Water is one of the most important molecules in human life, not only because makes up more than two thirds of human body weight, but also because all the cell and organ functions depend on water for their functioning. Something similar occurs in animals and plants. This means that without water most kind of life on earth would disappear in a few days.
Water is composed of one Oxygen atom and two Hydrogen atoms. Oxygen has six electrons in its valence shell, and needs two electrons to acquire great stability, those two electrons are taken from two Hydrogen atoms.
The molecule of H2O is a three dimensional structure, Oxygen is the centre of the molecule, around him there are two pairs of electrons, and two Hydrogen atoms with angles around 107º among them.
Oxygen and Hydrogen have electronegativity values of 3.5 and 2.1 respectively, when they are bonded the shared electrons are attracted to the Oxygen side, leaving the Hydrogen atoms with a slightly positive charge, and producing a final polar molecule.
When a water molecule is surrounded by another water molecule, they will interact each other through those slight charges. This interaction is called “Hydrogen Bonding”, which is a weak electrostatic attraction between slight positive and negative charges of different molecules.