Ice floating is one of the unique properties of water. Most other matter is denser when it is frozen or in a solid form and therefore would sink when place in the liquid form of the same matter. Ice floating is actually a very important biological property of water. If ice didn’t float, then lakes and other bodies of water would freeze solid all the way through instead of being protected by a later of ice at the top keeping the liquid underneath warmer. This would mean that anything that lived in the water would freeze and die. Therefore, all the seafood and fish that we like to eat would not exist.
Ice floats because of the nature of the hydrogen bonds that are found between the water molecules. Water is composed of two atoms of hydrogen and one atom of oxygen. These atoms form a covalent bond with each other meaning that each hydrogen atom shares a pair of electrons with the oxygen atom. The oxygen atom is a much more electronegative atom or an atom that likes to keep the shared electrons around it more of the time. Therefore, the electrons are not shared equally (they spend more time with the oxygen and less time with the hydrogen). This gives each hydrogen atom a partially positive charge and the oxygen a partially negative charge (but keep in mind the overall charge of the molecule is neutralneither negative or positive). These partial charges cause the hydrogen atoms of one molecule of water to be attracted to oxygen atoms of another water molecule. The weak bonds, which are the result of these attractions, are called hydrogen bonds. These bonds are much weaker than the covalent bonds that hold the water molecule together, if fact; they constantly break and reform between the molecules of water that are in liquid form.
When water is frozen or is in a solid form these hydrogen bonds are solidified so that they are no longer breaking and reforming. They become more stable and more permanent. This causes the water molecules to from a kind of crystal lattice with the hydrogen bonds between each molecule. This lattice causes the water molecules to be less dense and less compact in a solid form than when they are in liquid form, which causes the ice to be lighter than the water, therefore it floats.
Simply, because of hydrogen bonding, there are less water molecules in ice compared to the same amount of liquid water. Less molecules means less weight, and less weight means that ice floats.