Alkali earth metal.

The alkaline earth metal.

The elements such as beryllium, Magnesium, calcium, strontium, barium, radium which enter in the ns2 electronic configuration are called alkaline earth metal.The alkaline earth metal occurs in Group IIA (2) in this group.The group 2 elements show the same trend in properties as were serves with group 1. However, beryllium stands apart from the rest of the group, and differs much more from them than lithium does from the rest of Group I.The main reason for this is that the beryllium atom and Be²⁺ are both extremely small, and the relative increase in size from Be²⁺ to Mg²⁺ is four times greater than then the increase in size between Li⁺ and Na+. Berrylium and barium compounds are all very toxic. The elements form a well-graded metal but are less stable that Group 1. These elements are typically divalent and generally, form colourless ionic compounds. The oxides and hydroxides are less basic than those of Group 1: hence, their oxo salts(carbonates, sulphates, nitrates,) are less stable to heat. Magnesium is the important structural metal and is used in large amount. Several compounds are used in vast quantities. Limestone is used to make quicklime, Cao and cement are used in large amount for different purposes. Other compounds used on a large scale include gypsum, fluoride, magnesium. Mg²⁺ and Ca²⁺ are the essential elements in the human body, and Mg²⁺ is an important constituent of chlorophyll.

Property of alkaline earth metal.

• All the element present in this group are shiny.
• They are all silvery white in colour.
• They have low melting points.
• All the elements in this group have low boiling points.
• All the elements of this group low densities.
• These elements are somewhat reactive metals as standard temperature and standard pressure.
• This element readily loses their two outermost electrons to form cations

Biological importance of Ca⁺⁺ and Mg^++.

The biological importance of both these elements is discussed below.

• Both the element i,e Ca⁺⁺ and Mg⁺⁺ are involved in the neutralisation of negative charge which is accumulated due to the different metabolic reaction.
• These both ion transmitted the impulses through the nerve fibre.
• Ca⁺⁺ ions are important constituent for bond, teeth, and also used as apatite.
• Ca⁺⁺ ion are necessary for muscle contraction, blood clotting, regular beating of heart, division, growth, glycolysis, excretion-secretion.
• Mg⁺⁺ ion is the important constituent in chlorophyll in green parts of plants .
• Mg⁺⁺ ion is the active side for the enzyme. Phosphohydrolase and phosphor transferase which is involved in the reaction of ATP and energy is released.

Anomalous property of the Berrylium.

Beryllium is the first member of group 2. All elements in this group have a same electronic configuration. However, properties of beryllium differ very much to the rest elements of these elements. This is due to

1. It's very small size.
2. It has high charge density and has high polarising power.
3. High electronegativity.

The following properties of the beryllium explain anomalous behaviour.

• Beryllium form covalent compound but other elements in these group form ionic compounds.
• Salts of beryllium contains four molecules of H2O crystallisation. But salts of other elements contains 6 molecules of water of crystallisation.BeCl_2, 4H₂O, MgCl₂.6H₂O, CaCl₂.6H₂O.
• Halides and the hydride of beryllium are electron deficient and hence they act as lewis acid. Cl— Be— Cl→ only 4 electrons.
• Halides and the hydrides of the Be exist as the polymer.Example.

(BeH₂)_n, (BeCl₂)_n

Chlorophyll.

Chlorophyll is the most abduent biological complex formed by Mg²⁺ with porphyrin ring. In chlorophyll, Mg²⁺ remain bonded in the centre of porphyrin ring with four nitrogen atoms. Two types of chlorophyll found in higher plants.Mainly chlorophyll differ from one another in the substituent at ring II. Chlorophyll 'an' and 'b' work more effectively in the red and blue region of visible sunlight. In photosynthesis carbon dioxide and water combines in the presence of sunlight produce starch.Hence, chlorophyll makes available energy for the process of photosynthesis.

Calcium hydride.(CaH₂) = Hydrolith.

Preparation.

Calcium hydride is prepared by passing hydrogen gas through molten calcium metal.

$$Ca +H_2→ CaH_2$$

Properties.

1. It reacts violently with water to produce hydrogen gas.

$$CaH_2 + H_2O→ Ca(OH)_2 + H_2 ↑$$

2. It reacts with CO₂ to form calcium .

$$CaH_2 + CO_2\rightarrow (HCOO)_2Ca$$

3. Electrolysis of molten calcium hydride produce Ca and H_2. H₂ gas liberated at the anode.

$$CaH_2 → Ca^{++ }+ 2H^-$$

At anode the following reaction takes place.

$$H^- → He^-$$

$$2H → H_2↑$$

At cathode the following reaction takes place.

$$Ca^{++}+2e^-→ Ca$$

It confirms that CaH₂ contains hydrogen as halide ion (H)^-

Grignard reagent.

Preparation.

Grignard reagent is prepared by treating alkyl or aryl halide with Mg in an organic solvent like dry ether.

$$C_2H_5Br+Mg\xrightarrow[]{dry\, ether}\,Grignard\,reagent$$

Grignard reagent is probably the most versatile reagent in organic chemistry. It is used to prepare a wide range of organic compound. Some important example is given below.

$$RMgX+H^+→R-H\,(alkane)$$

$$RMgX+Cl_2→R-Cl\,(alkyl\, halide)$$

$$RMgX+O_2→ROH\,(Primary\, alcohol)$$

$$RMgX+RCOH→R_2CHOH\,(Secondary\, alcohol)$$

$$RMgX+R_2CO→R_3COH\,(Tertiary\, alcohol)$$

$$RMgX+CO_2+H^+→RCOOH\,(Carboxylic \,acid)$$

Lithium has strong tendency to form covalent compound than other elements in the group I. Give reason. How it resembles with Magnesium.

Lithium is the first element and Li⁺ ion is the smallest ion in this group. Because of the smallest size of the atom, its ion possesses the greatest polarising power out of all the alkali metal ion. This ion has strong tendency to form the cavalent compound.

Lithium resembles magnesium with which it is diagonally related. They have diagonal pair element. The resembles are due to a similarity in ionic size and high polarising power of Li⁺ and Mg²⁺ ions. Following postulates illustrate the similarity of Li and Mg.

1.Both of the metal are harder and has the high melting point than another member in their respective group.

2.Both Li and Mg reacts slowly with H₂O but another alkali metal reacts vigorously.

3. Li(OH) and (MgOH)₂ are slightly soluble in H₂O but other metal hydroxides are quite soluble in H₂O.

4. Li forms Li₃N and Mg form Mg₃N2. Both nitrides give NH₃ on reacting with H₂O.

$$Mg_3N_2+6H_2O→3Mg(OH)_2+NH_3$$

5. Li forms Li₂O but other gives peroxide and superoxide. Mg also gives MgO

Reference.

F.A.Cotton and Wilkinson G. Basic inorganic Chemistry. John,Wiley and Sons (Asia), 2007.

Lee., J.D. Concise Inorganic Chemistry. fifth edition. New Delhi: Oxford University Press., 2008.

Sharma, M.L and P.N Chaudhary. A textbook of B.S.C chemistry. Kathmandu Nepal: Ekta Books Thapathali Kathmandu, 2011.