We have to remember that the formula of <u>sulfur dioxide</u> is . So, the "S" atom is the central one and the "O" atoms are in the left and right of S. Additionally, "S" and "O" atom has <u>6 valence electrons each</u>. With this in mind, we can start with a structure in which the left oxygen has 2 lone pairs and a double bond, the S atom can have a double bond a single bond and a lone pair and the right oxygen with 3 lone pairs with a single bond <u>(structure A)</u>. Now, if we move the negative charge in the right (resonance) to the middle and the double bond to the left oxygen we will obtain <u>structure B</u>. Finally, we can move the negative charge in the left to the middle and we will obtain structure C. (See figure 1).