Chemistry · Ch 4 — Chemical Bonding and Molecular Structure

Hydrogen Bonding

4.9

Hydrogen Bonding

The Origin of the Hydrogen Bond

The story of the hydrogen bond begins with three elements: nitrogen, oxygen, and fluorine. These are the most electronegative elements on the periodic table. When any of these atoms forms a covalent bond with hydrogen, something dramatic happens to the electron pair they share.

Because nitrogen, oxygen, and fluorine pull so strongly on electrons, the shared pair in the $H-X$ bond (where $X = N, O, F$ ) is drawn far away from the hydrogen nucleus. This leaves the hydrogen atom with a significant partial positive charge, written as $H^{\delta+}$ . The electronegative atom, in turn, acquires a matching partial negative charge, $X^{\delta-}$ .

This charge separation sets the stage for a special kind of attraction. The partially positive hydrogen of one molecule is strongly attracted to the lone pair of electrons on the electronegative atom of a neighbouring molecule. This electrostatic attraction is the hydrogen bond.

Note

The hydrogen bond is an intermolecular force — it acts between molecules, not within a single molecule. The covalent $H-X$ bond is the strong bond inside the molecule; the hydrogen bond is the weaker, dotted-line connection between molecules.

The Hydrogen Bond in Hydrogen Fluoride

The classic example is hydrogen fluoride, $HF$ . In a sample of HF, the molecules do not exist as isolated units. Instead, they link together in a chain-like structure.

The arrangement looks like this:

\cdots H^{\delta+} - F^{\delta-} \cdots H^{\delta+} - F^{\delta-} \cdots H^{\delta+} - F^{\delta-} \cdots

In this diagram:

The solid line ( $-$ ) represents the strong covalent bond within each HF molecule.
The dotted line ( $\cdots$ ) represents the weaker hydrogen bond that bridges one molecule to the next.

The hydrogen atom acts as a bridge. It is held to its own fluorine atom by a covalent bond, and it is simultaneously attracted to the fluorine atom of the next molecule by a hydrogen bond.

Defining the Hydrogen Bond

Based on this behaviour, we can give a precise definition.

Important

Hydrogen bond — the attractive force which binds the hydrogen atom of one molecule with the electronegative atom (F, O or N) of another molecule.

The key requirement is that the hydrogen must be covalently bonded to one of the three highly electronegative atoms (F, O, or N). Only these three atoms are electronegative enough to create the strong $\delta+$ charge on hydrogen needed for a significant hydrogen bond.

Key Properties of the Hydrogen Bond

The textbook establishes several fundamental properties of this bond.

Important

Property 1: Relative Strength

The hydrogen bond is weaker than a covalent bond.

This is a critical distinction. A typical covalent bond has a strength of several hundred kJ/mol. A hydrogen bond, by contrast, has a strength of roughly 10–40 kJ/mol. It is a strong intermolecular force, but it is an order of magnitude weaker than the bonds that hold atoms together within a molecule.

Note

Why we know it is weaker: ice melts at 0 °C — the heat of fusion breaks a fraction of water's hydrogen bonds — while breaking water's O–H covalent bonds takes chemistry (electrolysis) or extreme temperatures. The two energy scales sit an order of magnitude apart: hydrogen bonds ≈ 10–40 kJ/mol against several hundred kJ/mol for covalent bonds.

Watch out

Common Mistake: Confusing Bond Types

Do not confuse the hydrogen bond with the covalent bond. The covalent bond is the intramolecular bond (inside the molecule), shown with a solid line. The hydrogen bond is the intermolecular attraction (between molecules), shown with a dotted line. They are fundamentally different in strength and nature.

The Bridge Analogy …

Start your 14-day free trial to unlock the full solution →

← Previous · 4.8Bonding in Some Homonuclear Diatomic Molecules Next · 4.9.1 →Cause of Formation of Hydrogen Bond