Question

Given below are two statements :
Statement I : The correct order in terms of atomic/ionic radii is \( Al>Mg>Mg^{2+}>Al^{3+} \).
Statement II : The correct order in terms of the magnitude of electron gain enthalpy is \( Cl>Br>S>O \).
In the light of the above statements, choose the correct answer from the options given below :

• A. Both Statement I and Statement II are false
• B. Both Statement I and Statement II are true
• C. Statement I is false but Statement II is true
• D. Statement I is true but Statement II is false

Hint:

For isoelectronic ions, remember: Higher positive charge = smaller radius.

Answer 1

The Correct Option is C

To determine the accuracy of the given statements, let's evaluate them individually, providing detailed explanations and reasoning.

Statement I: The correct order in terms of atomic/ionic radii is \(Al > Mg > Mg^{2+} > Al^{3+}\).

Explanation: 

• Atomic sizes:
  • The atomic radius of Al (Aluminum) is larger than that of Mg (Magnesium) because they belong to the same period (3rd period) where the atomic size decreases from left to right across a period due to increasing nuclear charge attracting the electron cloud closer to the nucleus. Thus, \(Mg > Al\) for atomic size.
• Ionic sizes:
  • \(Mg^{2+}\) is smaller than \(Mg\) because the loss of electrons leads to a higher effective nuclear charge per electron, pulling the electron cloud tighter to the nucleus.
  • Similarly, \(Al^{3+}\) is even smaller than \(Al\) and \(Mg^{2+}\).
  • The correct order based on ionic and atomic radii should be \(Mg > Al > Mg^{2+} > Al^{3+}\).

This means Statement I is false.

Statement II: The correct order in terms of the magnitude of electron gain enthalpy is \(Cl > Br > S > O\).

Explanation:

• Electron Gain Enthalpy: Electron gain enthalpy refers to the energy released when an electron is added to a neutral atom to form a negative ion.
• Typically, across a period, electron gain enthalpy becomes more exothermic (more negative) due to increasing nuclear charge. However, specific trends are affected by atomic size and electronic configuration.
• The trend for halogens (group 17) is as follows: Chlorine (Cl) has the most negative electron gain enthalpy, followed by Bromine (Br).
• For group 16 elements, Sulfur (S) has a more negative electron gain enthalpy than Oxygen (O) because the smaller size of oxygen leads to increased electron-electron repulsions in its compact 2p orbital.
• The given order \(Cl > Br > S > O\) is correct.

This means Statement II is true.

Conclusion: Based on the detailed analysis above, the correct answer is: Statement I is false but Statement II is true.