Quick answer
Nucleophiles fall into three classes by their electron source: negatively charged atoms with a lone pair (strongest), neutral atoms with a lone pair (moderate), and pi bonds (weakest). A full negative charge donates most readily, while loosely held pi electrons are the least reactive.
1. A nucleophile donates a pair of electrons, so its strength tracks how available that pair is.
2. Class 1 nucleophiles are negatively charged with a lone pair, making them the strongest.
3. Class 2 nucleophiles are neutral with a lone pair, so they are only moderate.
4. Class 3 nucleophiles donate a pi bond, and those loosely held electrons make them the weakest.
5. Down a column, polarizability raises nucleophilicity in protic solvent.
6. Summary
Charged lone pair = strongest · neutral lone pair = moderate · pi bond = weakest · polarizability wins down a column (I⁻ > Br⁻ > Cl⁻).
Quiz yourself
Tap a question to reveal the answer — free, no login.
Class 1 — negatively charged with a lone pair, so it is a strong nucleophile.
Ammonia is neutral (Class 2); hydroxide carries a full negative charge (Class 1), which donates its lone pair more readily.
The aromatic pi system — a Class 3 nucleophile whose loosely held pi electrons attack the electrophile.
Iodide — it is larger and more polarizable, so nucleophilicity rises going down the column.
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.