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List the Physical Properties of Alkali Metal, with brief explanation of most of the properties.

7 years ago


Answers : (1)



Physical Nature

1.    Density :

(a)    All are light metals, Li, Na and K have density less than water. Low values of density are because these metals have high atomic volume due to larger atomic size. On moving down the group the atomic size as well as atomic mass both increase but increase in atomic mass predominates over increase in atomic size or atomic volume and therefore the mass/volume ratio i.e., density gradually increases down the groups.

(b)    The density increases gradually from Li to Cs. Li is lightest known metal among all.

    Li = 0.534, Na = 0.972, K = 0.86, Rb = 1.53 and Cs = 1.87 g/ml at 20oC.

(c)    K is the lighter than Na because of its unusually large atomic size.

(d)    In solid state, they have body centred cubic lattice.

2.    M.Pt. and B.Pt. :

   All these elements possess low and in comparison to other group members.

                           Li        Na         K            Rb            Cs        Fr (K)    453.5   37.8        336.2     312.0    301.5       – (K)    1620    1154.4    1038.5    961.0    978.0      –

3.    Specific heat : It decreases from Li to Cs.

                                                Li         Na        K        Rb        Cs           Fr

    Specific heat (cal/g)    0.941     0.293    0.17   0.08      0.049      –

4.    State :

   All are silvery white, soft and light solids. These can be cut with the help of knife. When freshly cut, they have bright luster which quickly tarnishes due to surface oxidation.

5.    Conduction power :

All are good conductors of heat & electricity, because of loosely held valence electrons.

6.    Atomic and ionic radii :

(a)    Alkali metals have largest atomic and ionic radii than their successive elements of other groups belonging to same period.

(b)    Atomic ionic radii of alkali metals, however, increases down the group due to progressive addition of new energy shells. No doubt the nuclear charge also increases on moving down the group but the influence of addition of new energy shells predominates.

                                                        Li    Na     K        Rb    Cs      Fr

    Atomic radius (pm)                152  186  227   248    265    375

    Ionic radius of M+ ions (pm)60    95    133    148    169    –

7.    Ionisation energy & electropositive or metallic character :

8.    Oxidation Number and Valency :

These elements easily from univalent +ve by losing solitary ns1 electron due to low ionization energy values and form ionic compounds. Lithium salts are, however, covalent.

9.    Hydration of Ions :

  Hydration represents the dissolution of a substance in water by adsorbing water molecule by weak valency forces. The hydration is an exothermic process i.e. energy is released during hydration.

10.    Standard Oxidation Potential and Reducting Properties :

 Since alkali metals easily lose ns1 electron and thus they have high values of oxidation potential

11.    Characteristic flame colours : The alkali metals and their salts give characteristic colour to Bunsen flame. The flame energy causes an excitation of the outermost electron which on reverting back to its initial position gives out the absorbed energy as visible light. Li – crimson, Na – Golden yellow, K – Pale violet, Rb and Cs – violet. These different colours are due to different ionization energy of alkali metals. The energy released is minimum in the case of Li and increases in the order

12.    Electronegativities :

(a)    These metals are highly electropositive and thereby possess low values of electonegativities.

(b)    Electronegativity of alkali metals decreases down the group as the trend of numerical values of electronegativity given below on Pauling scale suggests.

                                             Li         N        K        Rb        Cs       Fr

    Electronegativity          0.98    0.93    0.82    0.82    0.79    –

Note :    Fr being radioactive elements and thus studies on physical properties of this element are limited.

13.    Occurrence : Alkali metals are very reactive and thus found in combined state.

14.    Extraction of alkali metals : Alkali metals cannot be extracted by the usual methods for the extraction of metals due to following reasons.

(i)    Alkali metals are strong reducing agents, hence cannot be extracted by reduction of their oxides or other compounds.

(ii)    Being highly electropositive in nature, it is not possible to apply the method of displacing them from their salt solutions by any other element.

15.    Alloy Formation

(a)    Alkali metals form alloys among themselves as well as with other metals.

(b)    Alkali metals dissolve in mercury to form amalgam with evolution of heat and the amalgamation is highly exothermic.

7 years ago

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