So first of all you take your periodic table and look at the valence electrons of Fluorine...it is 1. You don't need to look the valence electron of Cobalt because the "I" after Cobalt tells its valence electrons (it is a transition metal that is why). Now that Cobalt has a +1 charge and Fluorine has a -1 charge, it is automatically nuclear (means charge are cancelled out to 0). Therfore the answer is CoF.
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