Why does this make sense? 10. Why does this make sense? Why does this make sense? 8. Why does this make sense?

260 of valence-shell electrons is therefore 7 + 3 x 6 + 1 = 26. After putting in the single bonds and distributing the unshared electron pairs, we have For oxyanions—BrO 3 – , SO 4 2– , NO 3 – , CO 3 2– , and so forth—the oxygen atoms surround the central nonmetal atoms. Notice here and elsewhere that the Lewis structures of ions are written in brackets with the charge shown outside the bracket at the upper right. GO TO QUESTION 9 Æ EXCEPTIONS TO THE OCTET RULE The octet rule is so simple and useful in introducing the basic concepts of bonding that you might assume that it is always obeyed. It is limited in dealing with ionic compounds of the transition metals. The octet rule also fails in many situations involving covalent bonding. These exceptions to the octet rule are of several main types, including: 1. Molecules with an odd number of electrons 2. Molecules in which an atom has more than an octet Odd Number of Electrons In the vast majority of molecules, the number of electrons is even, and complete pairing of electrons occurs. In a few molecules, such as ClO 2 , NO, and NO 2 , however, the number of electrons is odd. Complete pairing of these electrons is impossible, and an octet around each atom cannot be achieved. GO TO QUESTION 10 Æ For example, NO contains 5 + 6 = 11 valence electrons. :N O: .. . 9. Why does this make sense? 10. Why does this make sense? : O Br O : : O : .. .. .. .. .. .. .. SAMPLE EXERCISE 2 Draw the Lewis structure for HCN. Solution Hydrogen has one valence-shell electron, carbon group 4A has four, and nitrogen group 5A has five. The total number of valence-shell electrons is therefore 1 + 4 + 5 = 10. Again, there are various ways we might choose to arrange the atoms. Because hydrogen can accommodate only one electron pair, it always has only one single bond associated with it in any compound. GO TO QUESTION 7 Æ C—H—N, therefore, is an impossible arrangement. The remaining two possibilities are H—C—N and H—N—C. The first is the arrangement found experimentally. You might have guessed this to be the atomic arrangement because the formula is written with the atoms in this order. Thus we begin with a skeleton structure that shows single bonds between hydrogen, carbon, and nitrogen: These two bonds account for four electrons. If we then place the remaining six electrons around N to give it an octet, we do not achieve an octet on C: We therefore try a double bond between C and N, using an unshared pair of electrons that we had placed on N. Again, there are fewer than eight electrons on C, so we try a triple bond. This structure gives an octet around both C and N: SAMPLE EXERCISE 3 Draw the Lewis structure for the BrO 3 – ion. Solution Bromine group 7A has seven valence electrons, and oxygen group 6A has six. An extra electron is added to account for the ion having a 1– charge. GO TO QUESTION 8 Æ The total number H C N H C N: .. .. H C N : .. .. H C N : 7. Why does this make sense? 8. Why does this make sense? 261 More than an Octet The largest class of exceptions consists of molecules or ions in which there are more than eight electrons in the valence shell of an atom. When we draw the Lewis structure for PCl 5 , for example, we are forced to “expand” the valence shell and place 10 electrons around the central phosphorus atom. Other examples of molecules and ions with “expanded” valence shells are SF 4 , AsF 6 – , and ICl 4 – . The corresponding molecules with a second-period atom, such as NCl 5 and OF 4 , do not exist. Let’s take a look at why expanded valence shells are observed only for elements in period 3 and beyond in the periodic table. Elements of the second period have only the 2s and 2p valence orbitals available for bonding. GO TO QUESTION 11 Æ Because these orbitals can hold a maximum of eight electrons, we never find more than an octet of electrons around elements from the second period. Elements from the third period and beyond, however, have ns, np and unfilled nd orbitals that can be used in bonding. For example, the orbital diagram for the valence shell of a phosphorus atom is as follows: Although third-period elements such as phosphorus often satisfy the octet rule, as in PCl 3 , they also often exceed an octet by seeming to use their empty d orbitals to accommodate additional electrons. GO TO QUESTION 12 Æ 11. Why does this make sense? 12. Why does this make sense?