![]() All halogens are nonmetals and display many similarities in chemical behavior and in the properties of their compounds with other elements. The remaining halogens do not exist in the free form, but are found primarily as anionic constituents in a large number of minerals and in marine water. Astatine, a radioactive element, does not occur naturally except in trace amounts. THE HALOGENS, members of Group 17 of the periodic table (Group VIIA, older nomenclature), include the five elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Scheele liberated chlorine by reaction of sodium chloride, sulfuric acid, and manganese dioxide, heatĤNaCl + 2H2 SO4 + MnO2 −→ 2Na2 SO4 + MnCl2 + 2H2 O + Cl2 (g), ![]() Chlorine was the first halogen to be isolated as a free element. Dibromoindigo (Tyrian purple) was employed as a dye by the Greeks and Romans, and medieval alchemists used aqua regia, a mixture of hydrochloric (HCl) and nitric (HNO3 ) acids, to dissolve gold. and the Bible contains numerous references to the importance of this substance. Archeological evidence indicates that rock salt (NaCl) was used by people as early as 3000 B.C. Compounds containing halogens have been known for thousands of years. Were discovered, the meaning was extended to include all members of this family. Higher positive values indicate a greater tendency for reduction. Standard reduction potential Potential difference developed between two electrodes, where the reaction at one electrode is the oxidation of hydrogen gas at 1 atm pressure to H+ at unit activity (about 1 M concentration) and the reaction at the second electrode is the reduction of interest. The number of electrons lost due to oxidation denotes a positive oxidation state, while the number of electrons gained due to reduction denotes a negative oxidation state. Oxidation state or oxidation number Number of electrons lost or gained by the element as a result of compound formation. Lewis base Electron-pair donor (nucleophile). Lewis acid Electron-pair acceptor (electrophile). P1: GLM Final Pages Encyclopedia of Physical Science and Technologyġ98 More negative lattice energies indicate that the ions are held more tightly in the crystal. Lattice energy Energy released when ions come together from infinite separation to form one mole of a crystal. More positive ionization potentials indicate that the electron is more tightly held. ![]() Ionization energy or ionization potential Energy required to remove the most loosely held electron from an isolated atom in the gas phase in order to form a positive ion in the gas phase. Covalent indicates that the atoms are held together through the sharing of electrons. Ionic indicates that the interaction between atoms is primarily electrostatic. Ionic and covalent Adjectives describing the way in which atoms are held together in a chemical compound. ![]() As the electronegativities of atoms A and B increase, the strength of the hydrogen bonding also increases. Hydrogen bonding Attraction of a partially positive hydrogen bonded to atom A (generally N, O, or F) in one molecule to a partially negative atom B either in the same or another molecule. Free radical Species with one or more unpaired electrons. The greater the difference in the electronegativities of two atoms bonded to each other, the greater is the charge separation in the bond. Electronegativity Ability of a bonded atom in a molecule to attract electrons to itself. More positive electron affinities indicate that the electron is more tightly held. Electron affinity Energy required to remove the most loosely held electron from an isolated negative ion in the gas phase in order to form a neutral atom in the gas phase. Carbonium ion Positively charged carbon ion that acts as a Lewis acid or electrophile. GLOSSARY Azeotrope Constant-boiling mixture formed during distillation, when the composition of the vapor phase becomes identical to the composition of the liquid phase. The Elements The Hydrogen Halides (Hydrohalic Acids) Other Inorganic Halides Interhalogen Compounds Halogen Oxides Oxyacids and Their Anions Other Inorganic Halogen Compounds Astatine Organic Halogen Compounds Halogen Chemistry Marianna Anderson Busch Baylor University Encyclopedia of Physical Science and Technology
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