+1

4+1+1+1+1

4 +1+1+ 7 + 7

7+7

1+ 6 + 7

6 + 6

6 + 1 +1

1+1+ 1 + 6-1

© 20 I 3 Flinn Scientific, Inc. All Rights Reserved.

Total Number of Valence Electrons

2

8

8

8

8

20

14

14

12

8

8

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Lewis Electron Dot Structure

H-H

.. H-Cl:

H-0:

..

I H

H-N-H I

H

H I

H-C-H I

H

H .. I ..

:Cl-C-CI: .. I .. H

.. .. :Cl-Cl: .. . .

.. H-0:

I : Cl:

.. .. :O=O:

[ .. ]+ H-i-H

IN4580

Teacher's Notes continued

Data Table 2. Covalent Compounds, continued

Valence Electrons Total Number . Formula " E h At f V: 1 EI t Lewis Electron Dot Structure ior ac om o a ence ec rons

12. NH; 5 + 1 + 1 + 1 + 1 - 1 8 [ H -t-Hr . .. l 2-:0:

.. I .. 13.so 2- 6+6+6+6+6+2 32 :o-s-o: 4 •• I ..

:0:

14. N2 5 + 5 10 : N = N:

H .. I ..

15 . CHCI3 4+1+ 7 +7+ 7 26 :9-T-9: :CJ:

H H I I

16.C2H 6 4+4+1+1+ 1 +1+1+1 14 H-C-C-H I I H H

. . .. 17. co2 4 + 6 + 6 16 Q = c = g

H I ..

18. CHpH 4 + 1+1+1+6+1 14 H-C-0-H I

H

.. .. 19. H20 2 1+1+6 + 6 14 H-0-0-H

H H 20. C2H4 4 + 4 + 1 + 1 + 1 + 1 12 I I

H-C=C-H

21. C2H2 4+4+1+1 10 H-C= C-H

.. . . .. 22. S02 6+6+6 18 Q=S-Q:

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Teacher's Notes continued

Answers to Questions

1. Use your completed Data Table 1 to write a general rule relating the placement of an element on the periodic table and the number of valence electrons.

The number of valence electrons is equal to the group number for each group of representative elements (Group IA to VIIIA).

2. What exceptions do you observe to the general rule that you wrote in #1? Why does this exception occur?

Helium is an exception to the rule listed in #1 because it is in group VIII but only has 2 valence electrons. Helium is a stable noble gas and has only 2 electrons total. Therefore, its outer shell is filled with only 2 valence electrons.

3. Why can any atom have no more than eight valence electrons?

Valence electrons are the electrons in the highest occupied energy level which includes only s and p orbitals. Since an s orbital can hold a maximum of 2 electrons and a p orbital can have a maximum of 6 electrons, then 8 is the maximum number of valence electrons in any atom. Note: There are exceptions to this statement which go beyond the scope of this kit. See references for further reading.

4. Write the Lewis electron dot structures for the following elements:

. . a. Barium, Ba Ba· d. Gallium, Ga ·Ga·

. b. Tin, Sn ·Sn· e. Potassium, K K·

.. c. Radon, Rn :Rn: f. Iodine, I : I

References

Herron, J. D.; Sarquis, J. L.; Schrader, C. L.; Frank, D. V.; Sarquis, M.; Kukla, D. A. Chemistry; D. C. Heath: Boston, MA, 1996; Chapter 13.

Wilbraham, A. C.; Staley, D. D.; Matta, M. S. Chemistry; Addison-Wesley: Menlo Park, CA, 1997; Chapters 14-15.

Materials for Lewis Electron Dot Models,Student Laboratory Kit are available from Flinn Scientific, Inc.

Catalog No. Description

AP4580 Lewis Electron Dot Models-Student Laboratory Kit AP4584 Magnetic Wand AP4585 Wire-rimmed Counting Chips

Consult your Flinn Scientific Catalog/Reference Manual for current prices.

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© 2013 Flinn Scientific, Inc. All Rights Reserved. IN4580

@ N 0

0

:!l 5· :> VJ ("")

n;· ;?.. 5i ("")

...... :>

" ~ Cl

(JQ

~ :::<l ~ " :;:i

" ~

z .p. V> 00 0

I --+>-• 1

The Periodic Table of the Elements 1

H 1.008

IA llA lllB

1 2 3

3 4 2 Li Be

6.941 9.012

11 12 3 Na Mg

22.99 24.31

19 20 21 4 K Ca Sc

39.10 40.08 44.96

37 38 39 5 Rb Sr y

85.47 87.62 88.91

55 56 57-71 6 Cs Ba La-Lu

132.9 137.3 * 87 88 89-103

7 Fr Ra Ac-Lr (223) (226) #

*

#

© 2010 Flinn Scientific, Inc . AP4580

IVB VB

4 5

22 23

Ti v 47.87 50.94

40 41

Zr Nb 91.22 92.91

72 73

Hf Ta 178.5 180.9

104 105

Rf Db (267) (268)

57 58

La Ce 138.9 140.1

89 90

Ac Th (227) 232.0

VIB VllB VlllB

6 7 8 9

24 25 26 27

Cr Mn Fe Co 52.00 54.94 55.85 58.93

42 43 44 45

Mo Tc Ru Rh 95.96 (98) 101.1 102.9

74 75 76 77

w Re Os Ir 183.8 186.2 190.2 192.2

106 107 108 109

Sg Bh Hs Mt (271) (272) (270) (276)

59 60 61 62

Pr Nd Pm Sm 140.9 144.2 (145) 150.4

91 92 93 94

Pa u Np Pu 231.0 238.0 (237) (244)

IB llB

10 11 12

28 29 30

Ni Cu Zn 58.69 63.55 65.38

46 47 48

Pd Ag Cd 106.4 107.9 112.4

78 79 80

Pt Au Hg 195.1 197.0 200.6

110 111 112

Ds Rg Cn (281) (280) (285)

63 64 65

Eu Gd Tb 152.0 157.3 158.9

95 96 97

Am Cm Bk (243) (247) (247)

lllA

13

5

B 10.81

13

Al 26.98

31

Ga 69.72

49

In 114.8

81

Tl 204.4

113

Uut (284)

66

Dy 162.5

98

Cf (251)

IVA VA VIA VllA

14 15 16 17

6 7 8 9 c N 0 F 12.01 14.01 16.00 19.00

14 15 16 17

Si p s Cl 2809 30.97 32.07 35.45

32 33 34 35

Ge As Se Br 72.64 74.92 78.96 79.90

50 51 52 53

Sn Sb Te I 118.7 121.8 127.6 126.9

82 83 84 85

Pb Bi Po At 207.2 209.0 (209) (210)

114 115 11 6 117

Uuq Uup Uuh Uus (289) (288) (293) (294)

67 68 69 70

Ho Er Tm Yb 164.9 167.3 168.9 173.0

99 100 101 102

Es Fm Md No (252) (257) (258) (259)

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2

He 4003

VlllA

18

10

Ne 20.18

18

Ar 39.95

36

Kr 83.80

54

Xe 131 .3

86

Rn (222)

118

Uuo (294)

71

Lu 175.0

103

Lr (262)

II

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