One molecule is a group of atoms and is the smallest unit of an element that can exist independently. By chemical affinity, the atoms are joined together to form molecules, which have a size of about a millionth of a millimeter. The affinity of the atoms depends on the amount of electrons that orbit around the nucleus. The atoms that are in the outer orbit eight electrons are not attached to any other element, as are balanced, they are called noble gases: helium, neon, argon, krypton, xenon, radon. Other atoms try to fill the number of eight electrons to be configured as the noble gases. That is, those with an electron are combined with those seven. For example, chlorine has seven electrons and combines with sodium has only one, to form a molecule of sodium chloride or common salt.
Since water molecules, ammonia, methane and other compounds did life on our planet. Formed simple organisms evolved over millions of years to reach the present diversity.
The Italian chemist Amedeo Avogadro, who lived between 1776 and 1856, defined the molecular constitution of matter: in equal volumes of gases at equal pressure and temperature are the same number of molecules.
The molecules of water, ammonia and methane is the most important compounds of nature, as the union of these arise after the three types of compounds essential to living things: carbohydrates, proteins and fats.