Structure of Water and Hydrogen Bonding

Structure of Water

A water molecule, H₂O, is composed of two hydrogen atoms bonded to one oxygen atom.
Atoms in a water molecule are held together by covalent bonds, where electrons are shared between atoms.
The structure of a water molecule is angular or V-shaped, not linear, due to the presence of two lone pairs of electrons on the oxygen atom.
Because oxygen is more electronegative (draws electrons towards itself) than hydrogen, the shared electrons spend more time closer to the oxygen atom. As a result, oxygen becomes slightly negative (δ-) and hydrogen becomes slightly positive (δ+). This is known as polarity.
The asymmetrical shape and polarity make water a dipolar molecule, or a molecule with a positive and negative end.

Hydrogen Bonding

Water molecules are attracted to each other due to the hydrogen bonds formed between each water molecule’s hydrogen atom (δ+) and an oxygen atom (δ-) of a different water molecule.
Hydrogen bonding is a type of intermolecular force – a force between different molecules - rather than a bond within a molecule.
It is weak compared to covalent bonds, but stronger than other intermolecular forces, which gives water some of its unique properties.
Each water molecule can form up to four hydrogen bonds with adjacent molecules: two bonds through its two hydrogen atoms and two more through the two lone pairs of electrons on the oxygen atom. This creates a network of water molecules connected by hydrogen bonds.

Properties of Water Resulting from Hydrogen Bonding

Cohesion: Water molecules stick together due to hydrogen bonding, creating a phenomenon known as cohesion. This quality helps water flow, making it possible for plants to transport water from their roots to their leaves.
High specific heat capacity: Hydrogen bonds absorb a lot of heat before they break, leading to a high specific heat capacity in water. This allows water to absorb heat without a significant change in temperature, providing a stable environment for aquatic organisms.
Density and the anomalous expansion of water: At temperatures above 4°C, water expands, becoming less dense as it warms. However, below 4°C, water begins to expand again, reaching its maximum density at 4°C. This leads to ice (solid state of water) being less dense than liquid water and thus, it floats. This unique property helps to insulate the water beneath, protecting aquatic life during winter.
Universal solvent: Due to its polarity, water can dissolve many substances, earning it the nickname of the universal solvent. This property is pivotal for the transport of nutrients, waste, and gases in biological systems.

Understanding these concepts about water’s structure and hydrogen bonding is crucial as it plays a central role in many biological processes and is often referred to as ‘the matrix of life’.