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UPSC Chemistry The s-Block Elements The s - Block Elements

The s - Block Elements

Category : UPSC

The s - Block Elements

 

1.           In Periodic Table

 

  • The s-block elements of the Periodic Table are those in which the last electron enters the outermost s-orbital. As the s-orbital can accommodate only two electrons, two groups (1 & 2) belong to the s-block of the Periodic Table.
  • Group 1 of the Periodic Table consists of the elements: lithium, sodium, potassium, rubidium, caesium and francium. They are collectively known as the alkali metals. These are so called because they form hydroxides on reaction with water which are strongly alkaline in nature.
  • The elements of Group 2 include beryllium, magnesium, calcium, strontium, barium and radium. These elements with the exception of beryllium are commonly known as the alkaline earth metals. These are so called because their oxides and hydroxides are alkaline in nature and these metal oxides are found in the earth's crust.
  • Among the alkali metals sodium and potassium are abundant and lithium, rubidium and caesium have much lower abundances. Francium is highly radioactive.

 

2.           Uses of s Block Elements

 

  • Lithium metal is used to make useful alloys, for example with lead to make "white metal’ bearings for motor engines, with aluminium to make aircraft parts.
  • And with magnesium to make armour plates. It is used in thermonuclear reactions. Lithium is also used to make electrochemical cells.
  • Sodium is used to make a \[Na/Pb\]alloy needed to make \[PbE{{t}_{4}}\] and \[PbM{{e}_{4}}\]. These organolead compounds were earlier used as anti-knock additives to petrol, but nowadays vehicles use lead-free petrol.
  • Liquid sodium metal is used as a coolant in fast breeder nuclear reactors.
  • Potassium has a vital role in biological systems. Potassium chloride is used as a fertilizer. Potassium hydroxide is used in the manufacture of soft soap. It is also used as an excellent absorbent of carbon dioxide.
  • Caesium is used in devising photoelectric cells.
  • Uses of Sodium Carbonate (Washing Soda), \[N{{a}_{2}}C{{O}_{3}}.10{{H}_{2}}O:\]
  • It is used in water softening, laundering and cleaning.
  • It is used in the manufacture of glass, soap, borax and caustic soda.
  • It is used in paper, paints and textile industries.
  • It is an important laboratory reagent both in qualitative and quantitative analysis.

 

3.           Sodium Chloride, NaCI and Other compounds of Na

 

  • The most abundant source of sodium chloride is sea water which contains \[2.7\]to \[2.9\]per cent by mass of the salt. In tropical countries like India, common salt is generally obtained by evaporation of sea water.
  • Crude sodium chloride, generally obtained by crystallisation of brine solution, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities.
  • Calcium chloride, \[CaC{{1}_{2}}\], and magnesium chloride, \[MgC{{1}_{2}}\] are impurities because they are deliquescent (absorb moisture easily from the atmosphere).
  • To obtain pure sodium chloride, the crude salt is dissolved in minimum amount of water and filtered to remove insoluble impurities. The solution is then saturated with hydrogen chloride gas. Crystals of pure sodium chloride separate out. Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution.
  • Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell. Uses of Sodium Hydroxide:
  • In the manufacture of soap, paper, artificial silk and a number of chemicals,
  • In the petroleum refining,
  • In the purification of bauxite,
  • In the textile industries for mercerising cotton fabrics,
  • For the preparation of pure fats and oils, and
  • As a laboratory reagent.

 

  • Sodium hydrogencarbonate is known as baking soda because it decomposes on heating to generate bubbles of carbon dioxide (leaving holes in cakes or pastries and making them light and fluffy).

 

4.           Biological Importance of Sodium and Potassium

 

  • A typical 70 kg man contains about 90 g of Na and 170 g of K compared with only 5 g of iron and 0.06 g of copper.
  • Sodium ions are found primarily on the outside of cells, being located in blood plasma and in the interstitial fluid which surrounds the cells. These ions participate in the transmission of nerve signals, in regulating the flow of water across cell membranes and in the transport of sugars and amino acids into cells.
  • Sodium and potassium, although so similar chemically, differ quantitatively in their ability to penetrate cell membranes, in their transport mechanisms and in their efficiency to activate enzymes. Thus, potassium ions are the most abundant cations within cell fluids, where they activate many enzymes, participate in the oxidation of glucose to produce ATP and, with sodium, are responsible for the transmission of nerve signals.
  • There is a very considerable variation in the concentration of sodium and potassium ions found on the opposite sides of cell membranes. As a typical example, in blood plasma, sodium is present to the extent of \[143\] mmol\[{{L}^{-1}}\], whereas the potassium level is only 5 mmol\[{{L}^{-1}}\] within the red blood cells.
  • These concentrations change to 10 mmol\[{{L}^{-1}}\] (\[N{{a}^{+}}\]) and 105 mmol\[{{L}^{-1}}\] (\[{{K}^{+}}\]). These ionic gradients demonstrate that a discriminatory mechanism, called the sodium-potassium pump, operates across the cell membranes which consumes more than one-third of the ATP used by a resting animal and about 15 kg per 24 h in a resting human.


 

5.           Alkaline Earth Metals

 

  • The group 2 elements comprise beryllium, magnesium, calcium, strontium, barium and radium. They follow alkali metals in the periodic table. These (except beryllium) are known as alkaline earth metals. The first element beryllium differs from the rest of the members and shows diagonal relationship to aluminium.
  • Calcium, strontium and barium impart characteristic brick red, crimson and apple green colours respectively to the flame. In flame the electrons are excited to higher energy levels and when they drop back to the ground state, energy is emitted in the form of visible light.
  • The electrons in beryllium and magnesium are too strongly bound to get excited by flame. Hence, these elements do not impart any colour to the flame.
  • Beryllium is used in the manufacture of alloys. Copper-beryllium alloys are used in the preparation of high strength springs. Metallic beryllium is used for making windows of X-ray tubes.
  • Magnesium forms alloys with aluminium, zinc, manganese and tin. Magnesium-aluminium alloys being light in mass are used in air-craft construction. Magnesium (powder and ribbon) is used in flash powders and bulbs, incendiary bombs and signals.
  • A suspension of magnesium hydroxide in water (called milk of magnesia) is used as antacid in medicine. Magnesium carbonate is an ingredient of toothpaste.
  • Calcium is used in the extraction of metals from oxides which are difficult to reduce with carbon. Calcium and barium metals, owing to their reactivity with oxygen and nitrogen at elevated temperatures, have often been used to remove air from vacuum tubes.
  • Radium salts are used in radiotherapy, for example, in the treatment of cancer.
  • Uses of Calcium Oxide or Quick Lime\[\left( CaO \right)\]:
  • It is an important primary material for manufacturing cement and is the cheapes form of alkali.
  • It is used in the manufacture of sodium carbonate from caustic soda.
  • It is employed in the purification of sugar and in the manufacture of dye stuffs.
  • Uses of Calcium Hydroxide or Slaked Lime \[(Ca{{(OH)}_{2}}\]:
  • It is used in the preparation of mortar, a building material.
  • It is used in white wash due to its disinfectant nature.
  • It is used in glass making, in tanning industry, for the preparation of bleaching powder and for purification of sugar.
  • Calcium Carbonate (\[CaC{{O}_{3}}\]) occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride. Uses of (\[CaC{{O}_{3}}\]) :

 

  • Building material in the form of marble and Manufacture of quick lime.
  • Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron.
  • Specially precipitated \[CaC{{O}_{3}}\] is extensively used in the manufacture of high quality paper.
  • It is also used as an antacid, mild abrasive in tooth paste, a constituent of chewing gum, and a filler in cosmetics.
  • Calcium Sulphate (Plaster of Paris), \[CaS{{O}_{4}}^{1/2}{{H}_{2}}O\]is a hemihydrate of calcium sulphate. It is obtained when gypsum, \[CaS{{O}_{4}}2{{H}_{2}}O,\], is heated to \[393\]K. The largest use of Plaster of Paris is in the building industry as well as plasters. It is used for immoblising the affected part of organ where there is a bone fracture or sprain. It is also employed in dentistry, in ornamental work and for making casts of statues and busts.

 

6.           Cement

 

  • Cement is an important building material. It was first introduced in England in 1824 by Joseph Aspdin. It is also called Portland cement because it resembles with the natural limestone quarried in the Isle of Portland, England.
  • The average composition of Portland cement is :\[CaO\], \[50-60\]per cent; \[Si{{O}_{2}}\], \[20-25\]per cent; \[A{{1}_{2}}{{O}_{3}}\], \[5-10\]per cent;\[MgO\], \[2-3\]per cent; \[F{{e}_{2}}{{O}_{3}}\], \[1-2\]per cent and \[S{{O}_{3}}\] 1-2 per cent. For a good quality cement, the ratio of silica (\[Si{{O}_{2}}\]) to alumina (\[A{{1}_{2}}{{O}_{3}}\]) should be between \[2.5\]and 4 and the ratio of lime \[\left( CaO \right)\]to the total of the oxides of silicon (\[Si{{O}_{2}}\]) aluminium (\[A{{1}_{2}}{{O}_{3}}\]) and iron (\[F{{e}_{2}}{{O}_{3}}\]) should be as close as possible to 2.
  • The raw materials for the manufacture of cement are limestone and clay. When clay and lime are strongly heated together they fuse and react to form 'cement clinker'. This clinker is mixed with \[2-3\]per cent by weight of gypsum (\[CaS{{0}_{4}}^{.}2{{H}_{2}}O\]) to form cement. Thus important ingredients present in Portland cement are dicalcium silicate (\[C{{a}_{2}}Si{{O}_{4}}\]) 26 per cent, tricalcium silicate (\[C{{a}_{3}}Si{{O}_{5}}\]) 51 per cent and tricalcium aluminate (\[C{{a}_{3}}A{{1}_{2}}{{O}_{6}}\]) 11 per cent.
  • When mixed with water, the setting of cement takes place to give a hard mass. This is due to the hydration of the molecules of the constituents and their rearrangement. The purpose of adding gypsum is only to slow down the process of setting of the cement so that it gets sufficiently hardened.

 

7.           Biological Importance of Magnesium and Calcium

 

  • An adult body contains about 25 g of Mg and 1200 g of Ca compared with only 5 g of iron and \[0.06\]g of copper. The daily requirement in the human body has been estimated to be \[200-300\]mg.
  • All enzymes that utilise ATP in phosphate transfer require magnesium as the cofactor. The main pigment for the absorption of light in plants is chlorophyll which contains magnesium.
  • About 99 per cent of body calcium is present in bones and teeth. It also plays important roles in neuromuscular function, intemeuronal transmission, cell membrane integrity and blood coagulation.
  • The calcium concentration in plasma is regulated at about 100 mgL\[^{-1}\]. It is maintained by two hormones: calcitonin and parathyroid hormone. Bone is not an inert and unchanging substance but is continuously being solubilised and redeposited to the extent of 400 mg per day in man. All this calcium passes through the plasma.

 

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