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You should add a double bond if it lowers the formal charges, since the most stable Lewis structure for a molecule is the one with the smallest formal charges on each atom. The number of bonds depends on which specific Lewis structure you're drawing, but in general you should never have more than three bonds between two atoms.
You need to check the formal charges of the atoms. The number of electrons drawn might be correct, but it will not be the most stable lewis structure until you make the formal charges as low as possible. A double bond can make the formal charges lower.
Just to reiterate, after all electrons are accounted for and single bonds are made, formal charges can then be calculated. This will indicate whether a double bond or even a triple bond should be made to lower formal charges. As you do more Lewis structures, patterns with certain elements, such as oxygen wanting to form a double bond while having two lone pairs, will become more apparent and adding double bonds will become easier.
I usually check to see if a double bond is needed through the formal charges or through the pairs. If there is an odd number and there is a single lone electron I usually find a way to make it a double bond. Formal charges are the best way to check if another bond is needed.
Hi ! I kind of see this as a game of seeing what best works. "What best works" is determined by the formal charge. That is to say, if the formal charge of EACH individual element is close to zero then it is MOST stable. However, if the molecule as an entirety has a charge, you would want the charge to be applied to the most electronegative element. In addition to that, you want to be able to add up all the charges and get the overall charge. For example, if the overall charge is -1, you can get an element with a-2 charge and another with a +1 charge, you can add them together and get an overall -1 charge. HOWEVER, that doesn't mean it is most stable. As such, I recommend you refer to these rules when determining the most stable structure.
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