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To determine the kind of bond, you must first determine the amount of valence electrons available for use in bonding. Once you have determined that, you need to find a bonding configuration that utilizes all available electrons. If necessary, you need to use the formal charge (LC=V-(L+B/2)) to determine which atoms to attach additional bonds to (Remember, a formal charge of zero or closest to zero is preferable).
You would usually add a double bond when the central atom does not have a full octet but you have to keep the exceptions in mind. In order to know how many double bonds to put you need to calculate the formal charge for each atom. The formal charges all need to add up to the overall charge of the molecule. For example if the molecule has an overall charge of 2- then you would add double bonds until the formal charges of all the atoms add up to -2.
When dealing with a limited number of valence electrons, you turn to double bonds to add more flexibility when wanting to achieve octets. For example, when given the compound ONF, first we find its valence electrons. O has 6, N has 5, F has 7, all of which total to 18. Now you know your limited number of electrons you will be dealing with is 18. Our goal is to establish an octet for O, N, and F. To do so we add one bond between O and N and a double bond between N and F. The rest of the elements are filled with lone pair electrons. Adding bonds help add more flexibility to your compound because as we learned, the electrons become shared between 2 elements to achieve an octet. Hope this helps.
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