The P-Block Elements

  • Question

    How would you account for the following?

    (i) NF3 is an exothermic compound but NCl3 is not.

    (ii) The acidic strength of compounds increases in the order:

    PH3 < H2S < HCl

    (iii) SF6 is kinetically inert.


    (i) As we move down the group 17, the size of the atom increases from fluorine to chlorine. The larger difference in the size of N and Cl results in the weakness of strength of N - Cl bond. On the other hand, the difference in size of N and F is small; consequently the N -F bond is quite strong. As a result, NF3 is an exothermic compound.

    (ii) In a period, the electronegativity decreases in the order Cl > S > P. As a result, the loss of H+ ions decreases. Thus, the acidic strength of the hydrides decreases in the following order. HCl > H2S > PH3

    (iii) The kinetic inertness of SF6 can be explained on the basis of its structure.


    As seen from the above structure, the six fluorides (F-) atoms protect the sulphur atom from attack by the regents to such an extent that even thermodynamically most favourable reactions like hydrolysis do not occur.

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