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Unformatted text preview: 1 Halogens (group 17) Shriver, Chapter 16 F, Cl, Br, I, At Halogen – from the Greek, for salt former All diatomic as elements F 2 and Cl 2 are gases, Br 2 is a liquid, I 2 is a volatile solid F 2 bp = -188 °C Cl 2 bp = -35 °C Br 2 bp = -59 °C I 2 bp = +184 °C A consequence of stronger dispersion forces with heavier elements (more electrons, more polarizable) A similar explanation for the relative boiling points of CF 4 (-128 °C) and CCl 4 (77 °C) All are colored, with the color determined by the HOMO-LUMO gap (filled π∗ to σ *) All are so reactive that the elemental forms do not occur in nature (instead, they are usually found as the halide salts, almost all of which are soluble) Electron configuration ns 2 np 5 , they require one electron to complete their octet Characterized by high electron affinity and high ionization energy F is most electronegative element; by definition its oxidation state is never positive All other halogens can have +ve oxidation states, even up to +7 The X-X bond strength decreases down the group (except F 2 ) Elemental properties are strikingly similar in group 17; F is anomalous but not dramatically so The lower electron affinity of F relative to Cl despite its higher electronegativity is caused by e--e- repulsion in the small F atom The same effect is responsible for the weak F-F bond, although X-F bonds are generally very strong Origins and uses Fluorine Name from Latin/French fluere for flux (flow); used to help make molten metals flow Use of fluorspar (fluorite, CaF 2 ) as flux described in 1529 In 1670, fluorspar treated with conc H 2 SO 4 was shown to etch glass Pale yellow gas, highly reactive even towards noble gases even water burns in F 2 ! Until recently, not freed from its compounds by any chemical reaction Therefore never found free in nature and extremely difficult to isolate Fluorine martyrs – 74 years of effort to isolate F2, resulting in many injuries and deaths HF is extremely dangerous: Its burns are painless (HF damages nerves) so not perceived immediately HF being weak acid in water, it is not dissociated, and therefore passes uncharged through lipid layers, causing deep burns 2 Reacts with calcium in cells causing hypocalcemia and cardiac arrest Therapy is injection with Ca salts HF spills over 75 in 2 (2.5% of body surface) can be fatal F 2 first formed in 1869, by electrolysis However, combines explosively with H 2 (and did, in that first experiment) First isolated in 1886 by Moissan, by electrolysis of HF/KF in a cell made completely of Pt, isolated from H 2 Moissan won the Nobel prize in 1906 2 CaF 2 + H 2 SO 4 → 2 HF + CaSO 4 HF + KF → KHF 2 2 KHF 2 → 2 HF + H 2 + F 2 (KHF 2 aids electrolysis by increasing conductivity, relative to HF) Can be stored in steel or monel (Ni/Cu), since surface is fluoride-passivated In 1986: first chemical method found: K 2 MnF 6 + 2 SbF 5 → 2 KSbF 6 + MnF 3 + ½ F 2 Large scale production necessitated by Manhattan project, to separate fissile U-235 (3%...
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