3.2.2 Group 2, the alkaline earth metals

• How do group 2 elements react?

  They each have 2 electrons in their outer shell. They lose these 2 valence electrons to form 2+ ions
• Why does atomic radius increase down the group?

  Each successive element has an additional energy level
• Why does first ionisation energy decrease down the group?

  1. Each successive element has an extra electron shell
  2. The extra inner shells shield the outer electrons from the attraction of the nucleus
  3. The extra electron shell means the valence electrons are further away from the nucleus, greatly reducing the nuclear attraction
  4. It is therefore easier to remover outer electrons, resulting in a lower first ionisation energy
• Why does reactivity increase down the group?

  1. First ionisation energy decreases due to increasing atomic radius and inner shielding
  2. Group 2 elements lose electrons when they react. The lower the 1st and 2nd ionisation energies (easier it is to lose electrons), the more reactive the element is
• Why does melting point generally decrease down the group?

  1. Group 2 elements have typical metallic structures, with positive ions in a crystal structure surrounded by delocalised valence electrons
  2. Going down the group the metal ions get bigger(the number of delocalised electrons and ion charge doesn't change)
  3. The larger the ionic radius, the further away the delocalised electrons are from positive nuclei and the less attraction they feel. Less energy is needed to break the bonds so melting point generally decreases
• What element is the exception to the trend in melting point down the group?

  Magnesium because the crystal structure changes
• How do group 2 elements react with water?

  • M (s) + 2H2O (l) -> M(OH)2 (aq) + H2 (g)
  • Oxidation state changes from 0 to +2 (oxidised)
  • Metal hydroxide and hydrogen
  • React more readily down the group

  
• How do solubility trends change down the group?

  Compounds of Group 2 elements that contain singly charged negative ions (e.g. OH-) increase in solubility down the group, whereas compounds that contain doubly charged negative ions (e.g. SO4²-) decrease in solubility down the group

  Mg(OH)2 is sparingly soluble

  BaSO4 is insoluble

  
• What is the test for sulfate ion?

  1. Add HCl to get rid of any sulfites or carbonates, which will also produce a white precipitate
  2. Add BaCl to a solution containing sulfate ions
  3. A white precipitate of BaSO4 forms

  
• How are Group 2 elements used to neutralise acidity?

  Known as the alkaline earth metals, many of their compounds are used as neutralising agents

  1. Calcium hydroxide (slaked lime, Ca(OH)2) is used in agriculture to neutralise acid soils
  2. Magnesium hydroxide (Mg(OH)2) is used in some indigestion tablets as an antacid (a substance which neutralises excess stomach acid)

  
• How is barium sulfate used in barium meals?


  A patient swallows the barium meal. The barium sulfate coats the tissues, making them show up on x-rays, showing the structure of organs. BaSO4 is opaque to x-rays, which normally pass straight through soft tissues. As BaSO4 is insoluble it won't be absorbed by the digestive system, so can be used even though it's toxic
• How is magnesium used in the extraction of titanium?


  1. Titanium(IV) oxide (TiO2) is converted to titanium(IV) chloride (TiCl4) by heating it with carbon in a stream of chlorine gas
  2. TiCl4 is then purified by fractional distillation, before being reduced by magnesium in a furnace at almost 1000°C

  
• What are flue gases?


  Gases emitted from industrial exhausts and chimneys
• How are calcium oxide and calcium carbonate used to remove sulfur dioxide from flue gases?

  1. Burning fossil fuels to produce electricity produces sulfur dioxide, which pollutes the atmosphere
  2. The acidic sulfur dioxide can be removed by reacting with an alkali - wet scrubbing
  3. Powdered calcium oxide(CaO) and calcium carbonate(limestone, CaCO3) can both be used for this
  4. A slurry is made by mixing the calcium oxide or calcium carbonate with water. It's then sprayed onto the flue gases. The SO2 reacts with the alkaline slurry and produces a solid waste product, calcium sulfite