halogen

The elements in group 17 of the periodic table are fluorine, chlorine, bromine, iodine, and astatine. They form the most typical salts with many metals, so in the 18th century, they were named halogens in France, after the Greek λα (halas) "salt" and γενν (genno) "to make". Halogen elements are chemically active, so they do not exist in nature as a single element. Chlorine is widely found in nature, followed by fluorine, and much less so by bromine and iodine. About 30 isotopes of astatine are known, but the longest-lived one is an unstable radioactive element with a half-life of 8.3 hours, and exists only in trace amounts. Halogens are very important elements because they form many useful compounds. The elements in this group have similar chemical properties, and as the atomic number increases, the electronegativity decreases and the melting point and boiling point increase. All of the elements are diatomic molecules _X2 , and are colored, and the gas has a pungent odor. At room temperature, fluorine and chlorine are gases, bromine is liquid, and iodine is solid. All of these elements have an s ² p ⁵ electron configuration in their outermost shells, and can easily gain an electron to become monovalent anions, forming ionic compounds with electropositive elements and covalent compounds with electropositive elements. Reactivity decreases with increasing atomic number. In particular, fluorine and chlorine often oxidize metals and nonmetals, changing them to higher oxidation states than those oxidized by bromine and iodine. The only oxidation state of fluorine that is stable in aqueous solution is -1, while the main oxidation states of chlorine, bromine, and iodine are -1, 1, 3, 5, and 7. They form various oxoacids, HXO _n , and all are strong oxidizing agents. Their compounds include inorganic compounds such as hydrogen halides, oxides, salts of halides, and interhalogen compounds, as well as organic compounds that undergo addition or substitution reactions to form many organic halides. All halogens except the radioactive element astatine are produced industrially on a large scale, with chlorine being by far the most widely produced, followed by iodine.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

周期表17族に属する元素で，フッ素，塩素，臭素，ヨウ素，アスタチンの5元素をいう．多くの金属ともっとも典型的な塩をつくるので，18世紀にフランスでギリシア語のλα(halas)“塩”およびγενν(genno)“つくる”にちなんでハロゲンと名づけられた．ハロゲン元素は化学的に活発なため，天然には単体で存在しない．塩素は天然に広く存在し，フッ素がこれにつぎ，臭素，ヨウ素ははるかに少なく，アスタチンは約30種の同位体が知られているが，もっとも寿命の長いもので半減期8.3 h の不安定な放射性元素で微量しか存在しない．ハロゲンは多くの有用な化合物をつくるので，非常に重要な元素である．この族の元素は化学的性質がよく似ており，原子番号の増加とともに，電気陰性度は減少し，融点，沸点は高くなる．単体はすべて二原子分子 X₂ で有色，気体は刺激臭をもつ．常温でフッ素，塩素は気体で，臭素は液体，ヨウ素は固体である．これらの元素はすべてその最外殻の電子配置はs² p⁵で，電子1個を得て一価の陰イオンになりやすく，陽性の元素とはイオン結合性の，陰性の元素とは共有結合性の化合物をつくる．反応性は原子番号の増加とともに減少する．とくにフッ素と塩素はしばしば金属や非金属を酸化し，臭素やヨウ素による酸化よりも高い酸化状態に変化させる．水溶液中で安定なフッ素の唯一の酸化数は－1で塩素，臭素，ヨウ素のおもな酸化数は－1，1，3，5，7である．種々のオキソ酸HXO_nをつくり，すべて強い酸化剤である．化合物はハロゲン化水素，酸化物，ハロゲン化物の塩，ハロゲン間化合物などの無機化合物のほか，有機化合物に付加または置換反応を起こして多くの有機ハロゲン化物をつくる．放射性元素アスタチンを除くすべてのハロゲンは工業的に大規模に生産されていて，そのなかでは塩素の生産が群を抜いて多く，ヨウ素がそれに続いている．

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