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MCQEasyJEE 2024Trends in Atomic Radius & Ionisation Energy

JEE Chemistry 2024 Question with Solution

The element having the highest first ionization enthalpy is:

  • A

    Si

  • B

    Al

  • C

    N

  • D

    C

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: The options are Si, Al, N, and C.

Find: The element with the highest first ionization enthalpy.

Ionization enthalpy is the energy required to remove an electron from a gaseous atom. It generally increases across a period and decreases down a group.

Among the given elements, Nitrogen (N) has a very high first ionization enthalpy because of its small size and stable half-filled pp-orbital configuration.

Hence, the order is

Al<Si<C<N\text{Al} < \text{Si} < \text{C} < \text{N}

Therefore, the element having the highest first ionization enthalpy is N. The correct option is C.

Periodic Trend Shortcut

Given: The same set of elements Si, Al, N, and C.

Find: Which one has maximum first ionization enthalpy.

Use periodic trends directly: first ionization enthalpy increases from left to right in a period and decreases down a group. Also, N is especially stable due to half-filled configuration.

So the order is

Al<Si<C<N\text{Al} < \text{Si} < \text{C} < \text{N}

Thus, Nitrogen has the highest first ionization enthalpy, so the correct option is C.

Common mistakes

  • Choosing C over N by only looking at left-to-right increase is incomplete. Although both are in the same period, N has extra stability due to its half-filled 2p32p^3 configuration, so it has higher first ionization enthalpy.

  • Choosing Si because it is larger is incorrect. Larger atomic size generally lowers first ionization enthalpy because the outer electron is farther from the nucleus. Down the group from C to Si, ionization enthalpy decreases.

  • Choosing Al by confusing metallic character with ionization enthalpy is wrong. Metals usually have lower ionization enthalpy because they lose electrons more easily. Al therefore cannot have the highest value here.

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