In nature, water exists in the liquid, solid, and gaseous states but under standard conditions (25°C and 1 atm of pressure) is primarily a liquid. 

It has the chemical formula of H₂O with the two hydrogen atoms connected to the oxygen atom by covalent bonds. Oxygen is more electronegative (holds onto its electrons more tightly) than most other elements, so the oxygen atom retains a negative charge while the hydrogen atoms are positively charged. The bent structure gives the molecule an electrical dipole moment and it is classified as a polar molecule. The molecules of water are constantly moving in relation to each other, and hydrogen bonds between the molecules are continually breaking and forming at intervals briefer than 200 femtoseconds (200 x 10^-15 seconds).

Arrangement of water molecules in the liquid phase showing that because of the molecules polarity they are able to form up to four hydrogen bonds with other neighbouring water molecules.

As the water cools, this movement slows and the molecules move gradually closer to one another. Eventually the solid state of water if formed, known as ice. Unlike most other substances, water’s solid form is less dense than its liquid form as a result of the hexagonal packing within its crystalline lattice. This lattice contains more space than when the molecules are in the liquid state and is called hexagonal ice. While this hexagonal ice is the major form to be found on Earth and the common one we are all familiar with there are two other types of ice that are of interest to electron microscopists, cubic and vitreous. These will be discussed fully in the next section.

Liquid water (left) consists of molecules connected by short-lived hydrogen bonds, with the molecules continuously moving. The hydrogen bonds in hexagonal ice (right) become permanent, resulting in an interconnected hexagonally shaped framework of molecules.