Hydrogen bonding

Hydrogen bonding is a strong type of intermolecular force. It forms between a hydrogen atom that is covalently bonded to a very electronegative atom (nitrogen, oxygen or fluorine) and a lone pair of electrons on a nitrogen, oxygen or fluorine atom in another molecule. Hydrogen bonds are the strongest forces between molecules, which is why water has such a high boiling point for its size and why ice floats.

What is a hydrogen bond?

A hydrogen bond is not a covalent bond. It is an attraction between molecules. Two things are needed:

1. A hydrogen atom covalently bonded to N, O or F: small atoms that are highly electronegative.
2. A lone pair on an N, O or F atom in a neighbouring molecule.

Nitrogen, oxygen and fluorine pull the shared electrons strongly towards themselves, so the hydrogen becomes very δ+. A hydrogen atom has no inner electrons, so its nucleus is left almost bare. The δ+ hydrogen is strongly attracted to a lone pair on a δ− atom nearby.

How to draw a hydrogen bond

• Show the partial charges, δ+ on the H and δ− on the N, O or F.
• Draw at least one lone pair on the δ− atom that receives the hydrogen bond.
• Draw the hydrogen bond as a dashed line from the H to the lone pair.
• Keep the three atoms (O-H, then the O it bonds to) in a straight line: the angle at the H is 180°.

Hydrogen bonding vs other intermolecular forces

AQA A-level Chemistry lists three forces between molecules: induced dipole-dipole forces (also called van der Waals, dispersion or London forces), permanent dipole-dipole forces, and hydrogen bonding.

Molecules that hydrogen bond also have induced dipole-dipole forces between them; the hydrogen bonds are extra. When a liquid boils, the intermolecular forces are overcome, not the covalent bonds inside the molecules.

Examples of hydrogen bonding

In biology, hydrogen bonds hold the two strands of DNA together, between complementary bases, and help hold proteins in shape. See what is DNA.

Practise reading the effect of intermolecular forces on boiling points in this question from the Chemical Bonding course.

Effects of hydrogen bonding

AQA highlights two: the low density of ice and the anomalous boiling points of some compounds.

Anomalous boiling points

Going down a group, boiling points usually rise, because bigger molecules have more electrons and stronger induced dipole-dipole forces. The hydrides of group 16 break this pattern at the top:

Water is the smallest molecule but has by far the highest boiling point, because extra energy is needed to break its hydrogen bonds. Hydrogen sulfide does not hydrogen bond, because sulfur is much less electronegative than oxygen. NH₃ and HF stand out in groups 15 and 17 in the same way.

Why ice floats

Each water molecule can form up to four hydrogen bonds: two through its hydrogens and two through the lone pairs on its oxygen. In ice, these hold the molecules in an open, regular lattice, with more space between molecules than in liquid water. When ice melts, some hydrogen bonds break and the molecules pack closer together. So ice is less dense than water and floats, and a frozen pond keeps liquid water underneath for living things.

Solubility

Small alcohols such as ethanol mix with water because they can form hydrogen bonds with water molecules. Hydrocarbons cannot, so they do not dissolve.

Worked example: comparing boiling points

Ethanol, CH₃CH₂OH (Mr 46), boils at 78 °C. Propane, C₃H₈ (Mr 44), boils at −42 °C. Explain the difference.

• Both have a similar number of electrons, so their induced dipole-dipole forces are similar.
• Ethanol has an O-H group, so its molecules form hydrogen bonds with each other. Propane only has C-H bonds and cannot.
• More energy is needed to overcome the hydrogen bonds between ethanol molecules, so ethanol boils at a much higher temperature.

Check your understanding.

Common mistakes

• Saying any molecule with hydrogen can hydrogen bond. The hydrogen must be bonded to N, O or F.
• Calling the O-H bond itself the hydrogen bond. The hydrogen bond is the attraction to the next molecule.
• Saying covalent bonds break when water boils. Only the intermolecular forces are overcome.
• Forgetting the lone pair when drawing the bond.
• Drawing the bond at an angle. X-H...Y is linear, 180° at the H.

Practise with a short timed paper

Intermolecular forces are in the first year of A-level Chemistry and in GCSE Chemistry as forces between molecules. The daily A-level Chemistry: Year 1 and GCSE Chemistry Higher papers are short timed papers of 8 questions, with a new one every day. Build water or ammonia in 3D in the Molecule Builder, and see covalent bonding for the bonds inside molecules.

Read next

Chemistry · Bonding and structureIntermolecular forces: the 3 types and boiling pointsThe three intermolecular forces: induced dipole, permanent dipole and hydrogen bonding, how they affect boiling points, and why water is unusual.