Problem 74 - A Complete Resource Book in Chemistry for JEE Main

Question

Correct order of ionization energy among the elements \(\mathrm{Be}, \mathrm{B}, \mathrm{C}, \mathrm{N}, \mathrm{O}\) is (a) \(\mathrm{B}<\mathrm{Be}<\mathrm{C}<\mathrm{O}<\mathrm{N}\) (b) \(\mathrm{B}<\mathrm{Be}<\mathrm{C}<\mathrm{N}<\mathrm{O}\) (c) \(\mathrm{Be}<\mathrm{B}<\mathrm{C}<\mathrm{N}<\mathrm{O}\) (d) \(\mathrm{Be}<\mathrm{B}<\mathrm{O}<\mathrm{N}<\mathrm{C}\)

Step-by-Step Solution

Answer

The correct order is option (a): \( \mathrm{B}<\mathrm{Be}<\mathrm{C}<\mathrm{O}<\mathrm{N} \).

Step 1: Understand Ionization Energy

Ionization energy is the energy required to remove an electron from an atom in its gaseous state. Generally, ionization energy increases across a period from left to right due to the increase in the number of protons, which leads to a stronger attraction between the nucleus and the electrons.

Step 2: Acknowledge Exceptions

While the general trend is that ionization energy increases across a period, there are exceptions caused by electron configuration. For example, \( \mathrm{Be} \) (beryllium) has a filled s subshell, making it more stable than \( \mathrm{B} \) (boron) which has a p subshell starting to fill, leading to \( \mathrm{Be} \) having a higher ionization energy than \( \mathrm{B} \). Similarly, \( \mathrm{N} \) (nitrogen) has a half-filled p subshell which is more stable than \( \mathrm{O} \) (oxygen), giving \( \mathrm{N} \) a higher ionization energy than \( \mathrm{O} \).

Step 3: Compare Ionization Energies

Starting with the lowest ionization energy, \( \mathrm{B} \) has the lowest followed by \( \mathrm{Be} \). Next comes \( \mathrm{C} \), then \( \mathrm{O} \) and finally \( \mathrm{N} \), which has the highest ionization energy of the five due to the stability of its half-filled p subshell.

Step 4: Determine Correct Order

From the analysis, the correct order of ionization energy from lowest to highest is \( \mathrm{B} < \mathrm{Be} < \mathrm{C} < \mathrm{O} < \mathrm{N} \). This corresponds to option (a) given in the exercise.

Key Concepts

Periodic Table Trends

When you look at the periodic table, you can notice several patterns or trends that help predict an element's properties. One of these trends is ionization energy, which typically increases as you move from left to right across a period. This happens because the number of protons in the nucleus increases, which strengthens the pull of the nucleus on the outer shell electrons. As a result, it requires more energy to remove an electron. This trend can guide us in predicting behaviors and characteristics of various elements. For instance, due to increasing nuclear charge, elements on the far right side of a period often have higher ionization energies than those on the left.

Electron Configuration Exceptions

Though the trend of increasing ionization energy from left to right on the periodic table is generally consistent, exceptions arise due to electron configurations. These exceptions mainly involve the stability of filled and half-filled subshells. Take beryllium (Be) and boron (B) for example: Beryllium has a filled s subshell, which is energetically favorable. However, boron starts filling its p subshell, leading to instability in comparison to a filled subshell. This makes beryllium's ionization energy higher than boron's, defying the usual trend. Similarly, nitrogen (N) possesses a half-filled p subshell, which provides additional stability compared to oxygen (O), giving nitrogen a higher ionization energy. Understanding these exceptions helps when predicting chemical properties and reactions.

Chemistry for JEE

When preparing for competitive exams like JEE, mastering periodic table trends and exceptions in electron configuration is crucial. These concepts form the basis for many questions and problem-solving scenarios. Pay close attention to the patterns across periods and groups, as recognizing trends allows for quicker analysis. Dive into the peculiarities of electron configurations, as they often serve as points of interest in JEE questions. Approach these topics with the understanding that exceptions are as important as the rules. Use study aids and practice problems to become familiar with these nuances, and always check how exceptions affect trends.

Ionization Energy Comparison

When comparing ionization energies, it is essential to consider the general trend as well as any exceptions. In the case of the elements Be, B, C, N, and O, the ionization energy does not simply increase smoothly.

• General Trend: Usually, ionization energy increases from boron to carbon and oxygen due to increased nuclear charge drawing electrons closer.
• Exceptions: Beryllium has a filled s subshell and therefore a higher ionization energy than boron. Similarly, nitrogen's half-filled p subshell results in higher stability than in oxygen, leading to a higher ionization energy.

Understanding these comparisons involves analyzing both the overarching patterns and how particular electron arrangements result in unique properties. Letting both factors inform your predictions can help accurately determine properties like ionization energy.