Water molecules join together by means of hydrogen bonding, weak attractions between partial positive and negative charges.
As a polar molecule (with oppositely charged ends), water is able to readily form bonds with other polar or charged molecules or atoms. Most importantly, it bonds to other water molecules using a special kind of interaction called a hydrogen bond, which is a weak intermolecular bond between a positively charged hydrogen atom and a negatively charged atom. The negatively charged atom is usually fluorine, oxygen, or nitrogen. A neutral hydrogen atom has only one electron. When a hydrogen bond forms between two water molecules, the hydrogen atom of one water molecule is attracted to the oxygen of the other water molecule. This attraction leaves the hydrogen nucleus with a partial positive charge. At the same time, the oxygen end of the other water molecule carries a partial negative charge. In a water molecule, each oxygen atom can form two hydrogen bonds with surrounding molecules and tends to orient itself so that each hydrogen atom is in alignment with it.
The hydrogen bonds formed by water molecules are some of the strongest intermolecular forces in small molecules. Most of the special properties of water, such as taking a lot of energy to change temperature, expanding as it freezes, and being attracted to itself, can be traced to the numerous hydrogen bonds formed among the individual water molecules.
Hydrogen Bonds in Water
As water switches among the three common states of matter (solid, liquid, and gas), the hydrogen bonding that exists between the molecules changes. In solid water, or ice, every oxygen atom is stuck bonded to two adjacent hydrogen atoms, leading to a regular and predictable crystal lattice pattern of the molecules. Water molecules in ice are locked in place apart from each other by the hydrogen bond frames. This makes ice less dense than water, so it floats. In liquid water, hydrogen bonds can form and break as the water molecules move around and interact with one another. The molecules can move closer together than in ice, so liquid water is denser. In its gaseous state, water molecules have even fewer bonds between them, so they are able to spread out and fill all of the space they occupy.