Group 2: The Alkaline Earth Metals

Atomic radius is the distance from the centre of the nucleus to the outermost electron. It is usually measured as the half the distance between the nuclei of two atoms. 

Overall trends: atomic radii increases, 1st ionisation energy decreases, melting point decreases.

- Each element has an extra shell of electrons: the outer electrons are further away, and there is more shielding, DESPITE an increasing nuclear charge.

- 1st ionisation enegery is the engergy required to remove one mole of electrons from one mole of gaseous atoms. It decreases because each element has an extra shell of electrons so the outer electrons are further away, and there is more shielding, DESPITE an increasing nuclear charge - it requires less energy to remove an electron.

- Melting point depends on the strength of the bonding within the metal. Down the group is easier to break metallic bonds as all group 2 metals "supply" 2 delocalised electrons. Each element has an extra shell of electrons so the delocalised electrons are further away, and there is more shielding, DESPITE an increasing nuclear charge. 

• Melting point depends on structure, radii and the pattern of ions.
  • - Be and Mg are hexagonal
  • Ca Sr and Ba are cubic, which arent as tightly packed as hexagonal, so they are expected to be lower.  
• Charge density gets larger in group 2 the smaller the ion is. As you go down the group, atomic radii increases but the chage stays the same, so the attraction between the nucleus and the electrons decreases, lowering the melting point
  • Mg has an unusually low mp, so is a useful reducing agent, and is used in the extraction of titanium. 

• Be - does not react
• Mg - reacts slowly, so we react it with steam. It burns with a bright white light. Magnesium hydroxide is sparingly soluble and a small amount will dissolve producing an alkaline solution:
  • Mg(s) + H2O(g) ----> MgO(s) + H2(g)
• Ca - reacts steadily. Calcium hyroxide is slightly soluble and will produce an alkaline solution that looks cloudy - any undissolved calcium will form a suspension:
  • Ca(s) + H2O(l) ----> Ca(OH)2 (s) + H2(g)
• Sr - reacts quickly: 
  • Sr(s) + H2O(l) ----> Sr(OH)2 (s) + H2(g)
• Ba - reacts fastest:
  • Ba(s) + H2O(l) ----> Ba(OH)2 (s) + H2(g)
• Sr and Ba are so reactive they must be stored in oil so they do not react with moisture in the air.

• To extract titanium from TiCl4
  • Titanium is strong, although steel is cheaper, titaniium is half as dense, it does not corrode - it forms a protective oxide when exposed to air. Ti makes up 0.66% of earth's crust, but it is difficult to extract. It is extracted from TiO2 and FeTiO3, but reduction involving carbon makes titanium carbide, which is brittle and useless. Reduction with an oxide produces impurities and is difficult and expensive. Titanium (IV) chloride is more easily reduced. It involves two steps: titanium oxide is converted to titanium chloride - 
    • TiO2(s) + 2Cl2(g) + 2C(s) ---> TiCl4(g) + 2CO(g)
  • Then titanium chloride is reduced with magnesium. TiCl4 is heated with liquid magnesium  at 1200oC in an inert atmosphere of helium or argon gas -
    • TiCl4(l) + Mg(l) ---> Ti(l) +2MgCl2(l)
  • The process can take up to 50 hours and 4 days to cool. It is a batch process. Cooled after each operaton and the process begins again.

• Hydroxide solubility gets higher down the group.
• Mg(OH)2 - neutralises acids so is added to toothpaste to stop decay -  bacteria produce acidic toxins. Is sparinly soluble, forms a white suspension called "milk of magnesia". Used as aan antacid - stomach acid is HCl:
  • 2HCl(aq) + Mg(OH)2(s) ---> MgCl2(aq) + 2H2O(l)
• Ca(OH)2 - used in farming to neutralise soil acidity, it increases to pH of soil, needs to be repeated - effectiveness is reduced over time cause of acid rain, plants and fertilisers.

• CaO and CaCO3 are used to remove SO2 from flue gases 
• Burning fossil fuels produces SO2 that pollutes the atmoshpere. This is acidic. It can be removed by reacting it with an alkali - known as wet scrubbing. CaO or CaCO3 can be used. A slurry is made by mixing the CaO or CaCO3 with water. Its sprayed onto flue gases. The SO2 reacts and produces a white solid: calcium sulfite - 
  • CaO(s) + 2H2O(l) + SO2(g) ---> CaSO3(s) + 2H2O(l)
  • CaCo3(s) + 2H2O(l) +SO2(g) ---> CaSO3(s) + 2H2O(l) + CO2(g)

• Sulfates decrease solubility down the group, eventually, BaSO4 is insoluble
• Soluble barium compounds are toxic. is insoluble, so x-rays cant pass through. Provides a contrast medium. Used to show the lower part of digestive system. Eventually passes through harmlessly

• We use the fact that barium sulfate is insoluble to test for sulfate ions - if one solution has barium ions and another contains sulfate ions, when they react, barium sulfate will form which is insolube. Barium chloride is soluble, so a solution with barium chloride will provide barium ions. However, if the reactiong solution contains carbonate ions, the barium will react with the carbonate to form barium carbonate, which will give a false test. SO dilute HCl or HNO3 is added to the barium chloride solution and when the two reactants react, the acid will produce with the carbonate ( acid + carbonate ---> salt + CO2 + water) and the barium can be left to react with the sulfate ion, to confirm the presence of sulate ions in the mixture.