Why is Ice lighter than Water?

Why is water the only thing, that when frozen, is less dense then it’s original liquid state?

This one’s quite tricky to understand, but it has to do with how water molecules behave. Water is a very polar molecule (has one side - the hydrogen side - positive, and one side -the oxygen side - negative) which when it is a liquid interact weakly, and pull the molecules together. When the kinetic energy, or temperature, of the molecules is sufficiently low, the molecules will be very closely packed. Below the freezing point of water, however, something kind of strange happens. I will have to explain hydrogen bonding before I go any further.

Hydrogen bonding occurs when hydrogen is near an atom with a pair of electrons not bound to anything else. Oxygen, nitrogen and sulphur are regular examples, but generally anything on the right hand side of the periodic table could potentially do this. When hydrogen is bonded to another atom, sometimes it receives an electron, sometimes it gives it’s only one away. When it gives one away, or shares it’s sole electron, it becomes positively charged away from the bond. If this hydrogen comes close enough to a non-bonding pair of electrons, it is attracted to the electrons, and can bond to them. This is not a true bond, but can be very strong, especially if there are lots of them (as in the case of DNA, which has litterally millions of them.)

Ok, so in water, these hydrogen bonds form at freezing point, and they form very distinct patterns, due to the arrangement of the bonds in space. The angle between the two hydrogens is about 104.5 degrees, and the 2 electron pairs in the oxygen are slightly further apart than 120 degrees, due to more repultion, due to being closer the the centre. With this, two hydrogen bonds can occur with each water molecule, and space appears between adjacent molecules due to the rigid formations that occur. This gives the property of waters solid form floating on it’s liquid form, very rare, but not limited to water. Technically sulphur hydride will display this, as well as selenium hydride, and everything which contains 2 pairs of unpaired electrons in the outside shell/orbital.

The fact that ice has these rigid constraints, but is flexible about it’s bonds gives rise to the immense variability of ice crystals, snowflakes etc.