Problem 58 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Read the following statement and explanation and answer as per the options given below : ASSERTION : The first ionization energy of Be is greater than that of B. REASON : \(2 p\) orbital is lower in energy than \(2 s\)

Step-by-Step Solution

Answer

Assertion is true, reason is false.

Step 1: Understanding Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom or ion. It is influenced by atomic structure and electron configuration.

Step 2: Identifying Electron Configuration

Beryllium (Be) has the electronic configuration of 1s² 2s², while Boron (B) has 1s² 2s² 2p¹. Notice that Boron has a p electron, whereas Beryllium's electrons are only in the s sublevel.

Step 3: Comparing Orbital Energies

The reasoning provided in the statement suggests that the 2p orbital is lower in energy than the 2s orbital. However, this is incorrect. The 2s orbital is actually lower in energy than the 2p orbital in the same shell in an atom.

Step 4: Explaining the Assertion

Since the 2s orbital is lower in energy than the 2p orbital, it requires more energy to remove an electron from the full 2s subshell in Be than from the 2p subshell in B. This means that Be has a higher ionization energy than B.

Step 5: Conclusion Assessment

The assertion is true because the first ionization energy of Be is indeed greater than that of B. However, the reason provided is incorrect because it inaccurately describes the relative energies of 2p and 2s orbitals.

Key Concepts

Electron Configuration

The electron configuration of an atom describes the distribution of electrons in the atomic orbitals. Understanding the electron configuration is essential for grasping concepts like ionization energy. For beryllium (Be), the electron configuration is:

• 1s² 2s²

This configuration means Be has four electrons total, with two electrons in the 1s orbital and two in the 2s orbital. Boron (B), on the other hand, has the following electron configuration:

• 1s² 2s² 2p¹

This means Boron has an additional electron in the 2p orbital after filling the 2s orbital. When it comes to ionization energy, understanding which orbital the valence electron occupies helps explain trends. Electrons in the 2s orbital are generally lower in energy and more stable, hence harder to remove, compared to those in the 2p orbital.

Energy Levels

Energy levels, also called electron shells, indicate the levels of energy that electrons in an atom can have. Each energy level contains subshells or orbitals where electrons reside. For example, in Be and B, the energy levels are the same up to the second shell ( 2s and 2p electrons). However, how the electrons are distributed in these subshells affects the ionization energy.

Significance of Energy Levels

• Energy levels influence chemical properties and reactivity.
• They determine the placement and behavior of electrons in an atom.

Ionization energy must overcome the attraction between the nucleus and the electron being removed. Higher-energy electrons are further from the nucleus and easier to remove than lower-energy electrons.

Orbital Energies

Orbital energies determine the stability of the electrons within the atom. Different types of orbitals (s, p, d, f) have different energy levels. In the second energy level, for example:

• 2s orbitals are lower in energy than 2p orbitals.
• An electron in a 2s orbital is more stable and requires more energy to be removed.

Explaining the Ionization Energy Difference

This insight explains why Be, having a complete 2s subshell, has a higher ionization energy than B. For B, the electron in the 2p orbital is higher in energy and thus easier to remove. The assertion that Be's ionization energy is greater than B's is correct. The incorrect reasoning was suggesting 2p orbitals were lower in energy than 2s. Understanding the actual energy hierarchy aids in making accurate predictions about atomic behavior.