Properties of Group 2 The alkaline earth metals

Electronic configurations All the alkaline earth metals have two electrons in their outer shell; Be = 1s2 2s2 Mg = 1s2 2s2 2p6 3s2 Ca = 1s2 2s2 2p6 3s2 3p6 4s2 Ba = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p65s2 When they react these electrons are lost, forming an M 2+ ion.

Group 2 elements are all reactive and are found as minerals. Eg; Emeralds are a beryllium silicate. Celestine is strontium sulphate. Barytes is barium sulphate.

Barium meals Soft tissue normally lets through X rays. But barium is opaque, its heavy atom absorbs X rays. A “barium meal”, barium sulphate, can be administered to examine the gut. Although barium salts are highly toxic the sulphate is safe to use as it is insoluble.

Testing for the presence of sulphate ions. Barium chloride is used as a reagent to test for the presence of sulphate ions. First dilute hydrochloric acid is added to remove any carbonate ions present. CO H + → CO 2 + H 2 O Otherwise white barium carbonate might be precipitated. BaCl 2 + SO 4 2- → BaSO 4 (s) + 2Cl - Barium sulphate forms a white ppt.

Bones and calcium The adult skeleton contains around 1 kg of calcium, as calcium phosphate Ca 3 (PO 4 ) 2. This explains why it is important to have a good source of calcium in the diet.

Be Mg Ca Sr Ba Strontium is in the same group of the periodic table as calcium. Ca Strontium (Sr) So it can replace Ca in bone minerals. Ca Sr Including radioactive Strontium-90 from nuclear tests!

Strontium has four stable isotopes 84 Sr 0.56% 86 Sr 9.86% 87 Sr 7% 88 Sr 82.58% Living organisms have no preferences between the Sr isotopes in their food and drink.

87 Sr forms from decay of 87 Rb, which has a half life of 4.75 x1010 years. The 86 Sr : 87 Sr ratio depends on the underlying rocks. It is greatest, c0.720, in ancient rocks of highland Britain >100 million years old, eg; granite and gneiss. In the younger rocks of lowland Britain it is c.704. These data can be used to track down people’s origins from their bones.

Case Study; Who were the “Beaker People who had helped build the megalithic stone circles…? …buried their dead under round barrows with their typical pots? And introduced metal into Britain?

In 2003 Wessex Archaeology excavated a mass grave which contained the remains of two adult men, an adolescent male and a child at Boscombe Down, near Stonehenge.

The Boscombe burials had very high ratios of 87 Sr: 86 Sr that are not matched in Wessex. But are typical of Cornwall, Man, Lake District and Highland Scotland.

Strontium and the diet. Herbivores have a Sr:Ca ratio c 5x more than the plants. Carnivores ingest less Sr and have even lower ratii. Producers ↑ ↑ Animals preferentially take up Ca rather than Sr to form bone. Omnivorves have intermediate ratii. ← ←

Analysis of Gladiator bones show high Sr, indicating vegetarian diets!

Radium Radium was discovered by Madame Curie, who noticed that the uranium ore pitchblende was 10x more radioactive than expected. From 10 tonnes of ore she extracted 1g of radium chloride, for which she won the Nobel Prize in 1903.

Physical properties Group 2 elements are metals. They have relatively high melting points and boiling points. They are all good conductors of heat and electricity. They are grey, silvery solids at 298K.

Reactivity Group 2 are less reactive than group 1. Na vigorously reacts with water. But Ca reacts very slowly. The difference is due to the amount of energy required to form the metal ion.

Energy needed Li519 kj/mol Na494 K418 Rb402 Cs376 Energy needed Be2660 kj/mol Mg2189 Ca1735 Sr1614 Ba1468

The combined ionisation energies decrease down the group. This means that reactivity increases as it becomes progressively easier to form the M 2+ ion. Energy needed Be2660 kj/mol Mg2189 Ca1735 Sr1614 Ba1468

Group 2 compounds Group 2 compounds are generally white ionic solids. Which dissolve to give colourless solutions.

Flame tests If a wire loop is dipped into a solution or into the solid compound a flame will be coloured. Calcium with a brick red flame. Strontium with a red flame. Barium with a green flame

Oxidation numbers. All uncombined metals have an oxidation number of O. In their compounds group 2 elements have an oxidation number of +2. Forming an ion M 2+ M → M e- As electrons are lost this is oxidation.

Reaction with air Group 2 elements rapidly react with air, forming an oxide film that prevents further reaction. They will burn in air, giving flames of characteristic colours. Eg; Magnesium has a white flame.

Oxidation of group 2 metals Eg; Mg + ½ O 2 → MgO Oxidation N o of Magnesium metal = 0. Oxidation N o of magnesium in its oxide = +2 Mg → Mg e- As its oxidation number becomes more positive magnesium has been oxidised. Oxygen is reduced; ½O 2 + 2e- → O 2-

Group 2 oxides Oxides are white, ionic solids with high MPs. They are prepared by the decomposition of the carbonates or nitrates. Due to its very high MP magnesium oxide is used as a refractory lining in furnaces.

General properties of oxides Generally metal oxides are basic They dissolve in water to form hydroxides; MO (s) + H 2 O → M(OH) 2 (aq) Non-metal oxides are acidic. They dissolve to form acids; XO 2 (s) + H 2 O → H 2 XO 3 (aq) A few, such as aluminium, are amphoteric. These demonstrate both acidic and basic properties.

Eg;Magnesium oxide dissolves slowly in cold water … …giving an alkaline solution. MgO (s) + H 2 O → Mg(OH) 2 (aq)

This is not a redox reaction; MgO (s) + H 2 O → Mg(OH) 2 (aq) Mg oxidation N o = +2 Hydrogen oxidation N o = +1 Oxygen oxidation N o = -2 All oxidation numbers have remained unchanged.

Reactions of magnesium hydroxide Mg(OH) 2 neutralises acids. Mg(OH) 2 + 2HCl→ MgCl 2 + 2H 2 O

Treating indigestion. HCl is secreted in the stomach to aid digestion. Excess acid causes indigestion. “Milk of Magnesia” contains magnesium hydroxide so neutralises excess acid.

Some basic reactions; Acids react with many metals to give hydrogen and a salt Eg; HCl + Na→ 1/2 H 2(g) + NaCl Acids neutralise alkalis to produce a salt and water Eg; HCl + NaOH→ HOH + NaCl (aq) Acids react with carbonates to produce a salt, carbon dioxide and water. 2HCl + CaCO 3 → CaCl 2 +CO 2(g) + H 2 O

Reaction of magnesium with HCl. Magnesium reacts with hydrochloric acid to give effervescence of hydrogen. Mg + 2HCl →MgCl 2 + H 2

This is a redox reaction; Mg + 2HCl →MgCl 2 + H 2 Mg oxidation N o = 0 Mg oxidation N o = +2 Magnesium is oxidised; Mg → Mg e - Hydrogen oxidation N o = +1 Hydrogen oxidation No =0 Hydrogen is reduced; 2H + + 2e - → H 2

Reactions of magnesium with water. Magnesium reacts slowly with cold water; Mg + 2H 2 O → Mg(OH) 2 + H 2 (The other members of its group react readily with cold water to give the corresponding sparingly soluble hydroxides.) Magnesium reacts more readily with steam to form its oxide; Mg + H 2 O (g) → MgO + H 2

Reactions of Ca, Sr and Ba with water Calcium reacts rapidly with cold water to give hydrogen and a white percipitate of calcium hydroxide. Ca + 2H 2 O → Ca(OH) 2 + H 2 Strontium and barium react in the same way, but more vigorously.

Reactions of magnesium carbonate Magnesium carbonate reacts with acids to give an effervescence of carbon dioxide. MgCO 3 + 2HCl →MgCl 2 + 2H 2 O + CO 2