Group 2 elements (The alkaline earth metals)

• What is the trend in atomic radius from Mg-Ba?

  The atomic radius increases down group 2. This is because more electrons are being added, causing extra shells and shielding.
• The atomic radius increases down group 2.
• What is the trend in first ionisation energy from Mg–Ba?

  First ionisation energy decreases down group 2. This is because of increased shielding, causing weaker attraction between the nucleus and outer electrons.
• First ionisation decreases down group 2:

  $\rightarrow$ Bigger molecules, increase in size.

  $\rightarrow$ Increase in shielding.

  $\rightarrow$ Outer electrons further away from the nucleus.

  $\rightarrow$ Weaker attraction.

  $\rightarrow$ Easier to remove.
• What is the trend in melting point in group 2?

  Melting point generally decreases as we go down the group.
• Group 2 elements have metallic structures with strong attraction between positive metal ions and a sea of negative delocalised electrons.
• Melting point decreases down group 2:

  $\rightarrow$ Size of metal ions increases but the number of electrons stays the same. (Nuclear charge also stays the same)

  $\rightarrow$ Larger ions mean greater distance between the positive nucleus and delocalised electrons.

  $\rightarrow$ Weaker attraction.

  $\rightarrow$ Meaning it is easier to break bonds, less energy is required.
• Magnesium is an exception to the trends in melting point because the strength of metallic bonding increases, stronger attraction between the oppositely charged ions.
• Beryllium does not react with water.
• Magnesium reacts very slowly with water, forming magnesium hydroxide.

  $Mg+$$2H_2O(l)\ \rightarrow\ Mg\left(OH\right)_2\ +$$H_2$
• Magnesium reacts readily with steam, forming the oxide of magnesium, this is formed instead of magnesium hydroxide because it is not stable at higher temperature and will decompose.

  $Mg\ +$$\ H_2O(g)\ \rightarrow\ MgO\ +$$\ H_2O$
• The reactivity of the group 2 metals reaction with water increases down the group.
• Calcium reacts with cold water, producing bubbles of hydrogen and a white precipitate, calcium hydroxide.

  $Ca\ +$$\ 2H_2O\ \rightarrow\ Ca\left(OH\right)_2\ +$$\ H_2$
• Strontium reacts with cold water, producing bubbles of hydrogen and a thin white precipitate, strontium hydroxide.

  $Sr\ +$$\ 2H_2O\ \rightarrow\ Sr\left(OH\right)_2\ +$$\ H_2$
• Barium reacts with cold water, producing bubbles of hydrogen and a very thin white precipitate, barium hydroxide.

  $Ba\ +$$\ 2H_2O\ \rightarrow\ Ba\left(OH\right)_2\ +$$\ H_2\$
• The relative solubility of Group 2 hydroxides increases down the group, they become more soluble.

  • $BaSO_4$ is insoluble.
  • $Mg\left(OH\right)_2$ is sparingly soluble, as the solution is slightly alkaline indicating that some hydroxide ions are dissolved in the water.
• The use of acidified $BaCl_2$ solution to test for sulfate ions:

  1. Add $1cm^3$ of dilute hydrochloric acid, the $HCl$ removes any other ions that may affect the test.
  2. Add $1cm^3$ of aqueous barium chloride solution
  3. If a thick white precipitate is formed, sulfate ions are present.
• Uses of group 2 elements and their compounds: Magnesium

  Magnesium is used in the extraction of titanium, it is very difficult to extract titanium from its ores (e.g $TiO_2$) due to it forming a brittle carbide.

  Titanium oxide is initially converted to the chloride which is subsequently reduced to titanium by reaction with magnesium.

  $TiCl_4\ +$$\ 2Mg\ \rightarrow\ Ti\ +$$\ 2MgCl_2$
• Uses of group 2 elements and their compounds: $Mg\left(OH\right)_2$

  Magnesium hydroxide is used to alleviate constipation and as an antacid, to neutralise excess acid in the gut. Due to it being sparingly soluble and only slightly alkaline, it will not irritate the oesophagus.
• Uses of group 2 elements and their compounds: $Ca\left(OH\right)_2$

  Calcium hydroxide is used to neutralise acidic soil. Low pH levels in soil causes soluble aluminium and manganese ion levels to increase toxic proportions, restricting root growth. This affects the plants chemical and biological properties.
• Uses of group 2 elements and their compounds: $CaO\ or\ CaCO_3$ = Catalytic converter

  Calcium oxide and carbonate is used to neutralise sulfur dioxide, which may be produced during the combustion of fossil fuels. This prevents acid rain.
• Uses of group 2 elements and their compounds: $BaSO_4$

  Barium sulfate can be eaten as part of a 'barium meal'. Barium is good at absorbing X-rays, therefore the outline of the gut can be located during X-rays.

  Although barium ions are very toxic, this technique is harmless because barium sulfate is insoluble and cannot be absorbed into the blood.