ChemistryICSE Class 10 · Chemical Bonding

Coordinate Bonding: Formation of H₃O⁺ and NH₄⁺ Ions (ICSE Class 10)

Updated 17 October 2026 · Fact-checked

A coordinate (dative) covalent bond is formed when one atom donates a lone pair of electrons, and another atom or ion that is short of electrons accepts it, so the shared pair comes from one atom only. A water molecule donates a lone pair of its oxygen atom to a hydrogen ion to form the hydronium ion, H₃O⁺. An ammonia molecule donates the lone pair of nitrogen to a hydrogen ion to form the ammonium ion, NH₄⁺.

Understand Coordinate Bonding: Hydronium and Ammonium Ions

1. Definition and idea. In an ordinary covalent bond each atom contributes one electron to the shared pair. In a coordinate bond both electrons of the shared pair come from the same atom, the donor, which must have a lone pair (two electrons not used in bonding). The acceptor has an empty space in its outer shell, such as a hydrogen ion H⁺ with no electron at all. Once formed, a coordinate bond behaves like any other covalent bond. It is drawn as an arrow from donor to acceptor: $A \to B$ (donor A, acceptor B).

2. The lone pair effect. The oxygen atom of water (2,6) uses two electrons to bond with two hydrogen atoms, so it has two lone pairs. The nitrogen atom of ammonia (2,5) uses three electrons for three N-H bonds, so it has one lone pair. These lone pairs can be donated.

3. Hydronium ion, H₃O⁺. When an acid dissolves in water, or when water molecules interact with each other, a water molecule donates one lone pair of oxygen to a proton (H⁺):
$H_{2} O + H^{+} \to H_{3} O^{+}$

Oxygen now has three bond pairs (two ordinary bonds and one coordinate bond) and one lone pair; the ion carries a positive charge.
All three O-H bonds are equivalent after formation.
It is the ion that makes an acid solution acidic. Acids produce hydronium ions: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ .

4. Ammonium ion, NH₄⁺. Ammonia donates the lone pair of nitrogen to a hydrogen ion:
$N H_{3} + H^{+} \to N H_{4}^{+}$

Nitrogen has four bond pairs and no lone pair, and the ion carries a positive charge.
Ammonia gas and hydrogen chloride give ammonium chloride, $N H_{3} + H C l \to N H_{4} C l$ . Inside NH₄Cl there are covalent and coordinate bonds in the NH₄⁺ ion and an electrovalent bond between NH₄⁺ and Cl⁻.
In water, ammonia takes a proton from water: $N H_{3} + H_{2} O ⇌ N H_{4}^{+} + O H^{-}$ , which is why the solution is alkaline.

5. The hydroxide ion, OH⁻. Water itself ionises very slightly: $H_{2} O + H_{2} O ⇌ H_{3} O^{+} + O H^{-}$ . One water molecule donates a lone pair to a proton lost by another water molecule; the molecule that loses H⁺ becomes OH⁻.

6. Properties of coordinate compounds. They have the same general properties as covalent compounds: low melting and boiling points and poor conduction in pure form. A compound with a coordinate bond need not be covalent overall, as the NH₄Cl example shows.

Coordinate Bonding: Hydronium and Ammonium Ions: formulas and key facts

Hydronium ion: $H_{2} O + H^{+} \to H_{3} O^{+}$
Ammonium ion: $N H_{3} + H^{+} \to N H_{4}^{+}$
Ionisation of water: $H_{2} O + H_{2} O ⇌ H_{3} O^{+} + O H^{-}$
Ammonia in water: $N H_{3} + H_{2} O ⇌ N H_{4}^{+} + O H^{-}$
Donor and acceptor: Donor: lone pair (O in H₂O, N in NH₃)
Acceptor: H⁺ with no electron
Arrow drawn from donor to acceptor

How to solve Coordinate Bonding: Hydronium and Ammonium Ions questions

Follow this order for a coordinate bond question.

1Draw the donor molecule with its lone pair (H₂O has two on O; NH₃ has one on N).
2Draw the acceptor as H⁺ (no electron).
3Show the lone pair being shared with H⁺, and draw an arrow from donor to acceptor.
4Show the final ion in square brackets with a plus sign outside the bracket.
5State which atom is the donor, the acceptor and the number of bond pairs and lone pairs in the new ion.

Coordinate Bonding: Hydronium and Ammonium Ions in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a few drops of ammonia solution turn red litmus blue

Suppose a student adds a drop of aqueous ammonia to some distilled water coloured with red litmus. The colour turns blue.

Use the idea of a lone pair to explain why the solution becomes alkaline.

Try it first, then show the solutionSolution

The nitrogen atom of NH₃ has a lone pair and takes a proton from a water molecule, forming the ammonium ion by a coordinate bond.
The water molecule that lost the proton becomes OH⁻, and the OH⁻ ions make the solution alkaline.

Answer: $N H_{3} + H_{2} O ⇌ N H_{4}^{+} + O H^{-}$ : the OH⁻ ions turn red litmus blue.

Memory hook: Lone pair on N catches H⁺, leaves OH⁻.

Tricks to remember Coordinate Bonding: Hydronium and Ammonium Ions

Short memory aids. Say each one aloud once, then use it on a question the same day.

One-sided sharing
Co-ordinate: Co (together) but one side gives both electrons; draw the arrow from the giver.
Use it when: Remembering the donor and acceptor roles.
Lone pairs: O has 2, N has 1
H₂O: O has two lone pairs. NH₃: N has one lone pair. Only one pair is donated at a time.
Use it when: Counting electrons correctly in H₃O⁺ and NH₄⁺.
Positive ions from a proton
H⁺ + water = H₃O⁺. H⁺ + ammonia = NH₄⁺. The H⁺ brings the charge and no electrons.
Use it when: Writing the equation and the charge.

Coordinate Bonding: Hydronium and Ammonium Ions: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Definition · 1 mark · Very short answerBoard level

Define a coordinate bond. State the condition on the atom that donates the pair.

Try it first, then show the solutionSolution

A coordinate bond is a covalent bond in which the shared pair is provided by one atom only.
The donor must have a lone pair.

Answer: A covalent bond in which both shared electrons come from one atom; the donor has a lone pair.

Example 2: Hydronium ion · 3 marks · Short answerBoard level

With the help of an electron dot diagram explain the formation of the hydronium ion. Name the donor and the acceptor.

Try it first, then show the solutionSolution

A water molecule has two bond pairs and two lone pairs on oxygen.
One lone pair of oxygen is donated to H⁺, which has no electron: $H_{2} O + H^{+} \to H_{3} O^{+}$ .
Water (oxygen atom) is the donor and the hydrogen ion is the acceptor. The ion has three bond pairs and one lone pair on O.

Answer: Oxygen of water donates a lone pair to H⁺, forming H₃O⁺ with three O-H bonds, one of them coordinate.

Example 3: Ammonium ion and its bonds · 3 marks · Short answerHarder board level

Explain the formation of the ammonium ion. Why are the four N-H bonds in NH₄⁺ identical even though one is coordinate?

Try it first, then show the solutionSolution

Nitrogen in NH₃ has one lone pair, which it donates to H⁺: $N H_{3} + H^{+} \to N H_{4}^{+}$ .
Nitrogen then has four bond pairs, no lone pair; the charge is positive.
After formation the bonding pair is shared by both atoms and cannot be told apart from the others, so all four N-H bonds are alike.

Answer: N donates its lone pair to H⁺; the four N-H bonds are equal once formed.

Example 4: Electron counting · 3 marks · Short answerHarder board level

Find for NH₄⁺ (N = 7, H = 1): (a) protons, (b) electrons, (c) total electrons in bonds.

Try it first, then show the solutionSolution

(a) Protons = 7 + 4 x 1 = 11.
(b) Neutral atoms would have 11 electrons; the + charge means one electron fewer, so 10 electrons.
(c) Four bond pairs = 8 bonding electrons; the other 2 electrons are the inner (K shell) electrons of nitrogen.

Answer: (a) 11 (b) 10 (c) 8.

Example 5: Ammonium chloride: three kinds of bond · 4 marks · Long answerHOTS / competency

Ammonium chloride contains covalent, coordinate and electrovalent bonds. Identify where each occurs and explain how the solid conducts electricity or not.

Try it first, then show the solutionSolution

Three N-H bonds are ordinary covalent bonds, and one N-H bond is a coordinate bond formed with H⁺.
The NH₄⁺ ion and the Cl⁻ ion are held together by an electrovalent bond.
In the solid the ions are fixed, so it does not conduct; in solution or when molten the ions are mobile and it conducts.

Answer: Covalent and coordinate inside NH₄⁺; electrovalent between NH₄⁺ and Cl⁻. Solid does not conduct; solution does.

Where students lose marks in Coordinate Bonding: Hydronium and Ammonium Ions

The slips examiners see most often, and what to write instead.

Mistake: Drawing the arrow the wrong way
The arrow in a coordinate bond points from the donor to the acceptor, not the reverse.
Wrong working
H -> N in NH₄⁺.
Right working
N -> H.
Do this: Draw the arrow from the atom with the lone pair.
Usually costs: 1 mark.
Mistake: Giving the hydrogen ion electrons
H⁺ has lost its electron; a diagram showing a dot on H⁺ is wrong.
Do this: Show H⁺ as an empty shell with no dot.
Usually costs: 1 mark.
Mistake: Forgetting the charge
H₃O⁺ and NH₄⁺ are shown without square brackets or without the + sign.
Do this: Put the structure in square brackets and write + outside.
Usually costs: 1 mark.
Mistake: Writing that the coordinate bond is different from the other bonds
After formation it cannot be distinguished from other covalent bonds.
Do this: Say: all four bonds in NH₄⁺ are alike.
Usually costs: 1 mark.

Exam tips

Name the donor, the acceptor and the species that carries the lone pair every time.
Always draw the lone pair on oxygen or nitrogen before forming the bond.
Link H₃O⁺ to acidity and the NH₃/H₂O equilibrium to alkalinity (OH⁻).
Do not call ammonium chloride "only covalent": mention the electrovalent bond to Cl⁻.

Practice questions on Coordinate Bonding: Hydronium and Ammonium Ions

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In the formation of the ammonium ion, the atom that donates the lone pair is:
(a) hydrogen
(b) nitrogen
(c) both hydrogen and nitrogen
(d) neither
Hint
Which has a lone pair?
Answer
(b) nitrogen.
Q2 · 1 mark · MCQBoard level
How many lone pairs are present on the oxygen atom in the hydronium ion?
(a) 0
(b) 1
(c) 2
(d) 3
Hint
Water had two; one was donated.
Answer
(b) 1.
Q3 · 2 marks · Short answerBoard level
What is meant by a lone pair? Give two molecules in which it is present.
Hint
Electrons not used in bonding.
Answer
A pair of outer electrons that are not shared with another atom. It is present on O in H₂O and on N in NH₃.
Q4 · 2 marks · Short answerHarder board level
Write the equation for the formation of the hydronium ion when hydrogen chloride gas dissolves in water. Name the type of bond formed between water and H⁺.
Hint
Water donates a lone pair.
Answer
$H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ ; a coordinate bond.
Q5 · 3 marks · Short answerHarder board level
Draw the structure of the ammonium ion showing the coordinate bond with an arrow, and state the number of bond pairs and lone pairs on nitrogen.
Hint
Arrow from N to H.
Answer
Nitrogen is at the centre with four bonds to H, one drawn as an arrow from N to H⁺; four bond pairs and no lone pair on N, charge + outside brackets.
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The ammonia molecule can form a coordinate bond with a hydrogen ion.
Reason (R): The nitrogen atom in ammonia has a lone pair of electrons.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Which atom is donor?
Answer
(a).
Q7 · 3 marks · Short answerHOTS / competency
Ammonia reacts with hydrogen chloride gas to form white fumes of ammonium chloride. After NH₄⁺ forms, why can the nitrogen atom not accept another H⁺ to give NH₅²⁺? Use the idea of a lone pair.
Hint
Count the lone pairs on N in NH₄⁺.
Answer
Nitrogen used its only lone pair to bond with the first H⁺. In NH₄⁺ it has four bond pairs and no lone pair, so there is nothing left to donate.
Q8 · 3 marks · Short answerHarder board level
Find for H₃O⁺ (O = 8, H = 1): (a) the total number of protons, (b) the total number of electrons, (c) how many electrons the oxygen atom has in its outer shell in the ion, counting bond pairs and lone pairs.
Hint
The ion has a charge of +1.
Answer
(a) 8 + 3 = 11 (b) 11 - 1 = 10 (c) 3 bond pairs + 1 lone pair = 4 pairs = 8 electrons.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is a donor atom?Show answer
The atom that provides both electrons of the shared pair.
2.Equation for the hydronium ion?Show answer
H₂O + H⁺ gives H₃O⁺.
3.Lone pairs on the N in NH₃?Show answer
One.
4.Which ion makes acids acidic?Show answer
The hydronium ion.
5.Symbol for a coordinate bond?Show answer
An arrow from donor to acceptor.

Coordinate Bonding: Hydronium and Ammonium Ions: frequently asked questions

What is the difference between a covalent and a coordinate bond?

In a covalent bond each atom gives one electron; in a coordinate bond one atom gives both electrons of the shared pair.

Why does the hydrogen ion accept a pair?

It has no electron and an empty first shell, so it can accept a lone pair.

Is H₃O⁺ the same as H⁺?

No. H⁺ is a bare proton; in water it is always bound to a water molecule as H₃O⁺.

How does the OH⁻ ion form?

Water ionises slightly, and a water molecule that loses H⁺ becomes OH⁻.