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The s-Block Elements

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Chapter 10 : The s-Block Elements

Group 1 Elements: Alkali Metals arrow_upward

  • The group 1 elements comprise Lithium, Sodium, Potassium, Rubidium, Caesium, Francium.

  • 10.1.1 Electronic Configuration

  • All the alkali metals have one valence electron, ns1 outside the noble gas core.

  • 10.1.3 Atomic and Ionic Radii

  • The alkali metal atoms have the largest sizes in a particular period of the periodic table.

  • 10.1.2 Ionization Enthalpy

  • The ionization enthalpies of the alkali metals are considerably low and decrease down the group from Li to Cs.

  • 10.1.4 Hydration Enthalpy

  • The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
  •                   Li+ > Na+ > K+ > Rb+ > Cs+

    10.1.5 Physical Properties

  • All the alkali metals are silvery white, soft and light metals. Because of the large size.

  • 10.1.6 Chemical Properties

  • Reactivity towards water: The alkali metals react with water to form hydroxide and dihydrogen.

  • Solutions in liquid ammonia: The alkali metals dissolve in liquid ammonia giving deep blue solutions.

  • 10.1.7 Use’s

  • Lithium metal is used to make useful alloys.
  • It is used in thermonuclear reactions.
  • Lithium is also used to make electrochemical cells.

  • 10.2 General Characteristics of the Compounds of the alkali Metals arrow_upward

  • General characteristics of some of their compounds are:
    • Oxides and Hydroxides
    • Halides

    10.2.1 Oxides and Hydroxides

  • On combustion in excess of air, lithium forms mainly the oxide, Li2 O (plus some peroxide Li2 O2 ), sodium forms the peroxide, Na2 O2 (and some superoxide NaO2 ) whilst potassium, rubidium and caesium form the superoxides, MO2 .

    10.2.2 Halides         

  • The alkali metal halides, MX, (X=F, Cl, Br, I) are all high melting, colourless crystalline solids.

  • 10.3 Anomalous Properties of Lithium arrow_upward

  • The anomalous behaviour of lithium is due to the:
    • Exceptionally small size of its atom and ion, and
    • High polarising power

    10.3.1 Points of Difference between Lithium and other Alkali Metals

  • Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals.
  • Lithium is least reactive but the strongest reducing agent among all the alkali metals. On combustion in air it forms mainly monoxide, Li2 O and the nitride, Li3 N unlike other alkali metals.

  • 10.3.2 Points of Similarities between Lithium and Magnesium

  • Both lithium and magnesium are harder and lighter than other elements in the respective groups.
  • Lithium and magnesium react slowly with water. Their oxides and hydroxides are much less soluble and their hydroxides decompose on heating. Both form a nitride, Li3 N and Mg3 N2 , by direct combination with nitrogen.
  • The oxides, Li2 O and MgO do not combine with excess oxygen to give any superoxide.

  • 10.4 Important Compounds of Sodium arrow_upward

  • Industrially important compounds of sodium include sodium carbonate, sodium hydroxide, sodium chloride and sodium bicarbonate.
  • Sodium Carbonate (washing soda) Na2 CO3 .10H2 O
  • Solvay process: Passing CO2 to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogen carbonate.

  • Sodium hydrogen carbonate crystal separates. To give sodium carbonate.
  • Uses:
  • It is used in water softening, Laundering and cleaning.
  • Sodium Chloride, NaCl
  • The most abundant source of sodium chloride is sea water which contain 2.7 to 2.9% by mass of the salt.
  • Crude sodium chloride, generally obtained by crystallisation of brine, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities.
  • Uses:
  • It is used as common salt or table salt for domestic purpose.
  • Sodium Hydroxide (Caustic Soda), NaOH
  • Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-kellner cell.
  • Uses:
  • It is used in the manufacture of soap.
  • Sodium Hydrogencarbonate(Baking Soda), NaHCO3
  • Sodium hydrogen carbonate is made by saturating a solution of sodium carbonate with carbon dioxide.
  • Uses:
  • It is used in fire extinguishers.

  • 10.5 Biological Importance of Sodium and Potassium arrow_upward

  • Sodium ions are found primarily on the outside of cells.
  • Potassium ions are the most abundant cations within cell fluids.
  • Example, in blood plasma, sodium is present to the extent of 143 mmolL–1, whereas the potassium level is only 5 mmolL–1 within the red blood cells.

  • 10.6 Group 2 Elements: Alkaline Earth Metals arrow_upward

  • The group 2 elements comprise beryllium, magnesium, calcium, strontium, barium and radium.

  • 10.6.1 Electronic Configuration

  • Their general electronic configuration may be represented as [noble gas] ns2 .

  • 10.6.2 Atomic and Ionic Radii

  • The atomic and ionic radii of the alkaline earth metals are smaller than those of the corresponding alkali metals in the same periods.

  • 10.6.3 Ionization Enthalpies

  • The alkaline earth metals have low ionization enthalpies due to fairly large size of the atoms.

  • 10.6.4 Hydration Enthalpies

  • The hydration enthalpies of alkaline earth metal ions decrease with increase in ionic size down the group.

  • 10.6.5 Physical Properties

  • The alkaline earth metals, in general, are silvery white, lustrous and relatively soft but harder than the alkali metals. Beryllium and magnesium appear to be somewhat greyish.

  • 10.6.6 Chemical Properties

  • Reactivity towards acids: The alkaline earth metals readily react with acids liberating dihydrogen.
  • M + 2HCl → MCl2 + H2

    10.6.7 Uses

  • Beryllium is used in the manufacture of alloys.
  • Magnesium-aluminium alloys being light in mass are used in air-craft construction.

  • 10.7  General Characteristics of Compounds of the Alkaline Earth Metals arrow_upward

  • General Character of Alkaline earth metals are:
    • Oxides of Hydroxides
    • Salts of Oxoacids

  • Oxides of Hydroxides: The alkaline earth metals burn in oxygen to form the monoxide.
  • Salts of Oxoacids
    • Carbonates: carbonates of alkaline earth metals are insoluble in water and can be precipitated by addition of a sodium of a soluble salt of these metals.
    • Sulphates: The sulphates of the alkaline earth metals are all white solids and stable to heat. BeSO4 , and MgSO4 are readily soluble in water.
    • Nitrates: The nitrates are made by dissolution of the carbonates in dilute nitric acid. Magnesium nitrate crystallises with six molecules of water.

    10.8 Anomalous Behavior of Beryllium arrow_upward

  • Beryllium has small atomic and ionic sizes and thus does not compare well with other member of the group.
  • It does not exhibit coordination number more than four as in its valence shell there are only four orbitals.

  • 10.8.1 Diagonal Relationship between Beryllium and Aluminum

  • Like aluminum, beryllium is not readily attacked by acids because of the presence of an oxide film on the surface of the metal.
  • Beryllium hydroxide dissolves in excess of alkali to give a beryllate ion, [Be(OH)4 ]2– just as aluminum hydroxide gives aluminate ion, [Al(OH)4 ].
  • Beryllium and aluminium ions have strong tendency to form complexes, Be

  • 10.9 Some Important Compounds of Calcium arrow_upward

  • Important compounds of calcium are calcium oxide, calcium hydroxide, calcium sulphate and cement.
  • Calcium Oxides or Quick lime, CaO
  • It is prepared on a commercial scale by heating limestone (CaCO3 ) in a rotary kiln at 1070-1270 K.
  • Uses:
  • It is used in the manufacture of sodium carbonate from caustic soda.
  • Calcium Hydroxide(Slaked lime), Ca(OH)2
  • Calcium hydroxide is prepared by adding water to quick lime, CaO.
  • Carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
  • Uses:
  • It is used in the preparation of mortar, a building material.
  • Calcium Sulphate (Plaster of Paris), CaSO4 .H2 O
  • It is obtained when gypsum, CaSO4 .H2 O, is heated on 393K.
  • Uses:
  • Cement is an important building material.
  • The largest use of Plaster of Paris is in the building industry as well as plasters.

  • Cement

  • Cement is an important building material.
  • Cement is a product obtained by combining a material rich in lime, CaO with other material such as clay which contains silica, SiO2 along with the oxides of aluminium, iron and magnesium.
  • Uses:
  • It is used in concrete and reinforced concrete, in plastering and in the construction of bridges, dams and buildings.

  • 10.10 Biological Importance of Magnesium and Calcium arrow_upward

  • 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.
  • About 99% of body calcium is present in bones and teeth.
  • All enzymes that utilize ATP in phosphate transfer require magnesium as the cofactor.

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