![]() ![]() 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 Example 3 can have two possible structures with the atoms in the same positions. 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: 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.If the Lewis structure must have nonzero formal charges, the arrangement with the smallest nonzero formal charges is preferable.A molecular structure in which all formal charges are zero is preferable to one in which some formal charges are not zero. ![]() A few guidelines involving formal charge can be helpful in deciding which of the possible structures is most likely for a particular molecule or ion: In many cases, following the steps for writing Lewis structures may lead to more than one possible molecular structure-different multiple bond and lone-pair electron placements or different arrangements of atoms, for instance. The arrangement of atoms in a molecule or ion is called its molecular structure. Using Formal Charge to Predict Molecular Structure ![]()
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