Hydrogen bonding
A hydrogen bond is a strong attraction between a hydrogen atom bonded to N, O or F and a lone pair on another N, O or F. It is the strongest type of intermolecular force, but still far weaker than a covalent bond.
Last updated
Analogy
Like a small magnet on a fridge: the H atom is the magnet, and lone pairs on N, O or F are the metal.
Example
H₂S boils at −60 °C.H₂O boils at 100 °C.Same shape, but only H₂Ohas hydrogen bonds.
In practice
Hydrogen bonds make water boil at 100 °C, not at about −80 °C, and make ice float.
Practice
Flashcards
- What is needed for a hydrogen bond?H bonded to N, O or F, and a lone pair on another N, O or F.
Interview questions
- Explain why water has a much higher boiling point than hydrogen sulfide.
- Water forms hydrogen bonds
- H₂S has weaker dipole-dipole and London forces
- More energy needed to separate water molecules
- Boiling point reflects intermolecular forces
Questions people ask
Why does ice float on water?
In ice, hydrogen bonds hold each water molecule in an open six-sided lattice with gaps. This takes up more space than the jumbled liquid, so ice is about 9 percent less dense than water. Lower density means ice floats, which protects fish by insulating lakes.
Do hydrogen bonds exist inside DNA?
Yes. Hydrogen bonds hold the two strands of DNA together by linking base pairs: two bonds between A and T, and three between G and C. They are weak enough that enzymes can unzip the strands to copy DNA, yet strong enough, in large numbers, to keep the helix stable.