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(left and center) and 2D (right) representations of the Terpenoid molecule Atisane .]] HISTORY Although the concept of molecules was first introduced in 1811 by Avogadro , and was accepted by many chemists as a result of Dalton's laws of Definite and Multiple Proportions (1803-1808), with notable exceptions ( Boltzmann , Maxwell , Gibbs ), the existence of molecules as anything other than convenient mathematical constructs was still an open debate in the physics community until the work of Perrin ( 1911 ), and was strenuously resisted by early Positivists such as Mach . The modern theory of molecules makes great use of the many numerical techniques offered by Computational Chemistry . Dozens of molecules have now been identified in Interstellar Space by Microwave Spectroscopy . OVERVIEW The science of molecules is called ''molecular chemistry'' or '' Molecular Physics '', depending on the focus. Molecular chemistry deals with the laws governing the interaction between molecules that results in the formation and breakage of Chemical Bond s, while molecular physics deals with the laws governing their structure and properties. In practice, however, this distinction is vague. In molecular sciences, a molecule consists of a stable system ( Bound State ) comprising two or more Atom s. The term ''unstable molecule'' is used for very Reactive species, i.e., short-lived assemblies ( Resonance s) of Electron s and Nuclei , such as Radicals , molecular Ion s, Rydberg Molecule s, Transition State s, Van Der Waals Complex es, or systems of colliding atoms as in Bose-Einstein Condensate s. A peculiar use of the term ''molecular'' is as a synonym to '' Covalent '', which arises from the fact that, unlike molecular covalent compounds, Ionic Compound s do not yield well-defined ''smallest particles'' that would be consistent with the definition above. No typical "smallest particle" can be defined for covalent Crystal s, or Network Solids , which are composed of repeating Unit Cell s that extend indefinitely either in a Plane (such as in Graphite ) or three-dimensionally (such as in Diamond ). CHEMICAL BOND See main article Chemical Bond In a molecule, the atoms are joined by shared pairs of Electron s in a chemical bond. It may consist of atoms of the same Chemical Element , as with Oxygen (O2), or of different elements, as with Water (H2O). SIZE Most molecules are much too small to be seen with the naked eye, but there are exceptions. DNA , a Macromolecule , can reach Macroscopic sizes. The smallest molecule is the Hydrogen molecule. The interatomic distance is 0.15 Nanometre s (1.5 Å ). But the size of its Electron Cloud is difficult to define precisely. Under Standard Conditions molecules have a dimension of a few to several dozen Å. EMPIRICAL FORMULA See main article Empirical Formula The empirical formula of a molecule is the '''simplest''' has the same ratio as ethanol, for instance. Molecules with the same Atom s in different arrangements are called Isomer s. The empirical formula is often the same as the molecular formula but not always. For example the molecule Acetylene has molecular formula C2H2, but the simplest integer ratio of elements is CH. CHEMICAL FORMULA See main article Chemical Formula The chemical formula reflects the exact number of atoms that compose a molecule. MOLECULAR MASS See main article Molecular Mass The Molecular Mass can be calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a 12 C Isotope atom. For Network Solids , the term Formula Unit is used in Stoichiometric calculations. MOLECULAR GEOMETRY See main article Molecular Geometry 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 because of their different structures. Stereoisomer s, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different Biochemical activities. MOLECULAR SPECTROSCOPY See Also: Spectroscopy Molecular spectroscopy deals with the response ( Spectrum ) of molecules interacting with probing signals of known Energy (or Frequency , according to Planck's Formula ). Scattering Theory provides the theoretical background for spectroscopy. The probing signal used in spectroscopy can be an Electromagnetic Wave or a beam of Particle s ( Electron s, Positron s, etc.) The molecular response can consist of signal absorption ( Absorption Spectroscopy ), the emission of another signal ( Emission Spectroscopy ), fragmentation, or chemical changes. Spectroscopy is recognized as a powerful tool in investigating the Microscopic properties of molecules, in particular their Energy Level s. In order to extract maximum microscopic information from experimental results, spectroscopy is often coupled with Chemical Computations . REFERENCES SEE ALSO
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