IONISATION POTENTIAL / kJ mol-1
| Element | Be | Mg | Ca | Sr | Ba |
| 1st | 899 | 738 | 590 | 550 | 503 |
| 2nd | 1757 | 1451 | 1145 | 1064 | 965 |
The charge-to-radius ratios of the Group IIA cations are; Be+� (estimated 17), Mg+� (3.3), Ca+� (1.8), Sr+� (1.2), Ba+� (1.0) and Ra+� (0.7). (Note: The charge to radius ratio is proportional to the charge per unit surface area.)
We know that beryllium reacts by covalent bonding, while the others form ionic compounds. However, magnesium can also form covalent bonds if the factors are right.
Tutorial 1
Why is the value of 17 for the charge-to-radius ratio Be+� given as an estimate? Answer
OXIDES AND HYDROXIDES
Magnesium oxide is relatively inert and does not dissolve in water. However it can absorb carbon dioxide from the air to give a carbonate. The oxides of the other members react with water to form the hydroxides.
In poor countries, villagers prepare calcium hydroxide (or lime) using this reaction to paint the walls of their huts. The reaction is fairly vigorous with large amount of heat being given off. The calcium hydroxide is also used here for its disinfectant property.
Except for beryllium hydroxide and magnesium hydroxide all the other hydroxides are soluble in water. The solubility increases with increase in the size of the cations.
Beryllium hydroxide is amphoteric. Magnesium is a weak base. The other hydroxides are strong bases.
OTHER SALTS
The common salts - sulphates, halides, and nitrates - can be easily prepared by the reaction of the hydroxides with the corresponding acids.
The solubility of the ionic compounds of the Group II elements decreases with increasing size of the cations; that is down the Group. This is because the hydration energies decrease relatively faster than the lattice energies, with increasing size of the cations. Of course the exception are the hydroxides mentioned earlier.
The cation being surrounded by six molecules of water.
SULPHATES
Calcium sulphate is slightly soluble in water. Commercially it is also known as the plaster of Paris. One of the applications is to make the cast to hold broken bones in position while it heal. The white material they apply on you when you have a broken bone and all your friends will sign their signatures on when they come to visit you. Here the calcium sulphate (with a molecule of water) will coordinate another molecule of water to form gypsum, the harden cast.
| CaSO4.H2O + H2O | � | CaSO4.2H2O |
| Plaster of Paris | Gypsum |
Calcium sulphate is slightly acidic and so the cast is fairly uncomfortable. Modern hospitals now use a plastic material to make the cast.
Strontium, barium, and radium sulphates are not soluble.
HALIDES
The halides are only slightly soluble in water except for the fluorides. The fluoride anion is comparatively so much smaller than the cations that in the crystal lattice the cations are squeezed against each other, hardly making any contact with the fluoride anion. So the lattice energies decrease more rapidly as compared with the hydration energies.
CARBONATES
The carbonates are only slightly soluble in water. This found an application in the analysis of carbonates and bicarbonates.
Calcium carbonate is also widely used as a health supplement for osteoporosis (a condition of loss of bony tissue). Visit your local pharmacy and read the labels on the bottles for calcium supplement. There are other forms too, like calcium citrate. As far as medical science is concern grinding your clean dried egg shell and sprinkling it into your food will serve the same purpose.
CHELATE COMPOUNDS
Calcium does not form covalent bonds. However it does complex with oxygen or nitrogen atoms in organic compounds to form chelate compounds. One outstanding chelate compound is calcium ethylenediaminetetraacetate, Ca(EDTA).
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Disodium ethylenediaminetetraacetic acid | Na2Ca(EDTA) | |||||
The chelating bonds are strong and the stoichiometry of the reaction is 1:1. This reaction is very useful in the quantitative analysis of calcium present in solutions.
ORGANOMETALLIC COMPOUNDS
MgBr2, MgI2, and magnesium perchlorate (MgClO4) are soluble in alcohols, ketones, and ethers (organic solvents having oxygen atoms which can share their valence electrons). Calcium chloride is also soluble in alcohols.
One important class of organometallic compound of magnesium is the Grignard reagent (RMgX) formed by reacting magnesium metal with organic halides. Example magnesium with ethyl chloride (CH3CH2Cl).
The covalent bonds formed are not very stable and so the reaction requires a very controlled environment where there is no other chemical impurities - like water moisture, carbon dioxide, etc. - present. Because on this instability it is highly reactive. We will discuss this further in the Organic Chemistry lessons.
