According to Lewis concept, an acid is

The correct answer is electron pair acceptor.

Key Points

• According to the Lewis concept, an acid is defined as a substance that accepts an electron pair.
• This definition expands the scope of acids beyond those that release protons (H⁺), as per the Brønsted-Lowry concept.
• Examples of Lewis acids include BF₃, AlCl₃, Fe³⁺, and H⁺, which can accept electron pairs from donors.
• Lewis acids play a crucial role in coordination chemistry, catalysis, and organic reactions like electrophilic additions.
• The concept is named after Gilbert N. Lewis, who introduced it in 1923 to broaden the understanding of acid-base behavior.

Additional Information

• Lewis Base:
  • A Lewis base is a substance that donates an electron pair to form a coordinate covalent bond.
  • Examples include NH₃, H₂O, OH⁻, and Cl⁻.
• Comparison with Brønsted-Lowry Theory:
  • While the Brønsted-Lowry theory focuses on proton transfer, the Lewis theory is based on electron pair interactions.
  • For example, NH₃ is both a Brønsted-Lowry base (proton acceptor) and a Lewis base (electron pair donor).
• Applications of Lewis Acids:
  • Lewis acids like AlCl₃ are widely used as catalysts in Friedel-Crafts reactions.
  • They are also employed in the synthesis of polymers and coordination compounds.
• Acid-Base Adducts:
  • A Lewis acid and a Lewis base can combine to form an acid-base adduct via a coordinate covalent bond.
  • For instance, BF₃ (acid) forms an adduct with NH₃ (base) to create BF₃·NH₃.