Which of the following statements are true? A. Unlike \ce{Ga}, that has a very high melting point, \ce{Cs} has a very low melting point. B. On the Pauling scale, the electronegativity values of \ce{N} and \ce{Cl} are not the same. C. \ce{Ar}, \ce{K+}, \ce{Cl-}, \ce{Ca^2+} and \ce{S^2-} are all isoelectronic species. D. The correct order of the first ionization enthalpies of \ce{Na}, \ce{Mg}, \ce{Al} and \ce{Si} is $\text{Si} > \text{Al} > \text{Mg} > \text{Na}$. E. The atomic radius of \ce{Cs} is greater than that of \ce{Li} and \ce{Rb}.

Solution & Explanation

\textbf{Answer:} (D) C and E only \textbf{Solution:} C is true: \ce{Ar}, \ce{K+}, \ce{Cl-}, \ce{Ca^2+} and \ce{S^2-} all have $18$ electrons, so they are isoelectronic. E is true: atomic radius increases down a group, so $\text{Cs} > \text{Rb} > \text{Li}$. A is false: \ce{Ga} does not have a very high melting point (it melts at about $30\,^\circ\text{C}$). D is false: the correct first-ionization-enthalpy order is $\text{Si} > \text{Mg} > \text{Al} > \text{Na}$ (\ce{Al} dips below \ce{Mg} due to the group-13 anomaly), not $\text{Si} > \text{Al} > \text{Mg} > \text{Na}$. Per the official key, only C and E are the intended true statements.