Question

For the second period elements the correct increasing order of first ionisation enthalpy is :

• A. Li < Be < B < C < N < O < F < Ne
• B. Li < B < Be < C < O < N < F < Ne
• C. Li < B < Be < C < N < O < F < Ne
• D. Li < Be < B < C < O < N < F < Ne

Answer 1

The Correct Option is B

The question asks for the correct increasing order of first ionisation enthalpy for elements in the second period of the periodic table. First ionisation enthalpy is the energy required to remove the outermost electron from a gaseous atom. The general trend is that ionisation enthalpy increases across a period due to increasing nuclear charge and decreasing atomic radius, making it harder to remove an electron.

1. Lithium (Li): As an alkali metal, Li has a relatively low ionisation enthalpy because it easily loses its single valence electron.
2. Boron (B) vs Beryllium (Be): Although Boron (B) comes after Beryllium (Be) in the periodic table, Beryllium has a full 2s subshell, which provides greater stability than the partially filled 2p subshell in Boron. This results in Beryllium having a higher ionisation enthalpy than Boron.
3. Carbon (C): Carbon has a higher ionisation enthalpy than both Boron and Beryllium due to increased nuclear charge and effective attraction between the nucleus and the outer electrons.
4. Nitrogen (N) vs Oxygen (O): Nitrogen has a half-filled 2p subshell which provides added stability compared to Oxygen. Consequently, Nitrogen has a higher ionisation enthalpy than Oxygen despite being positioned earlier in the periodic table.
5. Fluorine (F) and Neon (Ne): These elements have high ionisation enthalpies, with Neon having a completed octet, making it the most stable and thus having the highest ionisation enthalpy in the second period.

Based on these characteristics, the order of increasing first ionisation enthalpy is:

Li < B < Be < C < O < N < F < Ne

Each step of this order accounts for the stability and electron configuration effects that influence the ionisation enthalpy of each element. Therefore, the correct answer is:

Li < B < Be < C < O < N < F < Ne