Describe ammonium-ion structure and formation
Show how ammonia's lone pair accepts H⁺ and describe the resulting tetrahedral NH₄⁺ ion.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Explain the stated molecular, acid–base and atmospheric reaction pathways; balance equations and distinguish direct reactions from catalytic cycles. Written practice is self-checked
Before you begin
- Use covalent bond order, lone pairs, Brønsted–Lowry proton transfer, oxidation numbers and balanced equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many N–H bonds are present in NH₄⁺?
What you will learn
- Explain proton acceptance by the ammonia lone pair
- Draw or describe four N–H bonds
- State the tetrahedral shape and positive charge
A lone pair forms an N–H bond
NH₃ has three N–H covalent bonds and one lone pair on nitrogen. When ammonia accepts H⁺, the proton brings no bonding electrons; nitrogen donates its lone pair to form the fourth N–H bond.
The acid–base reaction can be written NH₃ + H⁺ → NH₄⁺. In water the proton donor is represented explicitly, for example NH₃ + H₃O⁺ → NH₄⁺ + H₂O.
Ammonium is tetrahedral
NH₄⁺ has four bonding pairs around nitrogen and no lone pair on the central atom. The four N–H bonds are equivalent in the ion and point toward the corners of a tetrahedron.
The ion has overall charge +1. Its eight valence electrons are shared across the four N–H bonds; the central nitrogen has a full octet. A coordinate origin for bond formation does not make one N–H bond permanently different.
Pause and explain
What shape does NH₄⁺ have?
Worked example
Describe the formation and shape of NH₄⁺ from ammonia.
Show the worked solution
- Ammonia has a lone pair on nitrogen.
- It accepts H⁺ by donating the lone pair to form a fourth N–H bond.
- NH₄⁺ has four equivalent bonding pairs, a tetrahedral shape and charge +1.
Answer NH₃ donates its lone pair to H⁺ to form NH₄⁺. The ion is tetrahedral, with four equivalent N–H bonds and an overall +1 charge.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Show the acid–base formation of ammonium from ammonia and a proton.
Give me a hint
Use the nitrogen lone pair to make a fourth N–H bond.
Compare my reasoning
- NH₃ is the proton acceptor.
- A lone pair is donated to H⁺.
- The product is NH₄⁺.
NH₃ + H⁺ → NH₄⁺.
Look for these in your work
- Reactants identified
- Lone-pair donation included
- Charge conserved
Describe the bonding and shape in NH₄⁺.
Give me a hint
Count bonding pairs and lone pairs around nitrogen after protonation.
Compare my reasoning
- There are four N–H bonding pairs.
- There is no lone pair on the nitrogen in NH₄⁺.
- Four electron-pair directions give a tetrahedral ion.
NH₄⁺ is tetrahedral with four equivalent N–H bonds and no lone pair on nitrogen.
Look for these in your work
- Four bonds stated
- Lone-pair count correct
- Shape named
A diagram shows three N–H bonds and one lone pair for NH₄⁺. Explain the error and state the charge.
Give me a hint
Compare NH₄⁺ with NH₃ after H⁺ has been accepted.
Compare my reasoning
- The lone pair has formed the fourth N–H bond.
- NH₄⁺ has four bonding pairs and no nitrogen lone pair.
- The ion's overall charge is +1.
The diagram shows NH₃, not NH₄⁺. Ammonium has four N–H bonds, no lone pair on nitrogen and charge +1.
Look for these in your work
- Error located
- Correct structure stated
- Charge stated
Common mistakes
- Showing a lone pair on nitrogen in NH₄⁺
- Calling NH₄⁺ trigonal pyramidal
- Treating one N–H bond as permanently distinct after formation
What can you explain now?
How many N–H bonds are present in NH₄⁺?
Compare with the explanation
Four
The incoming proton forms the fourth N–H bond.
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Make it stick
What changes at nitrogen when NH₃ accepts H⁺ to form NH₄⁺?
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