I AM A MOLE AND I LIVE IN A HOLE!

GROUP 2 

I) GENERAL PROPERTIES OF THE GROUP 2 ELEMENTS

The elements of Group 2 are called the alkaline earth metals. They are:

Beryllium - Be - [He]2s²

Magnesium - Mg - [Ne]3s²

Calcium - Ca - [Ar]4s²

Strontium - Sr - [Kr]5s²

Barium - Ba - [Xe]6s²

Radium - Ra - [Rn]7s²

The alkaline earth metals from magnesium to barium are silvery white metals, with low melting and boiling points compared with transition metals like iron. They are good conductors of heat and electricity.

The white colour that is often seen is an oxide film from the metal reacting with air, which prevents further reaction.

The metals burn in air, magnesium with a very bright white light, and the others with characteristic flame colours – calcium is brick red, strontium is crimson and barium is apple green.

The metals of Group 2 elements all have two electrons in their outer shell, and these are lost when a metal reacts. This means that they always form a 2+ ion in their compounds, such as Mg²⁺ and Ca²⁺.

This means that they are less reactive than Group 1 metals because they have to lose two outer shell electrons, whereas Group 1 metals lose only one.

To summarise:

• They are all metals
• They are good conductors of heat and electricity
• Their compounds are all white or colourless
• In all their compounds they have an oxidation number of +2
• Their compounds are ionic
• They are called alkaline earth metals because their oxides and hydroxides are basic
• They react with acids to give a salt plus hydrogen
• They are harder and denser that Group 1 metals
• They have higher melting points than Group 1 metals
• They exhibit stronger metallic bonding than Group 1 metals as two electrons are delocalised instead of one.

II) REACTIONS OF GROUP 2 ELEMENTS

Alkaline earth metals are powerful reducing agents (they are good at donating electrons to other atoms)

When Group 2 metals react with water, they reduce water to hydrogen and in the process are oxidised from an oxidation state of 0 (as elements) to a +2 oxidation state (as their ions).

The vigorousness of the reaction increases down the group as the outer shell electrons are more easily lost.

During the reaction, hydrogen gas and an aqueous solution of, for example, magnesium hydroxide is formed.

The aqueous solution is weakly alkaline, as magnesium hydroxide is sparingly soluble.

When water, as steam, is passed over heated magnesium there is a rapid reaction. Hydrogen gas is released during the reaction and a white solid (magnesium oxide) remains.

The reactions of the other alkaline earth metals with cold water all form a cloudy white precipitate of the hydroxide, which is sparingly soluble.

During the reaction with water, metal atoms are oxidised and the lost electrons are transferred to the hydrogen atoms in water molecules.

Their reactions with oxygen are vigorous. Magnesium ribbon burns with a bright white flame to form a solid white oxide.

Magnesium hydroxide is a weak alkali and is used in indigestion remedies (known as milk of magnesia) to neutralise excess acid in the stomach. It is also used in toothpastes to neutralise acids in the mouth which cause tooth decay.

Solid calcium hydroxide is used on acidic soil, to reduce the acidity of the soil, helping to increase crop yields.

It can be represented by the reaction:

Ca(OH)₂ (s) + H⁺ à Ca²⁺ (aq) + 2H₂O (l)

III) CHALK AND LIME CHEMISTRY

Chalk is chemically composed of CaCO₃. Limestone is a similar sedimentary deposit which also contains calcium carbonate. Both are used in large quantities to manufacture quicklime (calcium oxide) and cement.

 The thermal decomposition of calcium carbonate causes it to break down into calcium oxide and carbon dioxide:

CaCO₃ (s) à CaO (s) + CO₂ (g)

The carbonates of other Group 2 elements undergo thermal decomposition in a similar way. Down the group, the carbonates become more stable to thermal decomposition, so more energy is required to break them down.

Calcium oxide reacts vigorously with water to produce calcium hydroxide. If this solid calcium hydroxide is then added to water, it will partially dissolve, forming a saturated solution called lime water.

As calcium hydroxide is sparingly soluble, the concentration of hydroxide ions in the water is low, causing the solution to have a weakly alkaline pH (9-10).

Lime water is a reagent used to detect the presence of carbon dioxide gas. This is because the addition of carbon dioxide gas to calcium hydroxide solution forms a precipitate of calcium carbonate, causing the solution to turn milky.

Ca(OH)₂ (aq) + CO₂ (g) à CaCO₃ (s) + H₂O (l)

If carbon dioxide is continued to be added, the water reacts with it, forming a solution of carbonic acid. This then neutralises the calcium carbonate to calcium hydrogen carbonate, which is soluble, forming a clear solution.

GLOSSARY

Alkaline earth metals: the elements found in Group 2 of the Periodic Table.

Thermal decomposition: the breakdown of a compound by heat.