How it works
Each example is stored as a fixed set of atoms and σ bonds. A contributor is just a different placement of π bonds and lone pairs on that same skeleton, so atoms cannot move. Formal charge is worked out as valence electrons − lone-pair electrons − bonds. When you apply arrows, the tool moves the electron pairs, checks every atom for more than 8 electrons, and compares the result to the listed contributors. “Show me” searches for the smallest set of legal arrows (up to three) that reaches the target. The hybrid is the weighted average of the contributors: bond orders and charges are averaged by weight.
Weights and data
Equivalent contributors are weighted equally. For non-equivalent ones (enolate, amide, phenoxide), the weights are rounded teaching estimates consistent with natural resonance theory and computed charge distributions; they show trends, not exact values. Bond lengths are typical experimental values (X-ray or gas phase): acetate C–O 1.26 Å, carbonate 1.29 Å, formamide C–N 1.35 Å and C=O 1.22 Å, benzene 1.39 Å. Reference single/double lengths: C–C 1.54/1.34, C–O 1.43/1.21, C–N 1.47/1.27 Å. pKa values: phenol ≈ 10, cyclohexanol ≈ 16.