Question

The third ionisation enthalpy is highest in

• A. Alkali metals
• B. Alkaline earth metals
• C. Chalcogens
• D. Pnictogens

Answer 1

The Correct Option is B

1. Understand Ionization Enthalpy
Ionization enthalpy is the energy required to remove an electron from an isolated gaseous atom or ion. Successive ionization enthalpies (first, second, third, etc.) refer to the removal of subsequent electrons. The third ionization enthalpy is the energy required to remove the third electron.

2. Consider the Electronic Configurations

Alkali Metals (Group 1): General electronic configuration is ns¹. After removing one electron (first ionization), they attain a noble gas configuration. Removing a second electron requires significantly more energy (very high second ionization enthalpy). Removing a third electron would be even harder, but the third ionization energy is still extremely high.

Alkaline Earth Metals (Group 2): General electronic configuration is ns². After removing two electrons (second ionization), they attain a noble gas configuration. Removing a third electron would require breaking this stable noble gas configuration, requiring a very high amount of energy.

Chalcogens (Group 16): General electronic configuration is ns²np⁴. They need two more electrons to attain a noble gas configuration. Removing three electrons will not result in a noble gas configuration, and the ionization enthalpies will be lower than for alkali and alkaline earth metals after they have achieved noble gas configurations.

Pnictogens (Group 15): General electronic configuration is ns²np³. They need three more electrons to attain a noble gas configuration. Removing three electrons will result in a noble gas configuration, but with higher nuclear charge than alkali and alkaline earth metals.

3. Analyze the Third Ionization Enthalpy
Alkali metals will have extremely high second and third ionization energies due to the stable noble gas configuration they attain after the first ionization. Alkaline earth metals will have high third ionization energies since after two ionizations they reach a stable configuration. However, the increase is much more prominent in alkaline earth metals because they attain the noble gas configuration only after removing 2 electrons. So the third ionization involves removing an electron from a noble gas configuration.
 

Final Answer:
(B) Alkaline earth metals

Answer 2

The third ionisation enthalpy is highest in alkaline earth metals because after the removal of the second electron, the resulting ion achieves a stable noble gas configuration, making it more difficult to remove the third electron. 

This results in a very high third ionisation energy.

So, the correct answer is (B) : Alkaline earth metals.