A1.1 Water

Water (H₂O) is arguably the most important molecule in biology. Every living cell depends on it , not just as a container for reactions, but as an active participant in countless biological processes. Life as we know it almost certainly originated in water, and all known organisms require it to survive.

The unique properties of water arise from a single feature: the way its atoms are bonded together. Understanding water's molecular structure unlocks an explanation for everything from how trees grow tall to why aquatic animals need special adaptations to stay warm.

A water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms. The bonds are formed by the sharing of electrons between oxygen and hydrogen. However, this sharing is unequal.

Electronegativity: An atom's ability to attract electrons towards itself in a covalent bond. Oxygen is significantly more electronegative than hydrogen.

Because oxygen is more electronegative, it pulls the shared electrons closer to itself. This creates an uneven distribution of charge across the molecule:

• The oxygen atom develops a partial negative charge (δ−)
• Each hydrogen atom develops a partial positive charge (δ+)

This makes water a polar molecule , it has a positive end and a negative end, like a tiny molecular magnet.

Additionally, the two hydrogen atoms are not arranged in a straight line. The molecule has a bent shape, with a bond angle of approximately 104.5°. This bent geometry means the partial charges do not cancel each other out, reinforcing water's polarity.