Eli5: What is the atomic explanation of ice being less dense than liquid water?
In: Physics
Water molecules form a strong pseudo-bond with one another called a *hydrogen bond*. This isn’t as strong as the true bonds in molecules, but it is much stronger than typical forces between different molecules. This means that water molecules interact more strongly than molecules of most other materials, and it means that hydrogen bonds are by far the most important factor in the behavior of water molecules in bulk.
With respect to everyday ice, this bonding causes water to form an unusually open crystal structure. The hydrogen bonds want to align along the crystal’s axes, because otherwise there’s tension left over trying to pull them back into line. Because of the exact angles of the bonds in water, which line up nicely with some crystal lattices but not others, water is therefore encouraged to adopt one of the lattices where the angles are close to lined up. It turns out that the hexagonal pattern ice actually takes lines up pretty well: the 105 degree angles between the hydrogens in each water molecule line up quite closely with the roughly 109.5 degree angles of the lattice. It happens that this hexagonal lattice isn’t very good at packing the molecules close together, so ice isn’t very dense.
When water forms it’s crystal shape it takes up more area. While water molecules stand much closer together at room temp
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