It is not possible to write a single Lewis structure for NO 2 − NO 2 − in which nitrogen has an octet and both bonds are equivalent. Experiments show, however, that both N–O bonds in NO 2 − NO 2 − have the same strength and length, and are identical in all other properties. A double bond between two atoms is shorter (and stronger) than a single bond between the same two atoms. If nitrite ions do indeed contain a single and a double bond, we would expect for the two bond lengths to be different. Notice that the more likely structure for the nitrite anion in Example 7.8 may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O bonds: Also, it places the least electronegative atom in the center, and the negative charge on the more electronegative element (Guideline 4). However, the first arrangement of atoms is preferred because it has the lowest number of atoms with nonzero formal charges (Guideline 2). Note that the sum of the formal charges in each case is equal to the charge of the ion (–1). Possible Lewis structures and the formal charges for each of the three possible structures for the thiocyanate ion are shown here: The formal charges present in each of these molecular structures can help us pick the most likely arrangement of atoms. We can draw three possibilities for the structure: carbon in the center and double bonds, carbon in the center with a single and triple bond, and oxygen in the center with double bonds:Ĭomparing the three formal charges, we can definitively identify the structure on the left as preferable because it has only formal charges of zero (Guideline 1).Īs another example, the thiocyanate ion, an ion formed from a carbon atom, a nitrogen atom, and a sulfur atom, could have three different molecular structures: NCS –, CNS –, or CSN –. We know from our previous discussion that the less electronegative atom typically occupies the central position, but formal charges allow us to understand why this occurs. To see how these guidelines apply, let us consider some possible structures for carbon dioxide, CO 2. When we must choose among several Lewis structures with similar distributions of formal charges, the structure with the negative formal charges on the more electronegative atoms is preferable.Lewis structures are preferable when adjacent formal charges are zero or of the opposite sign.
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