Which of the following statements are true? A. Unlike $\mathrm{Ga}$, that has a very high melting point, $\mathrm{Cs}$ has a very low melting point. B. On the Pauling scale, the electronegativity values of $\mathrm{N}$ and $\mathrm{Cl}$ 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 $\mathrm{Si}$ is $\text{Si}>\text{Al}>\text{Mg}>\text{Na}$. E. The atomic radius of $\mathrm{Cs}$ is greater than that of $\mathrm{Li}$ and $\mathrm{Rb}$.

Solution & Explanation

\textbf{Answer:} (D) C and E only \textbf{Solution:} C is true: $\mathrm{Ar}$, $\mathrm{K}{}^{+}$, $\mathrm{Cl}{}^{-}$, $\mathrm{Ca}{}^{2+}$ and $\mathrm{S}{}^{2-}$ 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: $\mathrm{Ga}$ 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 $\text{Si}>\text{Mg}>\text{Al}>\text{Na}$ ($\mathrm{Al}$ dips below $\mathrm{Mg}$ 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.