Hydrogen Bond Length and Lattice Density [closed]

Question

The subject of the density of solid water is discussed on Physics Stack Exchange in this post.

In the post, it is said that..

the way the molecule is angled ensures that a specific (energetically favorable) orientation leaves a relatively large amount of open space

However, how can the orientation alone be responsible for density? With a short enough bond length, can not any solid lattice be more dense than its liquid form?

Would the density of a lattice structure not depend on the bond length as well? (In the case of water, would the density of the lattice not depend on the bond length of its (dipole-dipole Hydrogen bonds).

If Hydrogen bonds are indeed stronger than other types of intermolecular bonds, why is the bond length so long?

Answer 1

Here is a sketch of a layer of ice:

The hydrogen bonds are fairly short (177 pm, compared to a covalent O-H bond length of 100 pm), and the distance between hydrogen-bonded atoms is shorter than the minimal distance in the absence of the hydrogen bond (about 195 pm).

What should be apparent from the image is that there is a lot of space in the center of the hexagons (the picture does not show that every second water is a bit higher and the ones in between a bit lower). For a 3D view see http://biomodel.uah.es/en/water/p1.htm). Trying to squeeze in another water molecule would not work for two reasons. First, there is not enough space without disrupting existing hydrogen bonds. Secondly, that extra water molecule would have no hydrogen-bonding partners.

Liquid water has a less orderly structure and fewer hydrogen bonds, so the water molecules get closer on average. It does not, however, have shorter hydrogen bonds or let non-hydrogen bonded hydrogen and oxygen molecules approach closer than the non-bonded minimal distance.

Answer 2

The bond length, intermolecular forces, and molecular shape all determine the lattice structure, which in turn determines the density of the substance. Thus, with a small enough bond length, solids can be more dense than liquids due to a much more tight lattice structure.

Water is somewhat unique in its densities due to its bent molecular geometry. Most other substances tend to flow away from each other in their liquid form due to their kinetic energy, but when they become solid they don't have enough kinetic energy to move away from each other, making them closer to each other and thus denser in solid form. However, the attraction between water molecules causes them to use their kinetic energy while in a liquid state to get closer to each other. However, there are two lone electron pairs that belong to the oxygen atom in a water molecule, and when water becomes a solid, it no longer has enough kinetic energy to counter the lone electron pairs' repulsions, so the water molecules in ice are slightly farther away due to the repulsion, thus making ice less dense than liquid water. There are other molecules that are like water: more dense in their liquid form than solid form.

As an extra note, you said that hydrogen intermolecular forces are stronger than other intermolecular forces, but we should acknowledge that hydrogen intermolecular forces are weaker than ion-dipole and ion-ion intermolecular forces, explaining why things such as saltwater (ion-dipole) and table salt (ion-ion) have higher densities than water.