Rules List for Drawing Lewis Structures

Version 8

1. Calculate the total number of valence electrons for the molecule. Sum up the individual valence electrons for all of the atoms that make up the molecule. If it's a polyatomic ion, add or subtract the charge from the total number based on whether it's an anion or a cation.
2. Draw the skeleton structure for the molecule. The central atom is the least electronegative atom. The exception is hydrogen and it is never the central atom. Additionally, oxygen is never the central atom for oxyacids.
3. Make bonds between the surrounding atoms and the central atom. Each bond counts as two valence electrons.
4. Take the remaining valence electrons and distribute as many as needed to the surrounding atoms until their octets are filled. Remember that we modify the Octet Rule for hydrogen and call it the Duet or Modified Octet Rule. Therefore, 2 valence electrons satisfies the Duet Rule for hydrogen.
5. Distribute the remaining valence electrons to the central atom to fulfill its octet. Upon distribution of all of the valence electrons, if the central atom does not satisfy the Octet Rule, "borrow" lone pair(s) of electrons from one or more surrounding atoms and create multiple bonds until the Octet Rule is satisfied. If needed and if possible, draw multiple structures to indicate resonance. If there are remaining pairs of electrons after satisfying the Octet Rule and the central atom is in row 3 or greater in the Periodic Table, distribute them about the central atom (hypervalency). Remember that beryllium and boron form covalent compounds that have 4 and 6 electrons in their respective full valence shells (electron deficiency).
6. If asked to choose the best Lewis structure when given a choice, choose the one which has the smallest number of formal charges.