Advanced #25

The number of nearest neighbors increases on melting

Upon melting, the coordination number increases even though volume decreases.

Scientific Explanation

In most substances, molecules in the solid phase have more nearest neighbors than in the liquid, because they are packed more tightly. Water reverses this: in the ice crystal, each molecule has exactly four nearest neighbors in a strict tetrahedral arrangement (coordination number 4). Upon melting, this number rises to approximately 4.7 — more neighbors, not fewer.

This paradoxical behavior is explained by the unusually open structure of ice. The tetrahedral arrangement in ice Ih leaves gaps between the four neighbors that become partially filled in the liquid. When ice melts, the strict order collapses, and molecules that previously sat farther away in the second coordination shell move closer to the central molecule. Hydrogen bonds are partially bent or broken, but the greater disorder allows a denser packing with more neighbors in the immediate vicinity.

Neutron scattering measurements confirm this increase in coordination number during the transition from ice to liquid water. The effect is directly linked to the volume contraction upon melting (anomaly 16).

Nearest Neighbors Increase on Melting Comparison showing ice with 4 nearest neighbors in a tetrahedral arrangement versus liquid water with approximately 4.7 nearest neighbors, despite liquid water having a smaller volume per molecule. Ice (Ih) H₂O 4 neighbors (tetrahedral) → Melting Liquid Water H₂O ~4.7 neighbors (disordered, denser) More neighbors despite smaller volume per molecule Coordination Number: Ice vs. Liquid Water
Upon melting, the number of nearest neighbors rises from 4 to about 4.7 — despite the tetrahedral order in ice.

Everyday Relevance

This phenomenon explains at its core why ice floats on water. The open structure with only four neighbors per molecule makes ice more voluminous than the liquid, where molecules can pack closer together. What might seem like a technical detail of molecular physics has fundamental importance for aquatic ecosystems and the climate of our planet.