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Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting: *Octet rule is used with Lewis structures for main group elements, especially the lighter ones such as carbon, nitrogen, and oxygen, *Eighteen electron rule in inorganic chemistry and organometallic chemistry of transition metals, *Polyhedral skeletal electron pair theory for cluster compounds, including transition metals and main group elements such as boron including Wade's rules for polyhedral cluster compounds, including transition metals and main group elements and mixtures thereof. Atoms that do not obey their rule are called "electron-deficient" when they have too few electrons to achieve a ''noble gas configuration'', or "hypervalent" when they have too many electrons. Since these compounds tend to be more reactive than compounds that obey their rule, electron counting is an important tool for identifying the reactivity of molecules. ==Counting rules== Two methods of electron counting are popular and both give the same result. *The neutral counting approach assumes the molecule or fragment being studied consists of purely covalent bonds. It was popularized by M.L.H. Green along with the L and X ligand notation.〔http://www.columbia.edu/cu/chemistry/groups/parkin/mlxz.htm〕 It is usually considered easier especially for low-valent transition metals. *The "ionic counting" approach assumes purely ionic bonds between atoms. It rewards the user with a knowledge of oxidation states, which can be valuable. One can check one's calculation by employing both approaches, though it is important to be aware that most chemical species exist between the purely covalent and ionic extremes. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「electron counting」の詳細全文を読む スポンサード リンク
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