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 same.
C. Ar, K⁺, Cl⁻, Ca²⁺ and S²⁻ are all isoelectronic species.
D. The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na.
E. The atomic radius of Cs is greater than that of Li and Rb.
Choose the correct answer from the options given below:

CONCEPT:

Properties of Elements and Periodic Trends

• Melting Point:
  • The melting point of elements generally varies based on their metallic bonding and atomic structure.
  • Cesium (Cs) has a very low melting point compared to Gallium (Ga), which has one of the highest among the group 13 elements.
• Electronegativity:
  • Electronegativity is the ability of an atom to attract electrons in a chemical bond.
  • On the Pauling scale, nitrogen (N) has an electronegativity of 3.0, while chlorine (Cl) has an electronegativity of 3.16.
• Isoelectronic Species:
  • Isoelectronic species are atoms, ions, or molecules with the same number of electrons.
  • For example, Ar, K⁺, Cl⁻, Ca²⁺, and S²⁻ all have 18 electrons, making them isoelectronic.
• Ionization Enthalpy:
  • Ionization enthalpy is the energy required to remove an electron from a gaseous atom in its ground state.
  • The correct order of first ionization enthalpies for Na, Mg, Al, and Si is: Si > Al > Mg > Na, due to increasing nuclear charge and electron configuration stability.
• Atomic Radius:
  • The atomic radius increases down a group and decreases across a period.
  • Cesium (Cs) has a larger atomic radius than both lithium (Li) and rubidium (Rb) due to its position in the periodic table.

EXPLANATION:

• Statement A: "Unlike Ga that has a very high melting point, Cs has a very low melting point."
  • This statement is False. Gallium (Ga) has a relatively high melting point, while Cesium (Cs) has an almost equal melting point.
    
• Statement B: "On Pauling scale, the electronegativity values of N and Cl are same."
  • This statement is incorrect. Nitrogen (N) has an electronegativity of 3.04, and Chlorine (Cl) has an electronegativity of 3.16 on the Pauling scale, so they have different electronegativities.
• Statement C: "K⁺, Cl⁻, Ca²⁺ and S²⁻ are all isoelectronic species."
  • This statement is true. All these ions (K⁺, Cl⁻, Ca²⁺, and S²⁻) have the same number of electrons (18 electrons), making them isoelectronic species.
• Statement D: "The correct order of the first ionization enthalpies of Na, Mg, Al, and Si is Si > Al > Mg > Na."
  • This statement is incorrect. The correct order of ionization enthalpies is Si > Al > Mg > Na. Magnesium (Mg) has a higher ionization enthalpy than Aluminum (Al), and Sodium (Na) has the lowest ionization enthalpy due to its larger atomic size and lower nuclear charge compared to others.
• Statement E: "The atomic radius of Cs is greater than that of Li and Rb."
  • This statement is correct. While Cesium (Cs) has a larger atomic radius than Lithium (Li), it is also larger than Rubidium (Rb). The atomic radius increases as you move down a group in the periodic table, so Li < Na < K < Rb < Cs..

​Therefore, the correct answer is: Option 2) C and E only.