S-BLOCK Revision Notes - IIT JEE/NEET Preparation | Nucleon

S-BLOCK

  • Introduction
  • 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 group (1 & 2) belong to the s-block of
    the periodic table. Group 1 of the periodic table consists of the elements : lithium (Li), sodium (Na), potassium
    (K), rubidium (Rb), caesium (Cs) and francium (Fr). 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.

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  • Electronic Configuration
  • All the alkali metal have one valence electron, ns1 outside the noble gas core. The single valence electron
    is at a long distance from the nucleus and is only weakly held. Hence the loosely held s-electrons in the
    outermost valence shell of these elements make them the most electropositive metals.

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  • Ionization Enthalpy
  • The ionization enthalpies of the alkali metals are considerably low and decrease down the group from
    Li to Cs. This is because the effect of increasing size outweighs the increasing nuclear charge and the
    outermost electron is very well screened from the nuclear charge.

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  • Atomic and Ionic Radii
  • The alkali metal atoms have the largest size in a particular period of the periodic table. The atomic and
    ionic radii of alkali metal increase on moving down the group i.e., they increase in size while going from
    Li to Cs.

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  • Physical properties
  • All the alkali metal are silvery white, soft (because of having only one valency electron which participate in
    bonding) and light metals. Because of the larger size, these element have low density which increases
    down the group from Li to Cs.

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  • Reactivity towards air and water
  • The alkali metals tarnish in dry air due to the formation of their oxides which in turn react with moisture to
    form hydroxides. They burn vigorously in oxygen forming oxides. Lithium forms monoxide, sodium forms
    peroxide, the other metals form superoxide.

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  • Reactivity towards dihydrogen
  • The alkali metals react with dihydrogen at about 673 K (lithium at 1073 K) to form hydrides. All the metal
    hydrides are ionic solid with high melting points

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  • Reactivity towards halogens
  • The alkali metals readily react vigorously with halogens to form ionic halide , MX. However lithium halides
    are some what covalent. It is because of the high polarisation capability of lithium ion

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  • Reducing nature
  • The alkali metals, are strong reducing agents, lithium being the most and sodium the least powerful with
    the small size of its ion, lithium has the highest hydration enthalpy which account for its high negative E
    value and its high reducing power

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  • Solution in liquid ammonia
  • In dilute solutions the main species are metal ions (M+) and electrons, which are solvated (i.e. ammoniated).
    The blue colour, corresponding to a broad absorption band near 1500 nm that falls into the visible range, is
    attributed to the solvated electron.

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  • ANOMALOUS PROPERTIES OF LITHIUM
  • The anomalous behavior of lithium is due to the : (i) exceptionally small size of its atom and ion, and (ii) high
    polarising power (i.e., charge/ radius ratio ). As a result, there is increased covalent character of lithium
    compound which is responsible for their solubility in organic solvent. Further, lithium shows diagonal
    relationship to magnesium

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  • Points of Similarities between Lithium and Magnesium
  • The similarity between lithium and magnesium is particularly striking and arises because of their
    similar size: atomic radii, Li = 152 pm, Mg = 160 pm; ionic radii : Li+ = 76 pm, Mg2+ = 72 pm. The
    main points of similarity are :

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  • ANOMALOUS BEHAVIOUR OF BERYLLIUM
  • Beryllium the first member of the Group 2 metals, shows anomalous behaviour as compared to
    magnesium and rest of the members. Further, it shows diagonal relationship to aluminium

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  • Diagonal Relationship between Beryllium and Aluminium
  • The ionic radius of Be2+ is estimated to be 31 pm; the charge/radius ratio is nearly the same as that
    of the Al3+ ion. Hence beryllium resembles aluminium in some ways. Some of the similarities are

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  • SODIUM OXIDE
  • By reduction of nitrites and nitrates of sodium with metallic sodium :

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  • SODIUM PEROXIDE
  • It is formed by heating sodium in excess of air free from moisture and carbon dioxide or in excess
    of pure oxygen

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  • POTASSIUM SUPEROXIDE
  • It is prepared by burning potassium in excess of oxygen free from moisture.

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  • SODIUM HYDROXIDE OR CAUSTIC SODA
  • PREPARATION : It is most conveniently manufactured by one of the following processes.
    (a) Methods involving sodium carbonate as a starting material

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  • PROPERTIES OF NaOH
  • 1 It is a white crystalline solid and has soapy touch.
    2 It’s density is 2.13 g/mL and melting point is 318.4°C.

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  • POTASSIUM HYDROXIDE
  • It is prepared by electrolysis of KCl solution.
    KOH resembles NaOH in all its reactions. However KOH is much more soluble in alcohol. This accounts
    for the use of alcoholic KOH in organic chemistry.

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  • SODIUM CARBONATE OR WASHING SODA
  • Step - 1 (In ammonia absorber (A)) : Saturation of brine with ammonia and CO2

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  • POTASSIUM CARBONATE, POTASH OR PEARL ASH
  • The potassium carbonate like sodium carbonate, can not be prepared by Solvay process because the
    intermediate, KHCO3 formed is soluble in appreciable amount in water.

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  • SODIUM BICARBONATE OR BAKING SODA
  • It is obtained as the intermediate product in the Solvay ammonia soda process. Normal carbonate can be
    changed to bicarbonate by passing carbon dioxide through its saturated solution.

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  • SODIUM SULPHAT
  • 1  It is a white crystalline solid and effloresces readily in dry air to form anhydrous sodium sulphate.
    2  It is reduced to sodium sulphide when heated with carbon

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  • MAGNESIUM OXIDE OR MAGNESIA
  • Magnesium oxide when mixed with a saturated solution of magnesium chloride sets to a hard
    mass like cement known as magnesia cement or Sorel’s cement. The composition is
    MgCl2.5MgO.xH2O.

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  • MAGNESIUM HYDROXIDE
  • It is a white powder. It is sparingly soluble in water. It is basic in nature and forms salt with acids. It
    decomposes on heating. It readily dissolves in ammonium chloride solution and is, therefore, not precipitated
    in group IIIrd of qualitative analysis

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  • MAGNESIUM CHLORIDE
  • PROPERTIES :
    It is a colourless crystalline solid, highly deliquescent and highly soluble in water

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  • QUICK LIME and SLAKED LIME AND LIME WATER
  • 3Ca(OH)2 + 2Cl2  Ca(OCl)2. Ca(OH)2. CaCl2. 2H2O (bleaching powder).

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  • MAGNESIUM CARBONATE
  • It can be prepared by adding sodium bicarbonate to a hot solution of magnesium salt

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  • CALCIUM CARBONATE
  • It can be obtained by passing carbon dioxide through lime water or by adding sodium carbonate solution to
    CaCl2.

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  • MAGNESIUM SULPHATE
  • It occurs in nature as minerals kiesserite (MgSO4.H2O), epsom salt (MgSO4.7H2O)and kainite
    (KCl.MgSO4.3H2O).

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  • CALCIUM SULPHATE
  • It is found in nature as anhydride (CaSO4) and gypsum (CaSO4.2H2O)
    It can be prepared by reacting any calcium salt with either sulphuric acid or a soluble sulphate

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  • PLASTER OF PARIS
  • PREPARATION :
    It is obtained when gypsum, calcium sulphate dihydrate (CaSO4.2H2O), is heated at 120°C (393 K).

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