The hydrogen bond and the dative bond

This page covers a single Form 4 content standard: the hydrogen bond and the dative bond. These are the two special bonds of the Chemical Bond chapter. They look similar in name but they are very different in nature, and the exam likes to test whether you can tell them apart. Learn precisely what each one is, how it forms, and one clear example of each.

The dative (coordinate) bond

A dative bond, also called a coordinate bond, is a covalent bond in which the shared pair of electrons is contributed by only one of the two atoms. In an ordinary covalent bond each atom gives one electron to the shared pair; in a dative bond, one atom (the donor) supplies both electrons of the pair while the other atom (the acceptor) supplies none but still shares them.

For a dative bond to form, the donor must have a lone pair (an unshared pair of electrons) and the acceptor must have an empty orbital that can receive it. Once formed, a dative bond is identical to a normal covalent bond, the atoms simply share a pair of electrons. It is drawn with an arrow (→) pointing from the donor atom to the acceptor atom, to show where both electrons came from.

The classic example is the ammonium ion, NH₄⁺. An ammonia molecule (NH₃) has a nitrogen atom with a lone pair. When ammonia meets a hydrogen ion (H⁺), which has no electrons and an empty orbital, the nitrogen donates its lone pair to the H⁺. A new N–H dative bond forms, and the whole particle becomes NH₄⁺. All four N–H bonds in the ammonium ion are then the same.

The hydrogen bond

A hydrogen bond is a completely different thing: it is a weak force of attraction between molecules, not a bond formed by sharing or transferring electrons to make a compound. It forms when a hydrogen atom is covalently bonded to a small, highly electronegative atom, fluorine, oxygen or nitrogen, in one molecule, and is then attracted to a lone pair on the electronegative atom of a neighbouring molecule.

Because F, O and N pull the shared electrons strongly, the hydrogen becomes slightly positive and the F, O or N becomes slightly negative. The slightly positive hydrogen of one molecule is attracted to the slightly negative atom of the next. This attraction is much weaker than a covalent or ionic bond, but it is stronger than ordinary forces between molecules. Hydrogen bonding between water molecules is why water has an unusually high boiling point for such a small molecule, and why ice is less dense than liquid water so that it floats.

Worked example

Question. Explain how the dative bond forms in the ammonium ion, NH₄⁺, and describe how it is drawn.

Step 1, Identify the donor. In an ammonia molecule (NH₃), the nitrogen atom has three N–H covalent bonds and one lone pair of electrons that is not shared.

Step 2, Identify the acceptor. A hydrogen ion, H⁺, is a hydrogen atom that has lost its electron, so it has no electrons and an empty orbital ready to accept a pair.

Step 3, Form the bond. The nitrogen donates its lone pair into the empty orbital of the H⁺. Both electrons of the new N–H bond came from the nitrogen alone, so this is a dative (coordinate) bond.

Step 4, Show and interpret it. The dative bond is drawn as an arrow N → H, pointing from nitrogen (the donor) to the added hydrogen. The particle formed is NH₄⁺, and all four N–H bonds are now equivalent.

Answer. Nitrogen in NH₃ donates its lone pair to an H⁺ ion that has an empty orbital, forming a dative bond drawn as N → H; the ion produced is NH₄⁺.

Practice question

Question. Water molecules are attracted to one another by hydrogen bonds. (a) State the condition needed for a hydrogen bond to form. (b) Explain why hydrogen bonding gives water an unusually high boiling point.

Answer. (a) A hydrogen bond forms when a hydrogen atom is covalently bonded to a highly electronegative atom (fluorine, oxygen or nitrogen) and is attracted to a lone pair on the electronegative atom of a neighbouring molecule. In water, the hydrogen is bonded to oxygen. (b) The many hydrogen bonds between water molecules are extra attractions that must be overcome before the molecules can separate into vapour. Because more energy is needed to break these attractions, water boils at a much higher temperature than a molecule of its size otherwise would.

Exam tip

Do not confuse the two bonds, this is the most common error. A dative bond is a covalent bond within a molecule or ion, where both shared electrons come from one atom; show it with an arrow →. A hydrogen bond is a weak force between molecules, only where hydrogen is joined to F, O or N; it is not what holds a compound together. For the dative bond, always name the donor with a lone pair and the acceptor with an empty orbital. For the hydrogen bond, always state the F, O or N condition and remember it is weaker than covalent and ionic bonds.

Where this fits

This standard extends the covalent bond you met just before it, the dative bond is a special covalent bond, and it introduces an intermolecular force, the hydrogen bond, that helps explain physical properties in the Chemical Bond chapter and beyond. Review the definitions and diagrams with the revision notes and practise drawing the NH₄⁺ dative bond with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure you keep dative bonds (inside a particle, both electrons from one atom) and hydrogen bonds (between particles, needing F, O or N) firmly separate, which is exactly what avoids the trap the exam sets on this standard across SPM Chemistry.

Quick recap

• A dative (coordinate) bond is a covalent bond where both shared electrons come from one atom; drawn with an arrow →.
• It needs a donor with a lone pair and an acceptor with an empty orbital (example: NH₄⁺).
• A hydrogen bond is a weak force between molecules with hydrogen bonded to F, O or N.
• Hydrogen bonding explains water’s high boiling point and why ice floats.
• Dative = within a particle; hydrogen bond = between particles.