Build each structure yourself in the same 4 steps as the notes: count → skeleton → lone pairs → complete octets. Hints appear when something's off.
Step 1 · How many valence electrons are there in total?
Step 2 · Which atom goes in the centre?
Electrons placed: 0 / 0Left to place: 0
Each atom shows its electrons, e.g. 6/8 = needs 2 more, 8/8 = full, 10/8 = too many. Each bond counts 2 e⁻ per line; each lone pair counts 2.
Rules used by the checker
Count: add the valence electrons (H 1, C 4, N 5, O 6, S 6, Cl 7). Negative ion: add 1 per − charge. Positive ion: subtract 1 per + charge.
Skeleton: the central atom is the least electronegative one (it usually forms the most bonds). H and F are never central.
Lone pairs: complete the outer atoms first; leftover electrons go on the central atom.
Octets: if the central atom is short, move a lone pair from an outer atom into a double or triple bond. Every atom needs 8 electrons; H needs 2.
Best structure: formal charge = valence − lone-pair electrons − number of bonds. The best structure keeps formal charges as close to 0 as possible. For nitrate, carbonate and nitrite, any resonance form is accepted.