IB Chemistry HLTopic 6 — Electron Pair SharingPaper 1 & 2Core idea~9 min read
Lewis Acids and Bases
You already know acids as proton donors. Lewis looked at the same reactions and noticed the proton was not really the point — the electron pair was. That one change of viewpoint pulls acids, bases, nucleophiles and electrophiles into a single idea.
📚 What you need to know
A Lewis acid accepts a lone pair of electrons.
A Lewis base donates a lone pair of electrons.
The two join with a coordinate bond, where both electrons came from the base.
Lewis theory is broader than Brønsted–Lowry: it covers reactions with no proton anywhere.
Every Brønsted–Lowry base is also a Lewis base — you need a lone pair to grab a proton.
Nucleophile = Lewis base. Electrophile = Lewis acid. Same idea, organic vocabulary.
Water is amphoteric: it can behave as either, depending on what it meets.
The definition, in plain language
Forget protons for a moment. Ask instead: in this reaction, who supplies the pair of electrons and who takes it?
The one that supplies is the base. The one that takes is the acid. That is the whole of Lewis theory.
A Lewis acid–base reaction
A+ + :B– → A←:B
That arrow in the product is not a reaction arrow. It is a coordinate bond, drawn pointing from the atom that gave both electrons towards the atom that received them.
If you can spot a lone pair on one side and an empty slot on the other, you have found a Lewis acid–base pair. It really is that mechanical.
How this compares with Brønsted–Lowry
Brønsted–Lowry is not wrong; it is just narrower. It only recognises reactions where an H+ moves from one place to another.
Lewis says: proton transfer is one example of a lone pair being donated, but there are plenty of others. A copper ion pulling water molecules around itself involves no proton at all, yet it is exactly the same kind of electron-pair sharing.
Notice the asymmetry. The base lists barely change, but the acid list grows to include metal ions and electron-deficient molecules.
Wearing two hats at once
Species like OH– and NH3 are Brønsted–Lowry bases and Lewis bases. That is not a coincidence or a trick question — it is unavoidable.
To accept a proton you must have somewhere to put it, and the only thing a bare H+ can bond to is a pair of electrons. So the very act of being a Brønsted–Lowry base is the act of donating a lone pair.
The colour on the fourth hydrogen only records where its electrons came from. Chemically it behaves exactly like the other three.
Nucleophiles and electrophiles by another name
This is the bit worth pinning to your wall. Organic chemists and physical chemists use different words for the same two jobs.
Job
Physical chemistry name
Organic chemistry name
Examples
Donates a lone pair
Lewis base
Nucleophile
OH–, NH3, CN–, H2O
Accepts a lone pair
Lewis acid
Electrophile
H+, BF3, Cu2+, R+
One sentence to remember. Every nucleophile is a Lewis base and every electrophile is a Lewis acid. If a question asks you to identify the Lewis base in an organic mechanism, look for the nucleophile.
Water plays both sides
Water is amphoteric, which means it can act as an acid or a base depending on what it is up against.
Put water next to a strong Lewis acid such as Cu2+ and the oxygen lone pair is donated. Water is the base.
Put water next to a good Lewis base such as HCOO– and the δ+ hydrogen accepts a lone pair. Water is the acid.
So there is no such thing as “water is a base”. It depends entirely on the partner, which is why exam questions always give you a specific reaction to look at.
🧩 How to identify the Lewis acid and base in any equation
Find the new bond that appears in the product. Ignore everything else.
Ask where its two electrons came from. Look for a lone pair in a reactant that has disappeared.
That reactant is the Lewis base.
The species it bonded to is the Lewis acid.
Sanity check the acid: it should be positive, electron-deficient, or have a δ+ atom.
If a proton moved, you can also name them Brønsted–Lowry. If not, only Lewis works.
Worked examples
WORKED EXAMPLE
In the reaction HCOO– + H2O → HCOOH + OH–, identify the Lewis acid and the Lewis base, and justify each choice.
Find the new bond
An O–H bond has appeared on the methanoate, turning it into methanoic acid.
Where did the electrons come from?
From a lone pair on the methanoate oxygen. So methanoate is the donor.
Who accepted?
A hydrogen atom of water, which carries a δ+ and takes the pair as it separates from OH–.
Lewis base = HCOO– | Lewis acid = H2Oa proton did move, so you could equally call this a Brønsted–Lowry reaction
WORKED EXAMPLE
Explain why Cu2+ + 6H2O → [Cu(H2O)6]2+ can be described using Lewis theory but not using Brønsted–Lowry theory.
Check for a moving proton
Look at both sides. No H+ is transferred anywhere. Every water molecule is still intact.
So Brønsted–Lowry cannot describe it
That theory is defined entirely in terms of proton donors and acceptors.
Now check for electron pairs
Each water donates an oxygen lone pair into an empty orbital on the copper ion, forming six coordinate bonds.
Lewis acid = Cu2+ | Lewis base = H2Othis is the clearest example of why Lewis theory had to be invented
💡 Exam tip
Learn the two definitions word for word: acid = lone pair acceptor, base = lone pair donor. They are easy marks.
When you justify an answer, say which atom the lone pair came from, not just which molecule.
If a question asks why Lewis theory is “broader”, give a specific example with no proton transfer.
Name the bond formed as a coordinate bond or dative covalent bond. It is often a mark on its own.
Do not say “water is amphoteric” without adding what it does in the reaction in front of you.
For organic mechanisms, translate first: nucleophile → Lewis base, electrophile → Lewis acid.
⚠️ Common mix-up
Getting acid and base the wrong way round. The acid takes the electrons. It feels backwards because acids usually give something away.
Thinking Lewis acids must contain hydrogen. BF3 and Cu2+ have no hydrogen at all.
Assuming Brønsted–Lowry is “wrong”. It is a subset, not an error. Both descriptions can be correct at once.
Saying the base “donates electrons”. It donates a pair, and it does not lose them — they are now shared.
Drawing the coordinate arrow backwards. It points from the donor towards the acceptor.
Treating a coordinate bond as special afterwards. Once made, it is an ordinary covalent bond.
Up next: Lewis Acid and Base Reactions — putting the theory to work on BF3 and ammonia, and seeing what an empty orbital actually looks like.
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