Hydrogen Bonding Explained: Why Water Is So Strange

By rights, water should be a gas at room temperature. Look at the other hydrogen compounds of group 16: hydrogen sulfide (H₂S) boils at −60 °C, hydrogen selenide at −41 °C, hydrogen telluride at −2 °C. Extrapolate that trend upwards to the smallest, lightest molecule, H₂O, and you’d predict a boiling point somewhere far below zero.

Instead water boils at 100 °C. The reason is the hydrogen bond — and without it, Earth would have no oceans and no life as we know it.

What a hydrogen bond is

A hydrogen bond is an attraction between:

1. a hydrogen atom that is covalently bonded to N, O or F, and
2. a lone pair of electrons on another N, O or F atom nearby.

Written with a dotted line: O–H···O. The solid line is the ordinary covalent bond; the dotted line is the hydrogen bond.

Despite the name, it isn’t a full chemical bond. It’s the strongest type of intermolecular force — typically 5 to 40 kJ/mol, compared with around 460 kJ/mol for the O–H covalent bond inside a water molecule.

Why only N, O and F?

Two things make a hydrogen bond unusually strong:

• High electronegativity. N, O and F pull the shared electrons strongly towards themselves, leaving the hydrogen with a large partial positive charge (δ+). Hydrogen has no inner electrons, so that positive nucleus is almost exposed.
• Small size. N, O and F are small, so their lone pairs are concentrated and the neighbouring molecule can get very close.

Chlorine is nearly as electronegative as nitrogen, but it’s much larger, so its lone pairs are more spread out. HCl forms only weak dipole-dipole attractions instead.

Water’s hydrogen-bond network

Each water molecule has two hydrogens (each can be donated to a neighbour) and two lone pairs on its oxygen (each can accept a hydrogen). So each molecule can take part in up to four hydrogen bonds, building a three-dimensional network.

That network explains a remarkable list of water’s properties:

• High boiling point. Boiling water means breaking a huge number of hydrogen bonds.
• High heat capacity. Water absorbs a lot of heat for a small temperature rise, which moderates Earth’s climate and keeps your body temperature stable.
• High heat of vaporization. Evaporating sweat carries away a lot of heat.
• High surface tension. Water forms beads and supports insects walking on it.
• A great solvent for ionic and polar substances.

Why ice floats

Almost every substance is denser as a solid than as a liquid. Water is a spectacular exception: ice is about 9% less dense than liquid water.

When water freezes, the hydrogen bonds lock each molecule into a rigid hexagonal lattice, with every molecule hydrogen-bonded to four others at fixed angles. That arrangement has a lot of empty space in it. In the liquid, hydrogen bonds constantly break and re-form, letting molecules pack more closely.

The consequences are enormous. Lakes and seas freeze from the top down, and the floating ice insulates the water below, so aquatic life survives winter. Water also expands when it freezes, which is why it cracks rocks (frost weathering) and bursts frozen pipes.

Hydrogen bonds in living things

• DNA: the two strands of the double helix are held together by hydrogen bonds between base pairs — two between A and T, three between G and C. They’re strong enough to keep the helix stable, but weak enough that enzymes can unzip the strands to copy them.
• Proteins: hydrogen bonds hold helices and sheets in shape. Cooking an egg breaks many of them, which is why the proteins unfold and set.
• Cellulose: long glucose chains in plant fibres are bundled together by hydrogen bonds, which is part of why wood and cotton are so strong.

Other hydrogen-bonded substances

• Ammonia (N–H) boils at −33 °C, much higher than phosphine (PH₃) at −88 °C.
• Hydrogen fluoride (H–F) boils at about 20 °C, while HCl boils at −85 °C.
• Alcohols (O–H) have much higher boiling points than hydrocarbons of similar size. Ethanol boils at 78 °C; propane, of similar molar mass, at −42 °C.

Quick answers

Is a hydrogen bond a covalent bond? No. It’s an intermolecular attraction, roughly 10–20 times weaker than a typical covalent bond (though it has some partial covalent character).

Does CH₄ form hydrogen bonds? No. Its hydrogens are bonded to carbon, which isn’t electronegative enough.

Why does H₂O boil higher than HF even though F is more electronegative? Water can form up to four hydrogen bonds per molecule; HF, with only one hydrogen, forms about two on average.

Related

See intermolecular forces for how hydrogen bonds compare with other attractions, and oxygen’s element page for more on the element at the heart of water.