Question

Correct order of 2$^{nd}$ ionisation energy is:

• A. O<C<N<F
• B. C<N<O<F
• C. O<C<N<F
• D. C<O<N<F

Hint:

Half-filled and fully-filled subshells show extra stability, affecting ionisation energy trends.

Answer 1

The Correct Option is C

The question asks for the correct order of the second ionisation energy of several elements: Oxygen (O), Carbon (C), Nitrogen (N), and Fluorine (F). To determine the order, let's understand the concept of ionisation energy.

Ionisation Energy: Ionisation energy is the energy required to remove an electron from an atom or ion in its gaseous state. The second ionisation energy is the energy required to remove the second electron after the first has been removed.

The second ionisation energy is generally higher than the first because it is more difficult to remove an electron from a positively charged ion than from a neutral atom.

Trends in Ionisation Energy:

• Across a period from left to right, ionisation energy generally increases due to increasing nuclear charge, which pulls the electrons closer to the nucleus.
• Down a group, ionisation energy decreases because the outer electrons are further from the nucleus and feel less electrostatic force.

For the elements in question, let's consider their positions in the periodic table:

• Carbon (C): Group 14, Period 2
• Nitrogen (N): Group 15, Period 2
• Oxygen (O): Group 16, Period 2
• Fluorine (F): Group 17, Period 2

The second ionisation energy will be higher for elements that were stable after losing the first electron. Expectedly, after an electron is removed, the remaining electrons experience more attraction towards the nucleus, making subsequent removal more difficult.

Reasoning for Each Element:

• Oxygen (O): After removing one electron, Oxygen becomes O⁺. The remaining p³ configuration is relatively stable.
• Carbon (C): After removing one electron, Carbon becomes C⁺. Carbon does not achieve a stable configuration nor a half-filled or full shell upon losing its second electron, making subsequent removals moderately difficult.
• Nitrogen (N): After removing one electron, Nitrogen becomes N⁺. Nitrogen originally has a half-filled p orbital (p³) which is quite stable. Removing a second electron from this stable configuration is relatively difficult.
• Fluorine (F): After the removal of its first electron, the configuration approaches a stable state, so the removal of the second electron will require considerable energy.

Hence, the expected order of second ionisation energy, considering these elements, should increase across the period as O < C < N < F.

Correct Answer: O < C < N < F.