Oxoanion Lewis Structures CHEM 1212 F12
Of the polyatomic anions you memorized in CHEM 1211, many are composed of a nonmetal and a number of oxygen atoms. The charge of an oxoanion with a nonmetal in period 3 or greater is equal to the charge expected from that nonmetal in an ionic compound (the number of electrons needed to make the nonmetal isoelectronic with the nearest noble gas). You need to be able to draw the Lewis structure of these oxoanions quickly and correctly in this course, without having to resort to using formal charges to determine the best structure. Table I: Example oxoanions and their Lewis structure and formal charges shown ClO-­‐ ClO2-­‐ SO32-­‐ ClO3-­‐ ClO4-­‐ PO43-­‐ Inspect Table I to answer the following questions 1) Is there a relationship between the charge and the number of single bonds in each Lewis structure? If so, what is it? 2) Is there a relationship between the charge and the number of double bonds in each Lewis structure? If so, what is it? 3) How many electron domains are around each oxygen atom in each Lewis structure? 4) How many electron domains are around each central atom in each Lewis structure? 5) Which of the following statements best describes how to draw a Lewis structure for an oxoanion composed of a nonmetal from period 3 & beyond? • “You draw the Lewis structure by placing the nonmetal in the center and adding the number of oxygen atoms in the formula around that atom. Add double bonds to the same number of oxygen atoms as the charge and the rest are single bonds. Add lone pairs to the central atom until there are four electron domains around it. Make sure each oxygen has a full octet by adding lone pairs.” • “You draw the Lewis structure by placing the nonmetal in the center and adding the number of oxygen atoms in the formula around that atom. The charge tells you the number of single bonded oxygen atoms, with the rest being double bonded. Add lone pairs to the central atom until there are four electron domains around it. Make sure each oxygen has a full octet by adding lone pairs.” Oxoanion Lewis Structures CHEM 1212 F12
6) Which of these is the proper Lewis structure for the bromate anion? 7) Determine the three errors in the structure to the right for XO42-­‐. Draw the proper Lewis structure. 8) Draw proper Lewis structures for the periodate, hypobromite, sulfate and phosphite ions. The corresponding acid to an oxoanion can be drawn just as easily. Simply add a proton (H+) to each of the single bonded oxygen atoms to cancel the charge. Of course, H can only single bond, so each H+ added looks like a single bonded H (-­‐H). For example sulfurous acid is 9) For the ions listed in problem 7, write the corresponding acid’s name. 10) For the ions listed in problem 7, draw the corresponding acid’s Lewis structure. For carbonate, nitrate and nitrite, the above rules don’t follow. The reason is the central atom in these three ions cannot expand their octet and can only have 8 electrons around them. These three ions are common enough in chemistry that you should know them as well. Here they are: Carbonate CO32-­‐ Nitrate NO3-­‐ Nitrite NO2-­‐