Aspects of halogens

aspects of halogens

1. Group 17 T he Halogens: Fascinating aspects 1

2. fluorine chlorine bromine iodine I Br Cl F At The elements in group 17 of the periodic table, on the right , are called the HALOGENS . astatine 2

3. Why the name ….. HALOGEN?? * Halogen-metal compounds are salts occurring in sea water (e.g. NaCl ; sodium chloride), * halos = sea salts; genes =born. 3

4. STATE OF HALOGENS Astatine is radioactive and is one of the rarest of the chemical elements. 4

5. Halogen Greek name meaning Fluorine fluere Flow ( Fluorite melts when heated) Chlorine chloros Greenish yellow Bromine bromos Stench Iodine iodes Violet Astatine astatos Unstable (decay product of U, Th ) 5

6. poisonous and smelly . They are all toxic or harmful because they are so reactive GENERAL PROPERTIES OF THE HALOGENS Have 7 e- in their outer shells Exists as separate diatomic molecules ... F 2 , Cl 2 , Br 2 Colored and color darkens as we go down through the group non-metals do not conduct electricity never found free in nature because of their reactivity – they are found as compounds with metals ( eg . NaCl ). 6

7. Electron configuration of halogens Fluorine 1s 2  2s 2  2p 5 Chlorine  [Ne]3s 2  3p 5 Bromine  [ Ar ]3d 10  4s 2  4p 5 Iodine  [Kr]4d 10  5s 2  5p 5 Astatine [ Xe ]4f 14  5d 10  6s 2  6p 5 7

8. All halogens have seven electrons in their outer shell but one e- short of the nearest noble gas configuration. Electronic structure of halogens...... fluorine 2,7 (9) chlorine 2,8,7 (17) bromine 2,8,8,7 (35) They can easily attain a full and complete outer shell by gaining one electron. They all gain an electron in reactions to form negative ions with a -1 charge. So, they have similar chemical properties . Fluorine is the most electronegative element in the periodic table and it has no  d   orbitals in its valence shell, so it can't expand its valence shell. Cl 2 , Br 2 and I 2 have valence shell  d   orbitals and can expand their valence shells to hold as many as 14 valence electrons. 8

9. All halogen atoms require one more electron to obtain a full outer shell to become STABLE (8e-). How do halogen molecules exist? Each atom can achieve this by sharing one electron with another atom to form a single covalent bond . +  F F F F So all halogens exist as diatomic molecules : F 2 , Cl 2 , Br 2 and I 2 . 9

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11. Physical properties of halogens Ionization energy Electron affinity Electronegativity Oxidizing states Oxidizing power Metals are electropositive ( eg . Na) Non-metals are electronegative (Cl 2 ) 11

12. Ionization Energy (decreases down the group) I onization energy  ( IE ) is defined as the amount of energy required to remove the most loosely valence electron, of an isolated gaseous atom to form a  cation . If the outer valence electrons are far away from the nucleus, it does not take as much energy to remove them. So the energy required to pull off the outermost electron will be less for the elements at the bottom of the group since there are more energy levels. Iodine lose an e - and forms I + Also, the high ionization energy makes the element appear non-metallic. Iodine and astatine display metallic properties 12

13. Electronegativity (decreases down the group) Electronegativity  is a measure of the tendency of an atom to attract a bonding pair of electrons towards itself. The valence electrons will be at a distance from the nucleus as we go down the group and therefore, the nucleus and the electrons are  not that much attracted to each other. An increase in shielding is observed.   Electronegativity  therefore decreases down the group. So electrons can be easily removed from iodine. 13

14. Electron Affinity (decreases down the group) Electron affinity  of an atom or molecule is the amount of energy  released  or  spent  when an electron is added to a neutral atom or molecule in the gaseous state to form a negative ion i.e X - Since the atomic size increases down the group, electron affinity generally decreases but only gradually (At < I < Br <  F  <  Cl ) However, fluorine has a lower electron affinity than chlorine and this can be explained by the small size of fluorine, compared to chlorine. 14

15. Oxidizing states of halogens Halogen Oxidation States in Compounds Fluorine (always) - 1 Chlorine -1, +1, +3, +5, +7 Bromine -1, +1, +3, +4, +5 Iodine -1, +1 , + 5, +7 Astatine -1, +1, +3, +5, +7 -1 NaCl , +1 HClO , +3 BrF 3 , +5 HClO 3 , +7 IF 7 15

16. Oxidizing Power (decreases down the halogen group) The halogens are strong oxidizing agents. The oxidizing power decreases from fluorine to Iodine. Fluorine is the strongest oxidizing agent. It oxidizes other halide ions to halogens in solution or when dry. F 2 + 2 X -  2F - + X 2 ; X - = Cl - , Br - , I - (F 2 has high e- affinity) Halogen of low atomic number oxidizes the halide ion of higher atomic number. 16

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18. Oxidation or reduction? 18

19. How does electron structure affect reactivity? Reactivity of alkali metals decreases going down the group The atoms of each element get larger going down the group. This means that the outer shell gets further away from the nucleus and is shielded by more electron shells. The farther the outer shell is from the positive attraction of the nucleus, the harder it is to attract another electron to complete the outer shell. This is why the reactivity of the halogens decreases going down group 17 . decrease in reactivity F Cl Br Reactivity: F > Cl > Br > I 19

20. Difficulties encountered in the isolation of fluorine Fluorine is so reactive that it attacks the materials of all the vessels. For example, fluorine attacks carbon and silicon present in the glass vessels forming CF 4 and SiF 4 respectively. It also attacks the electrodes and vessels made of Pt to form PtF 4 . Lead, iron and tin vessels were also found to be unsuitable for the preparation. Isolation of fluorine from HF got failed because HF is so stable that it cannot be decomposed by oxidizing agents. 20

21. Isolation of Fluorine: Dennis ’ Method: F luorine is prepared by the electrolysis of fused potassium hydrogen fluoride. Electrolysis is carried out between graphite electrodes in a V-shaped electrically heated copper tube. The ends of the tube are covered with copper caps into which the graphite electrodes are fixed with Bakelite cement. The copper tube is thickly ba gged to prevent loss of heat. 21

22. Fluorine liberated at the anode is passed through the U-tube containing sodium fluoride. This removes the hydrogen fluoride vapors coming with fluorine. 22

23. Abnormal behavior of fluorine Fluorine differs from the other members of the halogen family. This is due to Small size High electronegativity Non availability of d orbitals in fluorine   23

24. Peculiarity of fluorine  Oxidation state : Halogens exhibit oxidation states of -1,0,+1,+3,+5 and +7 where as fluorine exhibits only -1 oxidation state Reactivity: Fluorine is the most reactive element among the halogens. This is due to strong electrostatic repulsion between the non bonding electrons present in fluorine molecule. The F- F bond is very weak. Behavior of hydrogen fluoride: Hydrogen fluoride has abnormally high melting and boiling points than the other hydrogen halides due to hydrogen bonding. Formation of HF 2 - ions : Due to hydrogen bonding the fluoride ion forms the anion HF 2 - . On the other hand halides do not form such ions. E.g., KHF 2 24

25. Peculiarity of fluorine ….. continued Maximum covalency : As fluorine lacks d orbitals it cannot have covalency more than one. But the other halogens can have covalency up to 7. ( eg . IF 7 ) Formation of SF 6 : Due to small size and high electronegativity , fluorine brings about the + 6 state in sulphur forming SF 6 . Other halogens do not form such hexahalides Complex formation: Flouride ion a greater tendency to form [FeF 6 ] 3- , [AlF 6 ] 3- Formation of polyhalides : Due to the non-availability of d- orbitals fluorine does not form any polyhalides . But other halogens form polyhalide ions. E.g.   [Br 2 ] + , [I 2 ] + , [Cl 3 ] + , [Br 3 ] + , [I 3 ] + [I 3 ] − , [Br 4 ] 2− , [I 4 ] 2− etc 25

26. Electropositive nature of iodine It has the lowest  electronegativity value , i.e. highly electropositive. It behaves like a metal Like the other halogens, it has one electron short of a full octet and reacts with many elements in order to complete its outer shell, although in keeping with periodic trends, it is the weakest oxidizing agent among the stable halogens Similarly, the iodide anion, I − , is the strongest reducing agent among the stable halogens, being the most easily oxidized back to diatomic I 2 . 26

27. Basic Iodine Metallic or basic properties increase down the group Among the halogens, iodine shows strong basic character. For e.g. Iodine has a metallic lustre has a tendency to form unipositive I + ion and tripositive I 3 + ion. 27 Atomic size increases from fluorine to iodine. Thus the nucleus of I 2 h olds the electrons in the orbit less firmly. So Iodine looses one e- easily f orming unipositive cation . I 2  2I + + 2e -

28. Evidence for basic iodine ICl acts as a strong electrophilic iodinating agent. This again confirms the basic character of iodine. 28

29.   IODINE TEST …. A TEST FOR STARCH Starch gives an intense " blue-black " color upon addition of aqueous solutions of the  triiodide anion (I 3 - ) , due to the formation of an intermolecular charge-transfer complex . In the absence of starch, the brown color of the aqueous solution remains. This interaction between starch and triiodide is the basis for iodometry. 29 KI + I 2  I 3 -

30. Uses of halogens Halogen Uses Fluorine In tooth pastes, Teflon® Chlorine purify water in wells, swimming pools etc as it kills bacteria (chlorination) Bromine Fire extinguishers, disinfectant, camer a films Iodine Iodized salt, in artificial rain, dyes, specialized soaps, lubricants, photographic film, tincture, and pharmaceuticals, test for starch, treatment of thyroid disorders 30

31. Facts about halogens in human … There are 3 to 6 grams of fluorine, 95 grams of chlorine, 260 milligrams of bromine, 10 to 20 milligrams of iodine in a 70 kg person. Chlorine is found in human blood in a concentration of 0.3%. 31

32. Interhalogen Compounds …. 32

33. Interhalogen Compounds These covalent compounds are formed when two different halogens react . These are formed due to the electronegativity difference among the halogens. Classification: Interhalogens have the general formula Ax n where n=1, 3, 5 &7. Type AX eg : ClF , BrF , BrCl , ICl , Ibr Type AX 3: eg : ClF 3, BrF 3, ICl 3 Type AX 5 eg : BrF 5 ClF 5 Type AX 7 eg : IF 7 When representing the compound, the less electro negativity element has to be written first. 33

34. Iodine mono chloride ….. ICl is formed by passing chlorine over solid iodine at temperature below 0 C. It is a red-brown chemical compound melts near room temperature. Because of the difference in the electronegativity of iodine and chlorine,  ICl  is highly polar ; I + Cl - * In organic synthesis, estimation of iodine No. of oils and as a source of I + . 34

35. Bromine trifluoride … BrF 3 It is obtained by mixing bromine vapor and fluorine in a stream of nitrogen at 20 o C.   It is a straw-colored liquid with a pungent odor.  It is a powerful  fluorinating agent   It is used to produce  uranium hexafluoride  (UF 6 ) in the processing and reprocessing of nuclear fuel. 35

36. Bromo pentafluoride … BrF 5 It is pale yellow liquid B y the direct reaction of bromine with excess fluorine at temp. over 150°C Br 2  + 5 F 2  → 2 BrF 5 It is an extremely effective fluorinating agent, converting most uranium compounds to the hexafluoride at room temperature like BrF 3 . 36

37. Iodine heptafluoride ….IF 7 It is a colorless gas It is prepared by passing a mixture of iodine pentafluoride vapors and fluorine through a platinum tube at 300 C. It is a strong oxidizing agent Used to prepare periodic acid. 37

38. Shapes of interhalogens 38

39. Pseudohalides and Pseudohalogens … 39

40. Pseudohalides & Pseudohalogens Pseudohalides are univalent negative inorganic radicals, composed of two or more electronegative atoms (ions), which exhibit reactions similar to those of the halide ions (X - ). E.g.  Cyanide CN - , Thiocyanide SCN - , Azide N 3 - Covalent dimers of such pseudohalides are called pseudohalogens  These possess properties similar to those of halogens. Examples., Cyanogens (CN) 2 - Thiocyanogen (SCN) 2 Selenocyanogen ( SeCN ) 2 etc. 40

41. Similarities between pseudohalogens and halogens  1. Dimeric and isomorphic nature: Cl 2 is isomorphous to (CN) 2 ; Br 2 is isomorphous to (SCN) 2 . 2. Like halogens, pseudohalogens combine with metals like silver, lead &mercury to form insoluble salts. AgCN , Pb (CNS) 2 (CN, CNS are pseudohalogens ) Ag + + Cl -  AgCl ; Ag + + CN -  AgCN 3. Pseudohalogens also react with alkalis Cl 2 + 2OH -  Cl - + OCl - + H 2 O; (SCN) 2 + 2OH -  SCN - + OSCN - + H 2 O   41

42. Similarities … continued 4. Formation of monobasic hydracids: Cl 2 + H 2  2HCl; (CN) 2 + H 2  2HCN 5. Addition to ethylenic bonds: Like halogens, pseudohalogens also form addition compounds with unsaturated hydrocarbons. 42

43. Preparation and properties of pseudohalogens … 43

44. Preparation of cyanogen (CN) 2 Dry cyanogen is obtained by heating a mixture of mercuric cyanide and mercuric chloride 2Hg(CN) 2  Hg 2 (CN) 2 + (CN) 2 It is a colorless, toxic gas with a pungent odor. The two cyano groups are bonded together at their carbon atoms: N≡C−C≡N Uses: an important intermediate in the production of many fertilizers. It is also used as a stabilizer in the production of nitrocellulose. 44

45. Preparation of Thiocyanogen (SCN) 2 Adding lead thiocyanate to Br 2 in methylene chloride solution at 0 o C. Pb (SCN) 2 + Br 2  PbBr 2 + (SCN) 2 PbBr 2 (solid) is filtered off under argon gas and the filtrate on evaporation yields free thiocyangen . 45

46. Compd Uses Na 3 AlF 6 Manufacture of aluminum BF 3 Catalyst CaF 2 Optical components, manufacture of HF, metallurgical flux ClF 3 Fluorinating agent, reprocessing nuclear fuels HF Manufacture of F 2 , AlF 3 , Na 3 AlF 6 , and fluorocarbons LiF Ceramics manufacture, welding, and soldering NaF Fluoridating water, dental prophylaxis, insecticide SF 6 Insulating gas for high-voltage electrical equipment SnF 2 Manufacture of toothpaste UF 6  Manufacture of uranium fuel for nuclear reactors 46

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48. DISPLACEMENT REACTIONS OF HALOGENS Reactive halogen displaces less reactive one from its salt solution… Cl 2 + 2NaBr  2NaCl + Br 2 48

49. SUMMARY OF GROUP I7 49