For example, the total bond number calculated by procedure (1)-( ) for C2H4O is 7 = A1 + A2 + A3 + A4 + A5 + A6. There are 4 hydrogen atoms (A1 +A2 + A3 + A4 = 1 + 1+ 1 + 1 = 4), then we only need to list the possible combination of the rest bond number, which is 7 -4 = 3. Thus, the two bonding spots between C, C, and O are single bond & double bond (3 = 1 + 2), or double bond & single bond (3 = 2 + 1). (7). Complete the octets on each atom (duet on hydrogen) for all the possible structures, which represent different resonance forms. (8). To check for the preferred resonance form, the formal charges on each atom in all proposed structures should be taken into consideration. A preferable Lewis structure is the one where more atoms have formal charges of zeros and negative formal charges should be on the more electronegative atoms.