A molecule is the smallest amount of any element or compound substance still possessing all the chemical properties of that substance which can exist. Molecules themselves are made up of even smaller particles called atoms, which define the basic elements such as hydrogen and oxygen.
The specific combinations of these atomic elements provide compound substances. One such compound is represented by the chemical formula H2O, having molecules made up of two atoms of hydrogen and one atom of oxygen.
The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst the most abundant elements in the universe. Carbon is another element of significant abundance, and is a key component in all organic matter.
Most mineral substances can exist in the three physical states (solid, liquid and vapour) which are referred to as phases. In the case of H2O, the terms ice, water and steam are used to denote the three phases respectively.
The molecular arrangement of ice, water, and steam is still not fully understood, but it is convenient to consider the molecules as bonded together by electrical charges (referred to as the hydrogen bond).
The degree of excitation of the molecules determines the physical state (or phase) of the substance.
Read the full story at: http://www.wermac.org