17

• Members of Group 17

  • Fluorine
  • Chlorine
  • Bromine
  • Iodine
  • Astatine
• Halogens


  Group 17 elements (Greek halo means salt and genes means born i.e., salt producers)
• Halogens


  • Highly reactive non-metallic elements
  • Great similarity amongst themselves
  • Regular gradation in their physical and chemical properties
• Astatine is a radioactive element
• Halogen abundance


  • Fluorine and chlorine are fairly abundant
  • Bromine and iodine are less abundant
• Sources of halogens


  • Fluorine is present mainly as insoluble fluorides (fluorspar CaF2, cryolite Na3AlF6 and fluoroapatite 3Ca3(PO4)2.CaF2)
  • Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium, but is mainly sodium chloride solution (2.5% by mass)
  • Certain forms of marine life contain iodine in their systems; various seaweeds, for example, contain up to 0.5% of iodine
  • Chile saltpetre contains up to 0.2% of sodium iodate
• Electron configuration of halogens

  Seven electrons in their outermost shell (ns2np5) which is one electron short of the next noble gas
• Atomic radii of halogens


  • Smallest in their respective periods due to maximum effective nuclear charge
  • Atomic and ionic radii increase from fluorine to iodine due to increasing number of quantum shells
• Ionisation enthalpy of halogens

  • They have little tendency to lose electron, thus they have very high ionisation enthalpy
  • Ionisation enthalpy decreases down the group due to increasing atomic size
• Electron gain enthalpy of halogens

  • Halogens have maximum negative electron gain enthalpy in the corresponding periods
  • Electron gain enthalpy becomes less negative down the group
  • Electron gain enthalpy of fluorine is less than that of chlorine due to small size of fluorine atom and strong interelectronic repulsions in the 2p orbitals
• Electronegativity of halogens


  • They have very high electronegativity
  • Electronegativity decreases down the group
  • Fluorine is the most electronegative element in the periodic table
• Halogens have maximum negative electron gain enthalpy in the respective periods of the periodic table because they have the smallest size in their respective periods and therefore high effective nuclear charge, allowing them to readily accept one electron to acquire noble gas electronic configuration
• Physical properties of halogens

  • Fluorine and chlorine are gases, bromine is a liquid and iodine is a solid
  • Melting and boiling points steadily increase with atomic number
  • All halogens are coloured due to absorption of radiations in visible region which results in the excitation of outer electrons to higher energy level
• Fluorine and chlorine react with water, bromine and iodine are only sparingly soluble in water but are soluble in various organic solvents
• The enthalpy of dissociation of F2 is smaller compared to that of Cl2 due to the relatively large electron-electron repulsion among the lone pairs in the F2 molecule
• Although electron gain enthalpy of fluorine is less negative as compared to chlorine, fluorine is a stronger oxidising agent than chlorine due to the low enthalpy of dissociation of F-F bond and high hydration enthalpy of F-
• Oxidation states of halogens

  • -1 oxidation state
  • +1, +3, +5 and +7 oxidation states (for chlorine, bromine and iodine)
• The higher oxidation states of chlorine, bromine and iodine are realised mainly when the halogens are in combination with the small and highly electronegative fluorine and oxygen atoms
• Fluorine exhibits only -1 oxidation state as it has no d orbitals in its valence shell and therefore cannot expand its octet
• Reactivity of halogens


  • All the halogens are highly reactive
  • Reactivity decreases down the group
• The ready acceptance of an electron is the reason for the strong oxidising nature of halogens
• Reactions of halogens with halide ions


  • F2 + 2X- → 2F- + X2 (X = Cl, Br or I)
  • Cl2 + 2X- → 2Cl- + X2 (X = Br or I)
  • Br2 + 2I- → 2Br- + I2
• The decreasing oxidising ability of the halogens in aqueous solution down the group is evident from their standard electrode potentials
• Reactions of halogens with water

  • Fluorine oxidises water to oxygen
  • Chlorine and bromine react with water to form corresponding hydrohalic and hypohalous acids
  • The reaction of iodine with water is non-spontaneous
• Anomalous behaviour of fluorine

  • Ionisation enthalpy, electronegativity, and electrode potentials are all higher for fluorine than expected from the trends set by other halogens
  • Ionic and covalent radii, m.p. and b.p., enthalpy of bond dissociation and electron gain enthalpy are quite lower than expected
  • Anomalous behaviour is due to its small size, highest electronegativity, low F-F bond dissociation enthalpy, and non availability of d orbitals in valence shell
• Properties of hydrogen halides

  • Hydrogen halides dissolve in water to form hydrohalic acids
  • Acidic strength varies in the order: HF < HCl < HBr < HI
  • Stability of the halides decreases down the group due to decrease in bond (H-X) dissociation enthalpy
• Reactivity of halogens towards oxygen

  • Fluorine forms two oxides OF2 and O2F2, with OF2 being thermally stable at 298 K
  • Chlorine, bromine and iodine form oxides in which the oxidation states of these halogens range from +1 to +7
  • Chlorine oxides (Cl2O, ClO2, Cl2O6 and Cl2O7) are highly reactive oxidising agents and tend to explode
  • Bromine oxides (Br2O, BrO2, BrO3) are the least stable halogen oxides and exist only at low temperatures
  • Iodine oxides (I2O4, I2O5, I2O7) are insoluble solids and decompose on heating
• Reactivity of halogens towards metals

  • Halogens react with metals to form metal halides
  • Ionic character of the halides decreases in the order MF > MCl > MBr > MI where M is a monovalent metal
  • Halides in higher oxidation state are more covalent than the ones in lower oxidation state
• Reactivity of halogens towards other halogens

  • Halogens combine amongst themselves to form interhalogens of the types XX', XX3', XX5' and XX7' where X is a larger size halogen and X' is smaller size halogen