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To make sure a lewis structure is at its lowest energy you should calculate the formal charge for each atom. The lewis structure with the most formal charges closest to zero is the most stable and has the lowest energy.
To find the lowest energy Lewis structure, you will need to examine formal charge. Lewis Structures with the lowest energy will have formal charges that cancel out to be zero or match the charge of the molecule if provided.
To show a lewis structure at its lowest energy, you'll need to calculate the formal charges and make sure that it's equal to zero or as close as you can. You'll use the formula FC=V-(B+d) for each of the elements. V is valence electrons, B is bonds, and d is dots :)
After you've worked to ensure that the atoms in the molecule you're drawing satisfy the octet rule (unless they're exceptions), you should calculate the formal charge of each atom in the molecule by subtracting the number of bonds it's formed and the number of unpaired electrons around it from the number of valence electrons that element typically has. You want the formal charges to be zero ideally (or as close to zero as possible if need be) to draw the lowest energy Lewis structure.
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