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Out of two given pairs, what are we looking for? I know you want a structure where negative charge is given to the more electronegative atom. What if one structure can result in no charge for any of the atoms. Is that the most preferred?
It depends on the charge of the molecule. If the charge is 0 (neutral) then we are looking for a Lewis Structure where preferably all the formal charges on the atoms are 0. This would be the most stable structure. However, if the molecule is charged, then we want to put the negative charge on the more electronegative atom like you said. An example of this would be ClO4- where one of the oxygen atoms has a formal charge of -1. We put this negative charge on Oxygen rather than Cl because Oxygen is more electronegative than Cl.
If there can be a resonance structure with no formal charges on any atoms, that will be the most stable structure. However, if this is not possible, you would indeed place the charge on the more electronegative atom.
The most important thing is to make sure that you are using the correct number of electrons. If there is multiple ways to do so, then you must look at formal charges and choose the structure that makes the most sense given what we know about electronegativity and bonding.
Structures with a charge are more likely to contribute to a resonance hybrid because the electrons are delocalized in resonance hybrids.
The sum of the formal charges of each atom in a Lewis structure must equal the charge of the overall ion. If the molecule is neutral, you want the sum of the formal charges to be 0. Usually more electronegative atoms are located at the outer edge of molecules. In resonance hybrids, electrons are delocalized which means that they are not associated with a single atom or covalent bond.
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