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             The halogens are in group 7 of the periodic table devised by Dimitri Mendeleev and have 7 electrons in their outer shell. The name halogen originates from the Greek for salt makers and is one of the more reactive non-metal groups. All of the halogens exist as diatomic molecules when elements with one covelant bond between the molecules. All halogens displace each other when they are more reactive (have a stronger electronegativity). For example:.
             2NaCl (aq) + F2 (aq) 2NaF (aq) + Cl2 (aq).
             In these reactions you would notice a colour change if there was a reaction.
             Fluorine is a pale yellow gas at room temperature and has a boiling point of 85K. It has an atomic radius of 0.0071 nm and an electronegativity of 4. It is the element at the top of group 7 and will displace all of the halogens. .
             Fluorine is the most reactive element in the group as it has the strongest polarising effect as it has the smallest atomic radius hence the strongest forces of attraction for electrons. Its atomic number is 9 and so therefore its electron configuration is 1s2 2s2 2p5. Due to the few shells it has there is little or no shielding and so therefore this also adds to its reactivity.
             One chemical test could be carrying out the reaction above and if there is a colour change from pale yellow to light green then it is likely to be fluorine. The test for the ion involves adding nitric acid to a compound of the substance followed by silver nitrate, if there is no precipitate it is the fluoride ion, as silver fluoride is soluble where as all other silver halides are insoluble and so form a precipitate. Fluorides are also soluble when dilute ammonia solution is added. .
             When fluorides are reacted with concentrated sulphuric acid it will form a salt .

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