An Ammonia Molecule Can Supply Both Electrons of a Bond by Itself
Learn what a coordinate bond is and why it needs a lone pair on one side and an empty space on the other, watch water and ammonia use their lone pairs to capture protons, build the hydronium, hydroxyl and ammonium ions electron by electron, and tell all three kinds of bond apart.
What happens when one atom supplies both electrons of a shared pair?
In every covalent bond so far, each atom put one electron into the shared pair. But look again at the ammonia molecule. Nitrogen has five outer electrons; three of them are shared with hydrogen atoms, and two are left over as a lone pair that nothing is using.
**Now bring up a hydrogen ion, . A hydrogen atom that has lost its only electron has no electrons at all — it is a bare proton, and it needs two electrons for a duplet.
The lone pair on nitrogen can fill that need completely. Nitrogen shares both of its spare electrons with the proton, and a new bond forms in which one atom has supplied both electrons. That is a coordinate bond, and the product is the ammonium ion**, .
Bonds of this kind are everywhere in everyday chemistry, even when their name is unfamiliar.
- Every acid dissolved in water works through one — water's oxygen captures the acid's proton as the hydronium ion
- Ammonium salts in fertilisers, such as ammonium sulphate, contain the ammonium ion
- The ammonia solution used to clean glass is alkaline because ammonia's lone pair takes protons from water
So this part answers four questions.
- What exactly is a coordinate bond, and what must the two partners have?
- Why are the lone pairs on water and ammonia so important?
- How do the hydronium, hydroxyl and ammonium ions form, electron by electron?
- How is a coordinate bond told apart from electrovalent and ordinary covalent bonds?
The link to the rest of the chemistry syllabus is direct. The hydronium ion defines an acid and the hydroxyl ion defines an alkali in the chapter on acids, bases and salts — so the diagrams on this page are the reason those definitions work.
This page covers the third part of the ICSE Class 10 Chemistry chapter on chemical bonding: the coordinate bond, the role of lone pairs, the hydronium, hydroxyl and ammonium ions, and the comparison of all three bond types.
**Now bring up a hydrogen ion, . A hydrogen atom that has lost its only electron has no electrons at all — it is a bare proton, and it needs two electrons for a duplet.
The lone pair on nitrogen can fill that need completely. Nitrogen shares both of its spare electrons with the proton, and a new bond forms in which one atom has supplied both electrons. That is a coordinate bond, and the product is the ammonium ion**, .
Bonds of this kind are everywhere in everyday chemistry, even when their name is unfamiliar.
- Every acid dissolved in water works through one — water's oxygen captures the acid's proton as the hydronium ion
- Ammonium salts in fertilisers, such as ammonium sulphate, contain the ammonium ion
- The ammonia solution used to clean glass is alkaline because ammonia's lone pair takes protons from water
So this part answers four questions.
- What exactly is a coordinate bond, and what must the two partners have?
- Why are the lone pairs on water and ammonia so important?
- How do the hydronium, hydroxyl and ammonium ions form, electron by electron?
- How is a coordinate bond told apart from electrovalent and ordinary covalent bonds?
The link to the rest of the chemistry syllabus is direct. The hydronium ion defines an acid and the hydroxyl ion defines an alkali in the chapter on acids, bases and salts — so the diagrams on this page are the reason those definitions work.
This page covers the third part of the ICSE Class 10 Chemistry chapter on chemical bonding: the coordinate bond, the role of lone pairs, the hydronium, hydroxyl and ammonium ions, and the comparison of all three bond types.
What is a coordinate bond, and how do you tell the donor from the acceptor?
A coordinate bond is a covalent bond in which both electrons of the shared pair come from one atom, the donor, and are shared with another atom or ion, the acceptor.
The definition. A coordinate bond is formed between two atoms when the shared pair of electrons is contributed entirely by one of them, so that both attain a stable configuration. It is also called a dative bond.
The two partners:
- The donor — an atom that has a lone pair of electrons available to share, and has already completed its octet
- The acceptor — an atom or ion that is short of two electrons and has room to receive a pair
Conditions for a coordinate bond:
- One atom must have at least one lone pair
- The other must need exactly a pair to complete its duplet or octet
How it is shown. A coordinate bond is drawn as an arrow pointing from the donor to the acceptor:
In a dot diagram, both electrons of the new pair carry the donor's symbol — two dots if the donor's electrons are dots — which is what makes the bond visible as coordinate.
Worked example — identifying the partners. In the reaction
- Donor: the nitrogen atom of ammonia, which has one lone pair
- Acceptor: the hydrogen ion, which has no electrons and needs two
- After bonding: nitrogen still counts eight electrons, and the new hydrogen counts two
Worked example — why methane cannot do this. Carbon in methane, , shares all four of its outer electrons with hydrogen atoms. It has no lone pair left, so methane cannot act as a donor, and does not form under ordinary conditions. No lone pair, no coordinate bond.
An everyday way to picture the difference. An ordinary covalent bond is like two friends each paying half for a shared auto-rickshaw ride. A coordinate bond is one friend paying the whole fare — once you are both inside, the ride is exactly the same for each of you.
That last point is the essential boundary case. Once a coordinate bond has formed, it cannot be told apart from the other covalent bonds. In the ammonium ion, all four N–H bonds are identical in length and strength, and there is no way of finding which hydrogen arrived as the proton. The difference between a coordinate bond and an ordinary covalent bond lies only in where the electrons came from, not in the bond that results.
The definition. A coordinate bond is formed between two atoms when the shared pair of electrons is contributed entirely by one of them, so that both attain a stable configuration. It is also called a dative bond.
The two partners:
- The donor — an atom that has a lone pair of electrons available to share, and has already completed its octet
- The acceptor — an atom or ion that is short of two electrons and has room to receive a pair
Conditions for a coordinate bond:
- One atom must have at least one lone pair
- The other must need exactly a pair to complete its duplet or octet
How it is shown. A coordinate bond is drawn as an arrow pointing from the donor to the acceptor:
In a dot diagram, both electrons of the new pair carry the donor's symbol — two dots if the donor's electrons are dots — which is what makes the bond visible as coordinate.
Worked example — identifying the partners. In the reaction
- Donor: the nitrogen atom of ammonia, which has one lone pair
- Acceptor: the hydrogen ion, which has no electrons and needs two
- After bonding: nitrogen still counts eight electrons, and the new hydrogen counts two
Worked example — why methane cannot do this. Carbon in methane, , shares all four of its outer electrons with hydrogen atoms. It has no lone pair left, so methane cannot act as a donor, and does not form under ordinary conditions. No lone pair, no coordinate bond.
An everyday way to picture the difference. An ordinary covalent bond is like two friends each paying half for a shared auto-rickshaw ride. A coordinate bond is one friend paying the whole fare — once you are both inside, the ride is exactly the same for each of you.
That last point is the essential boundary case. Once a coordinate bond has formed, it cannot be told apart from the other covalent bonds. In the ammonium ion, all four N–H bonds are identical in length and strength, and there is no way of finding which hydrogen arrived as the proton. The difference between a coordinate bond and an ordinary covalent bond lies only in where the electrons came from, not in the bond that results.
Why do the lone pairs on water's oxygen and ammonia's nitrogen make coordinate bonds possible?
Oxygen in water and nitrogen in ammonia each keep unshared electron pairs after completing their octets, and a lone pair is exactly what a hydrogen ion needs.
Counting the lone pairs.
- Oxygen, , has six outer electrons. In water it shares two of them with hydrogen atoms, leaving electrons as two lone pairs
- Nitrogen, , has five outer electrons. In ammonia it shares three, leaving electrons as one lone pair
- Carbon, , shares all four in methane, leaving no lone pair
Why a hydrogen ion is the ideal acceptor.
- A hydrogen atom has one electron; the hydrogen ion has lost it
- It has zero electrons and needs two for a duplet
- It cannot share in the ordinary way, because it has nothing to contribute
- So it can bond only by accepting a complete pair
A free hydrogen ion is so small and so strongly charged that it does not remain free in water. It attaches at once to the lone pair of a water molecule, and that is why acids in water are described in terms of hydronium ions rather than bare hydrogen ions.
Why water and ammonia accept only one proton each under ordinary conditions.
- After water accepts one proton, the hydronium ion carries a positive charge
- A second proton is repelled by that charge, even though oxygen still has one lone pair left
- Ammonia has only one lone pair to begin with, so after forming the ammonium ion it has none left at all
Ammonia gives its lone pair more readily than water does. Nitrogen is less electronegative than oxygen, so it holds its lone pair less tightly. That is why ammonia can take a proton from a water molecule, making the solution alkaline:
An everyday consequence. A bottle of ammonia solution used for cleaning glass turns red litmus blue. The alkalinity comes entirely from nitrogen's lone pair capturing protons from water and leaving hydroxyl ions behind.
Lone pairs also decide shape, which links back to the previous part. Water's two lone pairs bend the molecule; ammonia's one lone pair makes it pyramidal. After ammonia uses its lone pair to bond a proton, all four pairs round nitrogen are bond pairs, and the ammonium ion becomes a perfectly symmetrical tetrahedron — the same shape as methane.
One check that prevents a common error. A lone pair belongs to the atom whose outer electrons it is — oxygen or nitrogen — never to hydrogen. Hydrogen in water and ammonia already has its duplet and has no electrons to spare, so it can never be the donor.
Counting the lone pairs.
- Oxygen, , has six outer electrons. In water it shares two of them with hydrogen atoms, leaving electrons as two lone pairs
- Nitrogen, , has five outer electrons. In ammonia it shares three, leaving electrons as one lone pair
- Carbon, , shares all four in methane, leaving no lone pair
Why a hydrogen ion is the ideal acceptor.
- A hydrogen atom has one electron; the hydrogen ion has lost it
- It has zero electrons and needs two for a duplet
- It cannot share in the ordinary way, because it has nothing to contribute
- So it can bond only by accepting a complete pair
A free hydrogen ion is so small and so strongly charged that it does not remain free in water. It attaches at once to the lone pair of a water molecule, and that is why acids in water are described in terms of hydronium ions rather than bare hydrogen ions.
Why water and ammonia accept only one proton each under ordinary conditions.
- After water accepts one proton, the hydronium ion carries a positive charge
- A second proton is repelled by that charge, even though oxygen still has one lone pair left
- Ammonia has only one lone pair to begin with, so after forming the ammonium ion it has none left at all
Ammonia gives its lone pair more readily than water does. Nitrogen is less electronegative than oxygen, so it holds its lone pair less tightly. That is why ammonia can take a proton from a water molecule, making the solution alkaline:
An everyday consequence. A bottle of ammonia solution used for cleaning glass turns red litmus blue. The alkalinity comes entirely from nitrogen's lone pair capturing protons from water and leaving hydroxyl ions behind.
Lone pairs also decide shape, which links back to the previous part. Water's two lone pairs bend the molecule; ammonia's one lone pair makes it pyramidal. After ammonia uses its lone pair to bond a proton, all four pairs round nitrogen are bond pairs, and the ammonium ion becomes a perfectly symmetrical tetrahedron — the same shape as methane.
One check that prevents a common error. A lone pair belongs to the atom whose outer electrons it is — oxygen or nitrogen — never to hydrogen. Hydrogen in water and ammonia already has its duplet and has no electrons to spare, so it can never be the donor.
How do the hydronium, hydroxyl and ammonium ions form, shown electron by electron?
The hydronium and ammonium ions form when a lone pair on oxygen or nitrogen is donated to a hydrogen ion; the hydroxyl ion is what a water molecule becomes when its proton is taken away and it keeps both bonding electrons.
A counting rule that gives the charge on any of these ions. Add up the outer electrons the neutral atoms bring, compare with the electrons actually present in the ion, and the difference is the charge.
**1. The hydronium ion, .**
- Donor: one lone pair on the oxygen of water
- Acceptor: the hydrogen ion
- After bonding: oxygen has three bond pairs and one lone pair, still an octet, and each hydrogen has a duplet
- In the dot diagram, two of the bond pairs show one oxygen electron and one hydrogen electron; the third shows two oxygen electrons
- Charge: neutral atoms bring electrons; the ion holds ; charge
**2. The ammonium ion, .**
- Donor: the lone pair on nitrogen
- Acceptor: the hydrogen ion
- After bonding: nitrogen has four bond pairs and no lone pair
- Charge: neutral atoms bring ; the ion holds ; charge
Ammonium chloride forms when ammonia meets hydrogen chloride, the proton passing to nitrogen's lone pair:
**3. The hydroxyl ion, .
The hydroxyl ion forms on the other side of the same exchange. When a water molecule gives a proton to a lone pair — to another water molecule, or to ammonia — the proton leaves without** its electron:
- The oxygen keeps both electrons of the O–H bond that broke
- It now has one bond pair and three lone pairs — still an octet
- In the dot diagram, the three lone pairs and the shared pair are drawn round oxygen, with one electron marked differently to show the extra electron that came from the departed hydrogen, and the whole ion in brackets with a negative charge
- Charge: neutral O and H bring ; the ion holds ; charge
Stated precisely: the O–H bond remaining in the hydroxyl ion is an ordinary covalent bond. The hydroxyl ion is the partner produced whenever water donates a proton to a lone pair, which is why it appears alongside the hydronium and ammonium ions in these equations.
Worked check — the charges must balance across each equation.
- : charge . : . Balanced
- : charge . : . Balanced
An everyday link. A glass of plain drinking water contains a tiny number of hydronium and hydroxyl ions in exactly equal amounts, from the first equation above. Equal amounts mean the water is neutral — and adding an acid or an alkali tips that balance one way or the other.
A counting rule that gives the charge on any of these ions. Add up the outer electrons the neutral atoms bring, compare with the electrons actually present in the ion, and the difference is the charge.
**1. The hydronium ion, .**
- Donor: one lone pair on the oxygen of water
- Acceptor: the hydrogen ion
- After bonding: oxygen has three bond pairs and one lone pair, still an octet, and each hydrogen has a duplet
- In the dot diagram, two of the bond pairs show one oxygen electron and one hydrogen electron; the third shows two oxygen electrons
- Charge: neutral atoms bring electrons; the ion holds ; charge
**2. The ammonium ion, .**
- Donor: the lone pair on nitrogen
- Acceptor: the hydrogen ion
- After bonding: nitrogen has four bond pairs and no lone pair
- Charge: neutral atoms bring ; the ion holds ; charge
Ammonium chloride forms when ammonia meets hydrogen chloride, the proton passing to nitrogen's lone pair:
**3. The hydroxyl ion, .
The hydroxyl ion forms on the other side of the same exchange. When a water molecule gives a proton to a lone pair — to another water molecule, or to ammonia — the proton leaves without** its electron:
- The oxygen keeps both electrons of the O–H bond that broke
- It now has one bond pair and three lone pairs — still an octet
- In the dot diagram, the three lone pairs and the shared pair are drawn round oxygen, with one electron marked differently to show the extra electron that came from the departed hydrogen, and the whole ion in brackets with a negative charge
- Charge: neutral O and H bring ; the ion holds ; charge
Stated precisely: the O–H bond remaining in the hydroxyl ion is an ordinary covalent bond. The hydroxyl ion is the partner produced whenever water donates a proton to a lone pair, which is why it appears alongside the hydronium and ammonium ions in these equations.
Worked check — the charges must balance across each equation.
- : charge . : . Balanced
- : charge . : . Balanced
An everyday link. A glass of plain drinking water contains a tiny number of hydronium and hydroxyl ions in exactly equal amounts, from the first equation above. Equal amounts mean the water is neutral — and adding an acid or an alkali tips that balance one way or the other.
How do you distinguish electrovalent, covalent and coordinate bonds?
Ask what happens to the electrons: transferred completely gives an electrovalent bond, shared with one electron from each atom gives a covalent bond, and shared with both electrons from one atom gives a coordinate bond.
1. How the bond forms.
- Electrovalent — complete transfer of electrons from one atom to another
- Covalent — mutual sharing, each atom contributing one electron to every shared pair
- Coordinate — one-sided sharing, one atom contributing both electrons of the pair
2. Between which kinds of atom.
- Electrovalent — a metal of low ionisation potential and a non-metal of high electron affinity
- Covalent — non-metals of similar electronegativity
- Coordinate — a donor with a lone pair and an acceptor short of a pair
3. What is formed.
- Electrovalent — ions held in a lattice
- Covalent — neutral molecules
- Coordinate — often a charged ion such as or , because a neutral molecule has bonded to a charged one
4. How it is drawn.
- Electrovalent — separate ions in brackets with charges
- Covalent — a line, or a dot and a cross in the shared pair
- Coordinate — an arrow from donor to acceptor, or two of the donor's symbols in the shared pair
5. Examples.
- Electrovalent: , ,
- Covalent: , , ,
- Coordinate: one bond in each of and
Worked example — one compound with all three bonds. Identify the bonds in ammonium chloride, .
- **Between and — an electrovalent bond, the attraction between two oppositely charged ions
- Three N–H bonds — covalent, from the original ammonia molecule
- One N–H bond — coordinate, formed when nitrogen's lone pair captured the proton
Count**: bonds inside the ion, and ionic attraction between the ions. Once formed, the four N–H bonds are identical — the coordinate one is described separately only because of how it was made.
Worked example — sodium hydroxide. In the bond between and is electrovalent, and the O–H bond inside the hydroxyl ion is covalent. No coordinate bond is present.
Worked example — hydronium chloride in hydrochloric acid. In the solution, contains two covalent O–H bonds and one coordinate O–H bond, and it is surrounded by free ions.
Properties follow the particles, not the bond names. Ammonium chloride contains covalent and coordinate bonds inside its ions, but it is made of ions, so it behaves like an electrovalent compound: a white crystalline solid that dissolves in water and conducts in solution. An everyday example is the ammonium sulphate spread on fields, which dissolves readily in soil water exactly as a salt should.
The boundary case in one line. A coordinate bond is a special kind of covalent bond — the same shared pair, differing only in origin. It is a category of covalent bond, not a third, separate type of attraction, even though syllabuses list it alongside the other two.
1. How the bond forms.
- Electrovalent — complete transfer of electrons from one atom to another
- Covalent — mutual sharing, each atom contributing one electron to every shared pair
- Coordinate — one-sided sharing, one atom contributing both electrons of the pair
2. Between which kinds of atom.
- Electrovalent — a metal of low ionisation potential and a non-metal of high electron affinity
- Covalent — non-metals of similar electronegativity
- Coordinate — a donor with a lone pair and an acceptor short of a pair
3. What is formed.
- Electrovalent — ions held in a lattice
- Covalent — neutral molecules
- Coordinate — often a charged ion such as or , because a neutral molecule has bonded to a charged one
4. How it is drawn.
- Electrovalent — separate ions in brackets with charges
- Covalent — a line, or a dot and a cross in the shared pair
- Coordinate — an arrow from donor to acceptor, or two of the donor's symbols in the shared pair
5. Examples.
- Electrovalent: , ,
- Covalent: , , ,
- Coordinate: one bond in each of and
Worked example — one compound with all three bonds. Identify the bonds in ammonium chloride, .
- **Between and — an electrovalent bond, the attraction between two oppositely charged ions
- Three N–H bonds — covalent, from the original ammonia molecule
- One N–H bond — coordinate, formed when nitrogen's lone pair captured the proton
Count**: bonds inside the ion, and ionic attraction between the ions. Once formed, the four N–H bonds are identical — the coordinate one is described separately only because of how it was made.
Worked example — sodium hydroxide. In the bond between and is electrovalent, and the O–H bond inside the hydroxyl ion is covalent. No coordinate bond is present.
Worked example — hydronium chloride in hydrochloric acid. In the solution, contains two covalent O–H bonds and one coordinate O–H bond, and it is surrounded by free ions.
Properties follow the particles, not the bond names. Ammonium chloride contains covalent and coordinate bonds inside its ions, but it is made of ions, so it behaves like an electrovalent compound: a white crystalline solid that dissolves in water and conducts in solution. An everyday example is the ammonium sulphate spread on fields, which dissolves readily in soil water exactly as a salt should.
The boundary case in one line. A coordinate bond is a special kind of covalent bond — the same shared pair, differing only in origin. It is a category of covalent bond, not a third, separate type of attraction, even though syllabuses list it alongside the other two.
Exam tip
What does a complete coordinate bond answer need?
Name the donor, name the acceptor, show the lone pair before and the shared pair after, and state the charge with a reason.
- Write the configuration of the donor atom and show how many lone pairs it has before bonding
- Show the hydrogen ion with no electrons — never with a dot
- Mark the donated pair clearly, either with an arrow from donor to acceptor or with both electrons in the donor's symbol
- Complete every octet and duplet in the product, including the lone pair left on oxygen in
- Put the ion in brackets with its charge: , ,
- Check the charge by counting electrons brought against electrons present
- Balance charge across the equation
- For the hydroxyl ion, show three lone pairs and mark the extra electron
- State that all bonds become identical once a coordinate bond has formed
- Identify every bond type when asked about a compound such as ammonium chloride
The misconception to name. The hydrogen ion is not a hydrogen atom. A hydrogen atom has one electron and would form an ordinary covalent bond; a hydrogen ion has none and can only accept a pair. Drawing with an electron turns the coordinate bond into a covalent one and leaves the ion with the wrong charge.
A second trap. Saying water has one lone pair. Oxygen in water has two lone pairs, and in the hydronium ion it still has one left after bonding — omitting that remaining pair is the most common error in hydronium ion diagrams.
- Write the configuration of the donor atom and show how many lone pairs it has before bonding
- Show the hydrogen ion with no electrons — never with a dot
- Mark the donated pair clearly, either with an arrow from donor to acceptor or with both electrons in the donor's symbol
- Complete every octet and duplet in the product, including the lone pair left on oxygen in
- Put the ion in brackets with its charge: , ,
- Check the charge by counting electrons brought against electrons present
- Balance charge across the equation
- For the hydroxyl ion, show three lone pairs and mark the extra electron
- State that all bonds become identical once a coordinate bond has formed
- Identify every bond type when asked about a compound such as ammonium chloride
The misconception to name. The hydrogen ion is not a hydrogen atom. A hydrogen atom has one electron and would form an ordinary covalent bond; a hydrogen ion has none and can only accept a pair. Drawing with an electron turns the coordinate bond into a covalent one and leaves the ion with the wrong charge.
A second trap. Saying water has one lone pair. Oxygen in water has two lone pairs, and in the hydronium ion it still has one left after bonding — omitting that remaining pair is the most common error in hydronium ion diagrams.
Did you know
Why does ammonia solution smell so sharp while ammonium chloride has no smell?
Open a bottle of ammonia solution and the sharp, eye-watering smell reaches you at once. Solid ammonium chloride, a white salt that contains the same nitrogen and hydrogen atoms, has no smell at all. The difference is one coordinate bond.
Ammonia molecules are small, neutral and only loosely attracted to one another, so they escape easily from the solution into the air and reach your nose. Smell needs molecules that travel.
In ammonium chloride, each ammonia molecule has used its lone pair to capture a proton. It is no longer a neutral molecule but the charged ion , locked into a crystal lattice by its attraction to chloride ions. Ions held in a lattice do not drift into the air, so there is nothing to smell.
Now reverse the coordinate bond and the smell comes back. Warm ammonium chloride with sodium hydroxide solution:
The hydroxyl ion pulls the proton off the ammonium ion, the lone pair returns to nitrogen, and free ammonia gas escapes with its pungent smell. That is the standard laboratory test for an ammonium salt, and it works precisely because the coordinate bond can be made and unmade.
Heating the solid alone shows the same bond at work. Ammonium chloride heated in a dry test tube seems to vanish from the bottom and reappear as a white coating near the cooler mouth. In the hot zone the proton returns to chloride, giving ammonia and hydrogen chloride gases; in the cooler zone nitrogen's lone pair captures the proton again and the solid re-forms:
Bring a glass rod dipped in concentrated hydrochloric acid near an open bottle of ammonia solution and dense white fumes appear in mid-air. Those fumes are tiny crystals of ammonium chloride, each formed as an ammonia molecule donates its lone pair to a proton — a coordinate bond forming visibly, in the space between the rod and the bottle.
Ammonia molecules are small, neutral and only loosely attracted to one another, so they escape easily from the solution into the air and reach your nose. Smell needs molecules that travel.
In ammonium chloride, each ammonia molecule has used its lone pair to capture a proton. It is no longer a neutral molecule but the charged ion , locked into a crystal lattice by its attraction to chloride ions. Ions held in a lattice do not drift into the air, so there is nothing to smell.
Now reverse the coordinate bond and the smell comes back. Warm ammonium chloride with sodium hydroxide solution:
The hydroxyl ion pulls the proton off the ammonium ion, the lone pair returns to nitrogen, and free ammonia gas escapes with its pungent smell. That is the standard laboratory test for an ammonium salt, and it works precisely because the coordinate bond can be made and unmade.
Heating the solid alone shows the same bond at work. Ammonium chloride heated in a dry test tube seems to vanish from the bottom and reappear as a white coating near the cooler mouth. In the hot zone the proton returns to chloride, giving ammonia and hydrogen chloride gases; in the cooler zone nitrogen's lone pair captures the proton again and the solid re-forms:
Bring a glass rod dipped in concentrated hydrochloric acid near an open bottle of ammonia solution and dense white fumes appear in mid-air. Those fumes are tiny crystals of ammonium chloride, each formed as an ammonia molecule donates its lone pair to a proton — a coordinate bond forming visibly, in the space between the rod and the bottle.
Exam relevance
How are coordinate bonds used in JEE and NEET Chemistry?
This is foundation work for three chapters examined in both JEE Main and NEET Chemistry: Class 11 Chemical Bonding and Molecular Structure, Class 11 Equilibrium, and Class 12 Coordination Compounds.
Where the donor and acceptor lead in Equilibrium. Class 11 defines a Lewis base as an electron-pair donor and a Lewis acid as an electron-pair acceptor. Ammonia donating its lone pair to a proton is the textbook Lewis base, and questions that ask whether a species such as boron trifluoride or a metal ion can act as a Lewis acid are answered by exactly the reasoning on this page: does it have room for a pair?
Where the proton transfer leads. The same chapter describes acids as proton donors and bases as proton acceptors, and pairs such as and as conjugate acid-base pairs. The equations here — water giving a proton to ammonia, and water ionising into hydronium and hydroxyl ions — are the standard examples, and identifying conjugate pairs is a recurring objective question.
Where the dot diagrams lead in Chemical Bonding. Class 11 assigns formal charges to atoms in Lewis structures. The positive charge on the hydronium and ammonium ions is placed on oxygen and nitrogen by that method, and drawing correct Lewis structures of ions such as these, and of molecules with dative bonds such as carbon monoxide and ozone, is examined directly.
Where the ammonium tetrahedron leads. VSEPR theory explains why ammonia is pyramidal but the ammonium ion is tetrahedral, and why the bond angle opens up when the lone pair becomes a bond pair. Comparing shapes before and after protonation is a typical assertion-reason item.
Where the lone pair leads in Coordination Compounds. Class 12 builds an entire chapter on ligands — molecules and ions such as ammonia, water and chloride that donate lone pairs to a central metal ion. Tetraamminecopper(II) sulphate, which appears in the acids, bases and salts chapter, contains four coordinate bonds from ammonia to copper. Naming such compounds, counting coordinate bonds and working out oxidation states are standard in both exams.
Question types to expect. At this level: dot diagrams, donor and acceptor identification, and bond types in a compound. In competitive papers: Lewis acids and bases, conjugate pairs, formal charge, shapes of ions, and coordination number and naming.
The single trap that costs marks. Treating the coordinate bond in the ammonium ion as different from the other three. All four N–H bonds are equivalent, and a question that asks for the number of different bond lengths in expects the answer one.
Board versus competitive emphasis. The ICSE paper marks the dot diagram with donated pair and charge; a competitive paper marks a Lewis-acid classification, a conjugate pair or a ligand count. The transferable habit is asking, for every species, where its lone pairs are and whether it has room for a pair — the question that drives all three later chapters.
Where the donor and acceptor lead in Equilibrium. Class 11 defines a Lewis base as an electron-pair donor and a Lewis acid as an electron-pair acceptor. Ammonia donating its lone pair to a proton is the textbook Lewis base, and questions that ask whether a species such as boron trifluoride or a metal ion can act as a Lewis acid are answered by exactly the reasoning on this page: does it have room for a pair?
Where the proton transfer leads. The same chapter describes acids as proton donors and bases as proton acceptors, and pairs such as and as conjugate acid-base pairs. The equations here — water giving a proton to ammonia, and water ionising into hydronium and hydroxyl ions — are the standard examples, and identifying conjugate pairs is a recurring objective question.
Where the dot diagrams lead in Chemical Bonding. Class 11 assigns formal charges to atoms in Lewis structures. The positive charge on the hydronium and ammonium ions is placed on oxygen and nitrogen by that method, and drawing correct Lewis structures of ions such as these, and of molecules with dative bonds such as carbon monoxide and ozone, is examined directly.
Where the ammonium tetrahedron leads. VSEPR theory explains why ammonia is pyramidal but the ammonium ion is tetrahedral, and why the bond angle opens up when the lone pair becomes a bond pair. Comparing shapes before and after protonation is a typical assertion-reason item.
Where the lone pair leads in Coordination Compounds. Class 12 builds an entire chapter on ligands — molecules and ions such as ammonia, water and chloride that donate lone pairs to a central metal ion. Tetraamminecopper(II) sulphate, which appears in the acids, bases and salts chapter, contains four coordinate bonds from ammonia to copper. Naming such compounds, counting coordinate bonds and working out oxidation states are standard in both exams.
Question types to expect. At this level: dot diagrams, donor and acceptor identification, and bond types in a compound. In competitive papers: Lewis acids and bases, conjugate pairs, formal charge, shapes of ions, and coordination number and naming.
The single trap that costs marks. Treating the coordinate bond in the ammonium ion as different from the other three. All four N–H bonds are equivalent, and a question that asks for the number of different bond lengths in expects the answer one.
Board versus competitive emphasis. The ICSE paper marks the dot diagram with donated pair and charge; a competitive paper marks a Lewis-acid classification, a conjugate pair or a ligand count. The transferable habit is asking, for every species, where its lone pairs are and whether it has room for a pair — the question that drives all three later chapters.
Key takeaways
What must you be able to do from this part?
One new bond, two donors, three ions and a comparison.
- A coordinate or dative bond is a covalent bond in which both electrons of the shared pair come from one atom
- The donor has a lone pair; the acceptor is short of a pair
- It is drawn as an arrow from donor to acceptor, or with both electrons in the donor's symbol
- Once formed, it is identical to the other covalent bonds — all four N–H bonds in are the same
- Water's oxygen has two lone pairs; ammonia's nitrogen has one; methane's carbon has none and cannot donate
- The hydrogen ion has no electrons and can bond only by accepting a pair
- Ammonia donates more readily than water, so it takes protons from water:
- Hydronium ion: — three bond pairs and one lone pair on oxygen, charge
- Ammonium ion: — four bond pairs, no lone pair, tetrahedral, charge
- Hydroxyl ion: formed when water gives up a proton and keeps both bonding electrons — one bond pair and three lone pairs, charge
- Water ionises slightly: , in equal amounts, so pure water is neutral
- Electrovalent bonds transfer electrons, covalent bonds share with one electron from each atom, coordinate bonds share with both from one atom
- Ammonium chloride contains all three: one ionic attraction, three covalent N–H bonds and one coordinate N–H bond
- Properties follow the particles: compounds made of ions behave as ionic compounds even when their ions contain coordinate bonds
- Warming an ammonium salt with alkali releases ammonia, reversing the coordinate bond
The quickest self-test is three ions and a counting rule. Draw the hydronium, ammonium and hydroxyl ions from memory, mark every lone pair, and prove each charge by counting electrons — then name every bond in ammonium chloride without looking back.
- A coordinate or dative bond is a covalent bond in which both electrons of the shared pair come from one atom
- The donor has a lone pair; the acceptor is short of a pair
- It is drawn as an arrow from donor to acceptor, or with both electrons in the donor's symbol
- Once formed, it is identical to the other covalent bonds — all four N–H bonds in are the same
- Water's oxygen has two lone pairs; ammonia's nitrogen has one; methane's carbon has none and cannot donate
- The hydrogen ion has no electrons and can bond only by accepting a pair
- Ammonia donates more readily than water, so it takes protons from water:
- Hydronium ion: — three bond pairs and one lone pair on oxygen, charge
- Ammonium ion: — four bond pairs, no lone pair, tetrahedral, charge
- Hydroxyl ion: formed when water gives up a proton and keeps both bonding electrons — one bond pair and three lone pairs, charge
- Water ionises slightly: , in equal amounts, so pure water is neutral
- Electrovalent bonds transfer electrons, covalent bonds share with one electron from each atom, coordinate bonds share with both from one atom
- Ammonium chloride contains all three: one ionic attraction, three covalent N–H bonds and one coordinate N–H bond
- Properties follow the particles: compounds made of ions behave as ionic compounds even when their ions contain coordinate bonds
- Warming an ammonium salt with alkali releases ammonia, reversing the coordinate bond
The quickest self-test is three ions and a counting rule. Draw the hydronium, ammonium and hydroxyl ions from memory, mark every lone pair, and prove each charge by counting electrons — then name every bond in ammonium chloride without looking back.