ClO2, chlorine dioxide, can be illustrated in a Lewis structure by writing Cl with O on both sides, separated by double bond dashes. Cl has two dots above with one below, and the Os have two dots on their tops, bottoms and outer edges in the basic version, though variants are possible.
In order to draw a Lewis Structure, first determine the number of valence electrons (electrons represented on the borders of the elements in a Lewis Structure). There are seven electrons for a chlorine ion, in addition to the six valence electrons per oxygen ion. A total of 19 dots are therefore required in the Lewis structure initially. Connect the oxygen ions to make a chloride with chloride in the middle. Both oxygens should have two dots on top, side and bottom, while the chlorine has two dots above and one below.
Next, calculate the formal charge for each ion in the compound by taking the number of valence electrons the ion should have and subtracting the number of valence electrons on the preliminary Lewis structure. Then subtract the number of electrons involved in the bond (two in a single bond) divided by two. Oxygen in this compound has a formal charge of -1 (6-6-2/2). The formal charge of chlorine is calculated to be +2 (7 valence electrons minus 3 in structure, minus 4 electrons involved in bonds/2). To balance the charge in this depiction of a compound, two of the dots on both oxygen in the original Lewis dot structure can be erased and the original single bonds become double bonds from both oxygen to the chlorine.