In a normal covalent bond each atom chips in one electron. In a coordinate bond, one atom supplies both electrons and the other supplies none. That is the only difference — and once the bond has formed, you cannot tell it apart from any other covalent bond.
📚 What you need to know
A coordinate (dative) bond is a covalent bond where both shared electrons come from the same atom.
Two things are needed: an atom with a lone pair to give (the donor), and an atom that is electron deficient with an empty space in its outer shell (the acceptor).
It is drawn as an arrow that points from the donor to the acceptor.
Once formed, a coordinate bond is identical to a normal covalent bond — same length, same strength.
The classic SL example is the ammonium ion, NH4+.
Where the electrons come from
Normally a bond is a 50/50 arrangement: you bring one electron, I bring one, and we share the pair. A coordinate bond is more like a loan. One atom has a spare lone pair sitting there doing nothing. The other atom has a gap in its outer shell but no electrons to fill it with. So the first atom pushes its whole pair into the gap, and now both atoms share it.
You need both ingredients for this to work:
The donor — an atom with a lone pair. In practice this is almost always N, O or a halogen.
The acceptor — an atom that is short of electrons and has an empty orbital. Think H+, or the boron in BF3, or a metal ion.
The ammonium ion, NH4+
This is the one that comes up again and again, so learn it properly.
A hydrogen ion, H+, is just a proton. It has lost its only electron, so its outer shell is completely empty. It cannot share an electron because it does not own one.
Ammonia, NH3, has three bonding pairs and one lone pair on the nitrogen. That lone pair is exactly what H+ is missing. The nitrogen donates it, a fourth N–H bond forms, and the whole thing carries the + charge that came in with the hydrogen ion.
The green circle marks the lone pair being donated. In the product, the top bond has two red dots because both of its electrons came from the nitrogen.
Notice where the charge goes. The + belongs to the whole ion, not to the nitrogen — that is why we draw square brackets around the structure and put the charge outside, at the top right.
💡 The point examiners really want
All four N–H bonds in NH4+ are exactly the same — same length, same strength, same bond angle.
If you measured them you could not tell which one was the coordinate bond.
We only draw the arrow to record where the electrons came from. It says nothing about the finished bond being different.
Two more you should recognise
The same idea turns up all over the course, so it is worth seeing it in a couple of other places:
The hydroxonium ion, H3O+. Water has two lone pairs on its oxygen. One of them is donated to an H+, giving H3O+. This is what actually forms whenever an acid dissolves in water.
NH3 reacting with BF3. Remember boron only has 6 electrons in BF3, so it has room for more. Nitrogen donates its lone pair into that gap, and both atoms end up with a full octet.
The arrow always starts at the atom that owns the lone pair and points at the atom with the empty space.
WORKED EXAMPLE
Explain why ammonia can form a coordinate bond with H+ but methane cannot.
A coordinate bond needs a lone pair to donateNH₃: nitrogen has 5 outer electrons, 3 are used in bonds → 1 lone pair left.CH₄: carbon has 4 outer electrons, all 4 are used in bonds → no lone pairs.No lone pair means nothing to donateNH₄⁺ forms, but CH₅⁺ does not
WORKED EXAMPLE
A student says the coordinate bond in NH4+ must be weaker than the other three, because only one atom paid for it. Comment on this.
The student is wrongWhere the electrons started makes no difference once they are shared.Every N–H bond ends up as one shared pair between the same two nucleiAll four bonds are identical in length and strengthThe arrow is bookkeeping for us, not a property of the bond.
Looking ahead (HL). Coordinate bonding is also how transition metal complexes hold together — for example [Al(H2O)6]3+, where six water molecules each donate a lone pair from their oxygen to the central metal ion. At SL you don’t need the detail; you just need to recognise that coordinate bonding happens beyond simple molecules like NH4+.
⚠️ Common mix-up
The arrow is not a double bond. A coordinate bond is a single bond — one shared pair, just supplied by one atom.
Direction matters. The arrow points from the atom with the lone pair to the atom with the gap. Drawing it backwards loses the mark.
Don’t put the charge on the nitrogen in NH4+. Use square brackets with the charge outside.
A coordinate bond is still covalent. It is not a third type of bonding alongside ionic and covalent.
Up next: Molecular Shapes — using VSEPR theory to work out the 3D shape of a molecule and predict its bond angles. Everything you have learned about lone pairs is about to pay off.
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