3.2.2 group 2: alkaline earth metals

• group 2 elements are called alkaline earth metals
• all electron configurations for group 2 end in S2
• structure and bonding of any group 2 element

  • metallic bonding and giant metallic lattice
  • strong electrostatic forces between positive metal ions and delocalised electrons
• atomic radius: distance between the nucleus and outermost electrons
• trend in atomic radius down group 2

  • increases
  • electrons added to new shells which are further from the nucleus and so the electron sheilding increases
  • therefore electrostatic force of attraction between the nucleus and outermost electron decreases
• first ionisation energy: energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous unipositive ions
• trend in first ionisation energy down group 2

  • decreases
  • atomic radius increases and electron shielding increases
  • decrease in electrostatic force of attraction between nucleus and outermost electron
  • therefore less energy required to remove the outer electron
• trend in melting and boiling points down group 2

  • decreases
  • all have metallic lattice structure
  • atomic radius increases, electron shielding increases
  • electrostatic force of attraction between positive metal ions and delocalised electrons gets weaker
  • less energy required to overcome metallic bonding
• test for sulphate ions

  reagent- Barium Chloride solution

  1. acidify test solution with hydrochloric acid, remove impurities
  2. add barium chloride solution
  3. if sulphate ions are present a white precipitate of barium sulphate will form, as it is insoluble in water
• why can’t sulphuric acid be used to test for sulphate ions

  Ba2+ ions would react with the sulphate ions in sulphuric acid to give false positives
• equation to show the formation of the white ppt when testing for sulphate ions

  Ba2+ (aq) + SO4 2- (aq) ——> BaSO4 (s)
• test for barium ions

  1. add sulphuric acid
  2. if barium ions present white ppt will form
  3. write a simple ionic equation for the formation of BaSO4 (s)
• group 2 ion flame tests and colours

  • Ca2+ - brick red / orangey red
  • Sr2+ - red
  • Ba2+ - apple green / pale green
• test for Mg2+ cation:

  • add NaOH (aq) to the solution
  • If white precipitate of Mg(OH)2 (s) produced Mg2+ ions present
  • Write a simple ionic equation for the formation of Mg(OH)2 (s)
• ionic equation for the formation of Mg(OH)2

  Mg2+ (aq) + NaOH (aq) ——> Mg(OH)2 (s)
• ionic equation for the formation of BaSO4

  Ba2+ (aq) + H2SO4 (aq) ——> BaSO4 (s)