total number. For polyatomic ions, add electrons to the total number to account for negative charges; subtract electrons from the total number to account for positive charges.3.For each bond (dash) in the skeletal structure, subtract two electrons from the total valence electrons (determined in step 2) to determine the number of remaining electrons.4.Use the remaining electrons to complete the octets of the terminal atoms (those bonded to the central atom) by placing pairs of electrons on each atom. (Remember that an H atom only requires two electrons to complete its valence shell.) If there is more than one type of terminal atom, complete the octets of the most electronegative atoms first.5.If any electrons remain after step 4, place them in pairs on the central atom.6.If the central atom has fewer than eight electrons after completing steps 1 to 5, move one or more pairs from the terminal atoms to form multiple bonds between the central atom and the terminal atoms. (Unless the central atom is a Group 3A element.) Like Lewis dot symbols for atomic anions, Lewis structures for polyatomic anions are enclosed by square brackets.Page 43 of 961/30/2019
Figure 8.12 Drawing Lewis structures. Sample Problem 8.7 shows how to draw a Lewis structure.Page 44 of 961/30/2019
PRINTED BY: [email protected] Printing is for personal, private use only. No part of this book may be reproduced or transmitted without publisher's prior permission. Violators will be prosecuted.Page 344 Sample Problem 8.7 Draw the Lewis structure for carbon disulfide (CS2).StrategyUse the procedure described in steps 1 through 6 in Figure 8.12 for drawing Lewis structures.Setup1.Step 1: C and S have identical electronegativities. We will draw the skeletal structure with the unique atom, C, at the center. 2.Step 2: The total number of valence electrons is 16, six from each S atom and four from the C atom [2(6) + 4 = 16]. 3.Step 3: Subtract four electrons to account for the bonds in the skeletal structure, leaving us 12 electrons to distribute. 4.Step 4: Distribute the 12 remaining electrons as three lone pairs on each S atom. 5.Step 5: There are no electrons remaining after step 4, so step 5 does not apply. 6.Step 6: To complete carbon’s octet, use one lone pair from each S atom to make a double bond to the C atom. SolutionThink About ItCounting the total number of valence electrons should be relatively simple to do, but it is often done hastily and is therefore a potential source of error in this type of problem. Remember that the number of valence electrons for each element is equal to the group number of that element.Practice Problem Attempt Draw the Lewis structure for NF3.