Question

Which of the following has highest second ionisation energy?

• A. Chromium
• B. Calcium
• C. Iron
• D. Cobalt

Hint:

Half-filled and fully-filled configurations are stable.

Answer 1

The Correct Option is A

The question asks which element among the given options has the highest second ionisation energy. To answer this, let's first understand the concept of second ionisation energy.

Definition: Second ionization energy is the energy required to remove an electron from a singly positively charged ion (the M⁺ ion) to form a doubly positively charged ion (M²⁺).

Factors affecting ionization energy:

• Electronic configuration: Elements with stable electronic configurations tend to have higher ionization energies.
• Atomic size: Smaller atoms have higher ionization energies because electrons are closer to the nucleus and thus more strongly attracted.
• Nuclear charge: Higher nuclear charge increases ionization energy.

Let's analyze the electronic configurations of the given elements and their impact on the second ionisation energy:

1. Chromium (Cr): The electronic configuration of Chromium is [Ar] 3d⁵ 4s¹. Upon removing the first 4s electron, Chromium becomes Cr⁺ with a configuration of [Ar] 3d⁵. These 5 electrons in the 3d subshell are half-filled, providing greater stability. Removing another electron would disturb this stable half-filled 3d⁵ configuration, thus requiring more energy.
2. Calcium (Ca): The electronic configuration of Calcium is [Ar] 4s². After losing one electron, Ca⁺ becomes [Ar] 4s¹. Since removing the second electron would result in the stable argon-like noble gas configuration, it requires less energy.
3. Iron (Fe): The electronic configuration is [Ar] 3d⁶ 4s². Losing one 4s electron to form Fe⁺ results in configuration [Ar] 3d⁶ 4s¹, and losing another results in [Ar] 3d⁶, which is relatively stable but not as stable as Cr after the first ionisation.
4. Cobalt (Co): The electronic configuration is [Ar] 3d⁷ 4s². After losing one 4s electron, Co⁺ becomes [Ar] 3d⁷ 4s¹. Removing the second electron yields [Ar] 3d⁷, which is more stable than Fe but not as stable as the half-filled 3d⁵ of Chromium.

Conclusion: Among the given elements, Chromium has a half-filled d subshell after the first ionisation (3d⁵), leading to a significant increase in energy required to remove another electron. Therefore, Chromium has the highest second ionisation energy.