Nucleophiles as electron pair donors

Steps:

• Recall that a nucleophile is a species that donates electrons to form a new bond.
• Examine the definition: nucleophiles provide a lone pair or pi electrons to an electrophile.
• Evaluate choices against this core property.
• Confirm B matches the universal trait, while others do not.

Why B is correct:

• By definition, every nucleophile donates a lone pair of electrons to an electron-deficient center, as in SN2 reactions where Nu: attacks C+.

Why the others are wrong:

• A: Nucleophiles attack electrophilic centers like carbocations, not necessarily double bonds (e.g., in SN1 reactions).
• C: Nucleophiles can be molecules like NH3 or OH-, not just single atoms.
• D: Nucleophiles can be neutral (e.g., H2O) or positively charged (e.g., RNH3+), not always negative.

Final answer: B