Periodic classification

• Halogens


  Exist as discrete diatomic molecules in all phases (gas, liquid or solid)

  
• Halogens


  • Fluorine and chlorine are gases of pale yellow and greenish yellow colours respectively at room temperature and pressure
  • Bromine is a liquid of red-brown colour
  • Iodine is a metallic-appearing shiny greyish black solid
  • Have irritating odours, and they attack the skin
  • Bromine in particular causes burns that heal slowly

  
• Electronic configuration of halogens

  ns2 np5 (one electron short of the stable octet of the noble gases)

  
• Halogens


  • Fluorine (F)
  • Chlorine (Cl)
  • Bromine (Br)
  • Iodine (I)
  • Astatine (At)
• Halogens are called the "salt formers" from Greek hals, "salt" and gennan, "to form or generate"

  
• First four elements are the common elements of the halogen family but astatine is a rare halogen

  
• Astatine is radioactive and its most stable isotope has a half life of 8.3 hrs
• Topics covered in this chapter

  • The electronic configurations and the occurrence of halogens, the peculiar behaviour of fluorine
  • The volatility of halides and its explanation in terms of van der Waals forces
  • The relative reactivities of halogens as oxidizing agents
  • The properties of hydrogen halides, oxides and oxyacids of halogens
  • The comparison of thermal stability of hydrides in terms of bond energies
  • Reaction of chlorine with sodium hydroxide (hot/cold)
  • The preparation and reaction of bleaching powder
  • The commercial uses of halogens and their compounds as bleaches, refrigerants and aerosols
  • The electronic configurations, physical properties, inertness and isolation of noble gases from air
  • The properties of oxides, fluorides and oxyfluorides of xenon

  
• Van der Waals forces

  The intermolecular attraction is greater in the larger molecules having greater masses
• The van der Waals forces in iodine molecules are stronger than in the smaller molecules of the other halogens
• Ionization energy of halogens

  Fluorine atom holds its electrons tightly, while the electrons are least tightly bound in iodine

  
• The trend in ionization energy of halogens can be correlated with the sizes of the halogen atoms
• Electron affinity of halogens

  Large and negative, halogens gain electrons readily
• Standard electrode potential of halogens

  Large, positive, indicating high oxidizing power

  
• Electronegativity of halogens


  Fairly high

  
• The alkali and alkaline-earth metals include the most reactive electropositive elements and a study of their electronic configurations will help in understanding their properties
• Occurrence of halogens


  • Fluorine: Fluorospar (CaF2), Cryolite (Na3AIF6), Fluoroapatite (Ca5(PO4)3F)
  • Chlorine: Halite (NaCl), Carnallite (KCl. MgCl2.6H2O)
  • Bromine: Brine wells, sea water, NaBr, KBr, MgBr2
  • Iodine: NaIO3, NaIO4, deposits in Chile brine wells
• None of the halogens is found in free form in nature

  
• Alkali


  Means 'The Ashes' in Arabic, referring to the ashes of plants composed chiefly of sodium and potassium
• Alkali metals


  • Lithium
  • Sodium
  • Potassium
  • Rubidium
  • Caesium
  • Francium
• Peculiar behaviour of fluorine

  • Small size of F atom and of F- ion
  • High first ionization energy and electronegativity
  • Low dissociation energy of F2 molecule as compared to Cl2 and Br2
  • Restriction of the valence shell to an octet
  • Direct combination with inert gases
• Alkaline-earth metals


  • Beryllium
  • Magnesium
  • Calcium
  • Strontium
  • Barium
  • Radium
• Ionic fluorides have higher lattice energies than the other halides, making them insoluble in water
• The s-block elements are the metals in Group IA and Group IIA of the periodic table

  
• Fluorine is the only element that combines directly with noble gases like Kr, Xe, and Rn forming their fluorides
• The s-block elements are called the s-block because s-orbitals are being filled in their outermost shells
• Oxidizing power of halogens

  Decreases with increase in atomic number, with fluorine having the highest and iodine the least
• Group IA elements


  • Alkali metals (except hydrogen)
• The order of decreasing oxidizing power is F2 > Cl2 > Br2 > I2
• Group IIA elements


  • Alkaline-earth metals
• Alkali metals


  Have only one electron in 's' orbital of their valence shell, lose this electron to form monopositive ions M+, form ionic compounds, show +1 oxidation state
• Alkaline-earth metals


  Have two electrons in 's' orbital of their valence shell, lose these two electrons to form dipositive ions M2+, form ionic compounds, show +2 oxidation state
• Factors affecting oxidizing power of halogens

  Energy of dissociation, electron affinity of atoms, hydration energies of ions, heats of vapourization (for Br2 and I2)
• Fluorine can oxidize all the halide ions to molecular halogens

  
• Chlorine can oxidize both bromide and iodide ions, while bromine can oxidize only iodide ion

  
• Fluorine and chlorine can oxidize various coloured dyes to colourless substances
• Hydrogen halides (HX)


  • All halogens react with hydrogen forming hydrides
  • HF is a colourless volatile liquid, while other hydrogen halides (HCl, HBr, HI) are colourless gases at room temperature
  • They give fumes in moist air and are strong irritants

  
• Hydrogen fluoride attacks glass and can be handled in teflon containers or if absolutely dry, in copper or stainless-steel vessels kept under vacuum
• Liquid HF is strongly hydrogen bonded and is a viscous liquid, with its viscosity less than that of water due to the absence of a three dimensional network of H-bonds
• Gaseous HF consists of an equilibrium mixture of monomers and cyclic hexamers due to hydrogen bonding