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Unless specifically instructed to do so, I don't think you need to write the formal charge for each Lewis structure you write. When you are trying to determine which Lewis structure for the particular molecule is most stable and ideal, it may be useful to write the formal charges in order to compare different structures to determine the right one.
I wouldn’t necessarily write them out unless he asks for it on an exam. If you are doing it to check though, you can always do a short cut way, the equation would be number of valence electrons - sticks & dots. So the number of electrons of one element, minus the sticks and dots drawn around it. Works every time.
It is not necessary but it is a good way to figure out if your lewis structure can be improved or not. I would do it just in case. If the question asks specifically for you to add the formal charges, then do so, but if not then it is your choice.
Roni Touboul wrote:If we disregard the formal charges when drawing our lewis structure will it be wrong?
It could possibly be wrong if there is a better lewis structure that will give the most atoms a formal charge of zero, or if an atom that is not as electronegative as another has a negative formal charge. It would be best to calculate the formal charges of each atom to see if you have made the best lewis structure, and if it can be improved, change the structure.
usually you do not have to write the formal charge of each atom for points unless instructed to do so, but it will help you find the correct lewis structure by demonstrating which structure will have the lowest energy.
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