Which of the following elements has the highest first ionization potential?

Introduction / Context:
Ionization potential, also called ionization energy, is the energy required to remove the most loosely bound electron from a neutral atom in the gaseous state. The first ionization potential refers to removing the first electron. Understanding trends in ionization energy across periods and groups of the periodic table is a key part of atomic structure and periodicity. This question compares boron, carbon, nitrogen and oxygen to see if the learner remembers the trend and the special stability of certain electron configurations.

Given Data / Assumptions:
- The elements considered are boron (B), carbon (C), nitrogen (N) and oxygen (O).
- All are in the same second period of the periodic table.
- We are comparing first ionization potentials only.
- General knowledge of periodic trends and electron configurations is assumed.

Concept / Approach:
Across a period from left to right, ionization energy generally increases due to increasing nuclear charge and similar shielding, which pulls electrons more strongly. However, there are exceptions related to electron configurations and subshell stability. Nitrogen has a half filled 2p subshell with configuration 2p3, which is particularly stable. Oxygen has one more electron, causing pairing in a 2p orbital and slightly lower ionization energy than expected. Therefore, among B, C, N and O, nitrogen has the highest first ionization potential because of its stable half filled configuration.

Step-by-Step Solution:
Step 1: Write the second period order: B, C, N, O across the period. Step 2: Recall the general trend that ionization energy increases from left to right across a period as nuclear charge increases. Step 3: Consider electron configurations in the 2p subshell: boron has 2p1, carbon has 2p2, nitrogen has 2p3 and oxygen has 2p4. Step 4: Recognise that nitrogen has a half filled 2p3 configuration, which is especially stable and therefore requires more energy to remove an electron. Step 5: Note the known exception where oxygen has slightly lower first ionization energy than nitrogen because removing one electron relieves electron repulsion in a paired orbital. Conclude that nitrogen has the highest first ionization potential among the given elements.

Verification / Alternative check:
An alternative check is to recall approximate or relative values from data tables. Typical first ionization energies in kilojoules per mole follow the pattern: boron is lower, carbon is higher, nitrogen is higher still, and oxygen drops slightly below nitrogen. Graphs of ionization energy versus atomic number show a noticeable peak at nitrogen in the second period. This graphical evidence confirms that nitrogen has the highest first ionization potential among the listed elements.

Why Other Options Are Wrong:
Option A, oxygen, might be chosen if one applies the simple left to right trend without considering exceptions, but its first ionization energy is slightly lower than that of nitrogen because of electron repulsion in a paired 2p orbital. Option B, carbon, has a lower ionization potential than both nitrogen and oxygen since it has fewer protons and less effective nuclear charge. Option C, boron, is further to the left and has the lowest ionization energy among the four, as its outer electron is more weakly held. Therefore none of these options exceed nitrogen in first ionization potential.

Common Pitfalls:
A common error is to apply the periodic trend mechanically and assume that oxygen must have the highest ionization energy because it is furthest to the right among the given options. Another pitfall is forgetting the special stability associated with half filled and fully filled subshells. Students who do not recall the exception at nitrogen and oxygen may misinterpret such questions. Paying attention to both general trends and specific exceptions ensures a more accurate understanding of periodic properties.

Final Answer:
Among the given elements, the highest first ionization potential belongs to Nitrogen.