Question: 78. Arrange the following elements in increasing order of first ionization enthalpy:
Li, Be, B, C, N
Choose the correct answer from the options given below:
(1) Li < Be < B < C < N
(2) Li < B < Be < C < N
(3) Li < Be < C < B < N
(4) Li < Be < N < B < C
Answer: Option (2)
Explanation:
First ionization enthalpy is the energy required to remove the outermost electron from an isolated gaseous atom.
Across a period, ionization enthalpy generally increases due to increasing nuclear charge and decreasing atomic size.
Lithium has the lowest ionization enthalpy because it has the largest atomic size and least effective nuclear charge among the given elements.
Boron has lower ionization enthalpy than beryllium because the electron removed from boron is from the 2p orbital,
which is higher in energy and less penetrating than the 2s electron removed from beryllium.
Carbon has higher ionization enthalpy than both boron and beryllium due to increased nuclear charge and smaller size.
Nitrogen has the highest ionization enthalpy because it has a stable half-filled 2p^{3} electronic configuration.
Therefore, the increasing order of first ionization enthalpy is
Li < B < Be < C < N