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One rule of thumb is to calculate the number of valence electrons in a molecule. Fulfill the octet rule for every atom in the Lewis structure. If you still have some electrons left over, it is highly likely that there is an atom with an expanded octet. Usually elements in the p-block, in period 3 or higher can accommodate expanded octets. Thus, add lone pairs of electrons to those atoms within a molecule and as always, calculate the formal charge.
Most of the time when you have a molecule that can form an expanded octet, such as Xenon, you know that it has eight valance electrons. In bonding, you're trying to reduce formal charge, so you can form double bonds between Xenon and other atoms to reduce the formal charge as much as possible.
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