Question: 47. Which of the following statements are true? A. Unlike Ga that has a very high melting point, Cs has a very low melting point. B. On Pauling scale, the electronegativity values of N and Cl are not the same. C. \mathrm{Ar}, \mathrm{K}^{+}, \mathrm{Cl}^{-}, \mathrm{Ca}^{2+} and \mathrm{S}^{2-} are all isoelectronic species. D. The correct order of the first ionization enthalpies of \mathrm{Na}, \mathrm{Mg}, \mathrm{Al}, and Si is \mathrm{Si}>\mathrm{A} \ell>\mathrm{Mg}>\mathrm{Na}. E. The atomic radius of Cs is greater than that of Li and Rb . Choose the correct answer from the options given below:
(1) A, B and E only
(2) C and E only
(3) C and D only
(4) A, C and E only
Answer: Option (2)
Explanation:
Statement C and statement E are true; the others are false.
Statement A: Gallium (Ga) actually has a very low melting point, close to room temperature, similar to caesium (Cs).
So it is incorrect to say that Ga has a very high melting point while Cs has a very low melting point.
Hence, statement A is false.
Statement B: On the Pauling scale, the electronegativity values of nitrogen and chlorine are equal.
Both have a value of about 3.0.
Therefore, the statement that their electronegativities “are not the same” is incorrect.
Hence, statement B is false.
Statement C: Check whether they are isoelectronic (same number of electrons).
\mathrm{Ar} has 18 electrons.
\mathrm{K}^{+} has 19 - 1 = 18 electrons.
\mathrm{Cl}^{-} has 17 + 1 = 18 electrons.
\mathrm{Ca}^{2+} has 20 - 2 = 18 electrons.
\mathrm{S}^{2-} has 16 + 2 = 18 electrons.
All have 18 electrons, so they are isoelectronic.
Hence, statement C is true.
Statement D: First ionization enthalpy generally increases across a period, but there are exceptions.
The actual order for period 3 is \mathrm{Si} > \mathrm{Mg} > \mathrm{Al} > \mathrm{Na}.
Magnesium has higher ionization enthalpy than aluminium due to its stable \mathrm{ns}^2 configuration.
The statement given, \mathrm{Si} > \mathrm{Al} > \mathrm{Mg} > \mathrm{Na}, places \mathrm{Al} above \mathrm{Mg},
which is incorrect. Hence, statement D is false.
Statement E: Atomic radius increases down a group. In group 1:
\mathrm{Li}<\mathrm{Na}<\mathrm K<\mathrm{Rb}<\mathrm{Cs}Thus, Cs has a larger atomic radius than both \mathrm{Li} and \mathrm{Rb}.
Hence, statement E is true.
Only statements C and E are true,
so the correct answer is Option (2).