The term ‘molecule’ was coined by Frene philosopher Rene Descartes in the 1620s. A molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bond Molecules are distinguished from polyatomicions in strict sense. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules and biomolecules.

A molecule may consist of atoms of the same chemical element, as with oxygen (O2), or of different elements, as with water (H2O). Atoms and complexes connected by non-covalent bonds such as hydrogen bonds or ionic bonds are generally not considered single molecules

Molecular size :  Most molecules are far too small to be seen with the naked eye, but there are exceptions, DNA (a genetic material of various eukaryotic livings), a kind of macromolecule, can reach macroscopic sizes, as can molecules many polymers. The smallest molecule is the diatomic hydrogen (H2), with an overall length of roughly twice the 74 picometers (0.74A) length. Molecules chiefly used as building blocks for organic synthesis have a of a several dozen A. Some of the largest molecules are macromolecules or supermolecules. Sing molecules cannot usually be observed by
light, but molecules and even the of individual atoms may be traced in some circumstances by use of an atomic force microscope.

Molecular Formula ::  The empirical formula of a compound is the simplest integer ratio of the chemical elements that constitute it. For example, water (H2O) is always composed of 2 :1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen and oxygen in 2 : 6 : 1 ratio. However, this does not determine the kind of molecule uniquely, dimethyl ether has the same ratio as ethanol. The molecular formula reflects the exact number of atoms that compose the molecule and so characterises different isomers.

Molecular Geometry ::  Most chemical reactions proceed in two directions, as products convert back into reactants and at the same time reactants convert into products. This situation is called chemical equilibrium. Molecules have fixed equilibrium geometries-bond lengths and angles-about which they continuously oscillate through vibrational and rotational motions. A pure substance is composed of molecules with the same average geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties, chiefly its reactivity. Isomers share a chemical formula of various chemical elements and organic matters but normally have very different properties because of their different structures.

Molecules of Life :: Bonded atoms form larger molecules. Living things are composed mostly of water and organic (C-containing) molecules. Biochemicals (organic molecules found in organisms) are also rich in oxygen, nitrogen, hydrogen, sulphur and phosphorus. Many biological molecules so larger that they are termed macromolecules.
The plant pigment chlorophyll. for instance, contains carbon atoms, hydrogen, magnesium, 4 nitrogen and 5 oxygen atoms. Carbon monoxide (Co) and carbon dioxide (Co2) are often not considered because of their simple structures. However, even these small molecules can have profound effects on living beings.