Step 1: count valence electrons
Sum up the valence electrons of every atom. For an ion, add one electron per negative charge and subtract one per positive charge. For SO₄²⁻: S(6) + 4·O(6) + 2 (for the charge) = 32 electrons.
Step 2: draw the skeleton
Put the least electronegative atom in the middle (usually). Hydrogens go on the outside — never in the middle. Connect each atom with a single bond. Each bond uses two electrons; subtract them from your total.
Step 3: fill octets with lone pairs
Distribute the remaining electrons as lone pairs, starting with the most electronegative atoms on the outside, until every atom has eight (two for H). If you run out before everyone is full, you'll fix it in step 4.
Step 4: convert lone pairs to multiple bonds
If the central atom doesn't have an octet yet, "borrow" a lone pair from a neighbor and turn it into a second bond. You may need triple bonds (N₂, CO) or multiple double bonds (CO₂, SO₃).
Step 5: formal charge check
Formal charge = valence electrons – nonbonding electrons – (bonding electrons / 2). Compute it for every atom. Sum them up — must equal the overall charge of the molecule/ion. The structure with formal charges closest to zero (and negative charges on the most electronegative atoms) is the major resonance contributor.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.