A molecule is the smallest portion of a pure element or compound that retains a set of unique chemical and physical properties. A molecule usually consists of two or more atomsbonded together, with the exception of noble gases which are composed of single atom molecules.
(left and center) and 2D (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.]]
Most molecules are much too small to be seen with the naked eye, but there are exceptions. A grain of salt, or the diamond on an engagement ring, are giant crystal lattices, repetitive molecules with atomic bonding (either ionic or covalent bonding) connecting the entire structure. Another example of a macroscopically-sized molecule is the DNA, a macromolecule.
A property of molecules is the integerratio of the elements that constitute the compound, the empirical formula. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers.
Chemical formula on the other hand reflects the exact number of atoms that compose a molecule. The molecular mass is calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a 12Cisotope atom.
(left and center) and