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When drawing structures, how do I know when to add a double bond instead of a lone pair around an atom? For example, in H2CO, how do I know to add a double bond between C=O instead of giving O 3 lone pairs?
Just kidding, remember the order of priorities. Make sure you have the correct number of electrons, then pay attention to the octet rule. After that, double bonds can be created to ensure you have the lowest energy lewis structure meaning fewest formal charges, and with any negative formal charges on more electronegative atoms if possible.
You would add a double bond instead of a lone pair around an atom when adding the bond would bring the formal charge of the atom closer to 0 or when you want to complete the octet of one of the bonded atoms but cannot add a lone pair because this would lead to a Lewis structure that has more electrons than the molecule. In H2CO, giving O 3 lone pairs would complete its octet, but you will have already used up the twelve electrons from the molecule in the drawing (3 bonds=6 electrons + 6 electrons from the 3 lone pairs), so you would not be able to add a lone pair to C to complete its octet. Therefore, you would take a lone pair off of O and make it a bond to C. This would complete the octet for both C and O without adding any extra electrons.
If you have an odd number of electrons like when oxygen makes one bond and then has 5 extra electrons there is one extra electron to create 3 lone pairs of electron so you can create a double bond to have 4 electrons which is an even number for the lone pairs
Regarding double bonds in molecules with resonance, he said to compute the formal charges for each atom to find the lowest state possible in each of the possible resonance structures. So just use the equation, FC=V-(L+S/2).
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