Center Atom in Structure:
The atom with the highest ionization energy OR there is only ONE of it.
OR
LEAST electronegative atom
A line means TWO electrons in valence.

Step when drawing Lewis Structure:
Calculate the Sum of the valence electron of all of the atoms involved and write it down.
But the atoms together so they achieve octet, but the total amount of electrons has to be the SUM of the valence electrons from the first step.

Resonance:
If a molecule like NO3 can be written in different equally valid ways.
This means that the actual structure is more of a COMBINATION of the 2+ structures.
So a BLEND is better for describing, which would be the resonance structure.
The way to show a blend is to show a “double-headed” arrow between the two.
ALSO:
This means that every bond is a hybrid between covalent and ionic, so technically you can draw Cl2 as either ionic Lewis OR covalent.
Diatomic elements are 99.99% covalent, but also 0.01% ionic

Formal Charge:
The LOWER (closest to 0) the number the more likely it is to be the ACTUAL Lewis structure.
This applies to PERFECT covalent bonds, Ex. Diatomic stuff like H2 and O2, but can also be used for others.
To find the formal charge subtract the valence electrons of the atom, so 6 for O and the sum of the electron dots and the bond lines.

Expanded Valence Shell:
If a central atom has an empty d-orbital, so it can carry 10 or 12 electrons.
You need to have n >= 3 AND something with d-block
REASONS:

  1. More atoms might just attach than they should
  2. Some single bonds might be swapped out with higher-order bonds

Smaller Valence Shell:
Beryllium and Boron only need 6 valence electrons.