A double-headed arrow between Lewis structures indicates that they are resonance forms. The actual electronic structure of the molecule (the average of the resonance forms) is called a resonance hybrid of the individual resonance forms. We call the individual Lewis structures resonance forms. Is the average of a double bond and a single bond. The actual distribution of electrons in each of the nitrogen-oxygen bonds in NO 2 − Instead, we use the concept of resonance: if two or more Lewis structures with the same arrangement of atoms can be written for a molecule or ion, the actual distribution of electrons is an average of that shown by the various Lewis structures. In which nitrogen has an octet and both bonds are equivalent. It is not possible to write a single Lewis structure for NO 2 − Have the same strength and length, and are identical in all other properties. Experiments show, however, that both N–O bonds in NO 2 − 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. The electrons involved in the N–O double bond, however, are in different positions: You may have noticed that the nitrite anion in can have two possible structures with the atoms in the same positions. 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: CNS –, NCS –, 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.
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