Skip to notes
wise otter.Chemistry
Contents
9476 / 2027
Theories of Acids and Bases

Lesson 3 of 3

Lewis: follow an electron pair

Explain acid-base reactions without requiring water or proton transfer.

A-Level 9476 (2026-2027)

  • I can understand and apply the Lewis theory.Syllabus

    Syllabus 9476, 4(c). Understand and apply the Lewis theory.

The Lewis base supplies an electron pair

A Lewis acid accepts a pair; a Lewis base donates one.

A Lewis acid accepts an electron pair to form a covalent bond. A Lewis base donates a pair. This definition includes proton-transfer reactions because H+ can accept a lone pair, but it also explains reactions with no proton transfer.

Ammonia donates to boron trifluoride

A lone pair on the nitrogen of NH3 is directed towards the electron-deficient boron of BF3. The arrow starts at the electron pair and ends at boron. No hydrogen ion is transferred, and no water is required.

NH3 is the electron-pair donor; BF3 is the acceptor. This is a non-aqueous acid-base reaction.

In BF3, boron has only three shared pairs in its simple bonding description and can accept another pair. Nitrogen in NH3 has a lone pair. Donation forms an N-B bond in the neutral adduct H3N→BF3. Both centres then have four bonding regions. The arrow records the origin of the bonding pair; the final bond is covalent.

Apply the definition to the actual event
Reaction featureAcidBase
H+ accepts a lone pair from water.H+ is a Lewis acid.H2O is a Lewis base.
NH3 transfers a proton from water to itself.H2O is the Bronsted-Lowry donor.NH3 is the proton acceptor and uses its lone pair.
BF3 accepts the nitrogen pair from NH3.BF3 is a Lewis acid.NH3 is a Lewis base.
A metal ion binds a water ligand through oxygen.The metal ion accepts the pair.The water ligand donates a lone pair.

Watch out for this

A student says: "NH3 donates a lone pair to BF3, so NH3 is a reducing agent and BF3 is reduced." What is wrong with this?

Show the answer

An electron pair becomes shared in a new bond. Do not automatically call its donor a reducing agent. Acid-base classification follows proton or electron-pair donation; oxidation and reduction require a change in oxidation state.

Check your understanding

Why does BF3 + NH3 -> F3B-NH3 need the Lewis definition of acids and bases?

  1. No proton moves; BF3 accepts a lone pair from NH3 to form a B-N bond.
  2. BF3 releases F- ions in water, so it acts as an Arrhenius acid here.
  3. NH3 gives a proton to BF3, so BF3 acts as the Bronsted base here.
Show the answer

No proton moves; BF3 accepts a lone pair from NH3 to form a B-N bond.

Bronsted-Lowry needs a proton transfer, and there is none here. The Lewis definition covers electron-pair acceptors (BF3) and donors (NH3).