The s-Block Elements

  • Question 173
    CBSEENCH11007151

    What is meant by the diagonal relationship of elements? Discuss the diagonal relationship of beryllium with aluminium.

    Or

    Beryllium exhibits some similarities with aluminium. Point out three such properties.

    Solution
    When an element shows similarities to another element in the next higher group, and in the next period, these are said to exhibit diagonal relationship. For example, Li resembles Mg, Be resembles Al and B resembles Si.

    Diagonal relationship of beryllium with aluminium:
    Since beryllium (Be2+) and aluminium (Al3+) have similar charge/radius ratio, they exhibit diagonal relationship. They resemble as follows:
    (i) Both these elements dissolve in strong alkalies to liberate hydrogen and forming beryllate's and aluminates.
                       Be space plus space 2 NaOH space rightwards arrow space space stack Na subscript 2 BeO subscript 2 space plus space straight H subscript 2 left parenthesis straight g right parenthesis with sodium space beryllate below
2 Al space plus space 2 NaOH space plus space 2 straight H subscript 2 straight O space rightwards arrow space space 2 NaAlO subscript 2 space plus space 3 straight H subscript 2 left parenthesis straight g right parenthesis

    (ii) Both have strong tendency to form covalent compounds. Both BeCl2 and AlCl3 are covalent.
    (iii) Both form non-volatile, hard oxides (BeO and Al2O3) having very high melting points.
    (iv) Both Be and Al form fluoro-complex anions 
    BeF subscript 4 superscript minus space and space AlF subscript 4 superscript minus
    (v) Carbides of both the metals react with water liberating methane gas.
    Be subscript 2 straight C space plus space 4 straight H subscript 2 straight O space rightwards arrow space space 2 Be left parenthesis OH right parenthesis subscript 2 space plus space CH subscript 4
Al subscript 4 straight C subscript 3 space plus space 12 straight H subscript 2 straight O space rightwards arrow space space 4 Al left parenthesis OH right parenthesis subscript 3 space plus space 3 CH subscript 4

    Question 174
    CBSEENCH11007152

    Discuss the diagonal realtionship of Be and Al with regard to:
    (i) action of alkali    (ii) structure of chlorides.

    Solution

    (i) The action of alkali: Both the metals dissolve in strong alkalies to form soluble complexes and liberate hydrogen.
                       Be space plus space 2 OH to the power of minus space space rightwards arrow space space stack BeO subscript 2 superscript 2 minus end superscript with Beryllate below space plus space straight H subscript 2
2 Al space plus space 2 OH to the power of minus space plus space 2 straight H subscript 2 straight O space rightwards arrow space space space stack 2 AlO subscript 2 with Aluminate below space plus space 3 straight H subscript 2
    (ii) The structure of chlorides: Structure of BeCl2: In the solid state, it exists as polymeric chain structure. In vapour state, beryllium chloride exists in the dimeric form which decomposes at 1200K into monomeric form. 

    The structure of AlCl3: Aluminium chloride exists as a dimer (Al2Cl6). In this dimeric structure, each aluminium atom forms one co-ordinate bond by accepting a lone pair of electrons from the chlorine atoms covalently bonded to the other aluminium atom.

    Both the aluminium atoms complete their octet. 

    Question 175
    CBSEENCH11007153

    Give any three points of similarities between beryllium and aluminium and two points of dissimilarities between beryllium and barium ?

    Solution

    Points of similarities:
    (i) BeO as well as Al2O3 are high melting solids.
    (ii) Anhydrous chlorides of Be as well as Al are covalent, soluble inorganic solvent and act as Lewis acids.
    (iii) Both Be and Al form fluoro-complex anions BeF subscript 4 superscript minus and AlF subscript 4 superscript minus in solution.
    Points of dissimilarities:
    (i) Beryllium does not impart any colour to bunsen flame owing to its small atomic and ionic size in which electrons are held more tightly ; on the other hand, barium imparts apple green colour to the bunsen flame.
    (ii) Beryllium is least and barium is most reactive in the group. 

    Question 176
    CBSEENCH11007154

    How beryllium chloride is prepared? Give its two important properties

    Solution

    Preparation. It is prepared by heating beryllium oxide and carbon mixture in an atmosphere of chlorine. 
    BeO space plus space straight C space plus space Cl subscript 2 space rightwards arrow with 870 minus 1070 space straight K on top space BeCl subscript 2 space plus space CO
    Properties:
    1. It is covalent in nature and fumes strongly in the moist air due to hydrolysis. 
    BeCl subscript 2 space plus space 2 straight H subscript 2 straight O space rightwards arrow space space Be left parenthesis OH right parenthesis subscript 2 space plus space 2 HCl
    2. In the vapour phase, it exists as a dimer which dissociates into monomers at about 1200 K.
    stack left parenthesis BeCl subscript 2 right parenthesis subscript 2 with Dimer below space rightwards arrow with 1200 space straight K on top space stack 2 BeCl subscript 2 with Monomer below

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