Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The five successive ionization enthalpies of an element are 800,2427 , 3658, 25024 and \(32824 \mathrm{~kJ} \mathrm{~mol}^{-1}\). The number of valence electrons in the element is : (a) 5 (b) 4 (c) 3 (d) 2

Step-by-Step Solution

Answer

The number of valence electrons is 3.

Step 1: Understanding Ionization Enthalpy

Ionization enthalpy refers to the amount of energy required to remove an electron from an atom in the gaseous state. The values given indicate how much energy needed for each successive electron removal.

Step 2: Analyzing Energy Values

Look at the given ionization enthalpy values: 800, 2427, 3658, 25024, and 32824 kJ/mol. Notice the large jump between the third and fourth values (from 3658 to 25024). Such a significant increase typically indicates the removal of an electron from a new, inner electron shell.

Step 3: Determining Valence Electrons

The large increase between the third and fourth ionization enthalpies suggests the removal of the first core electron, indicating that the element has 3 valence electrons. These electrons are easier to remove compared to accessing the inner core electrons.

Key Concepts

valence electrons

Valence electrons are the outermost electrons of an atom and play a vital role in chemical reactions and bonding. These electrons reside in the atom's highest energy level. Since they are farthest from the nucleus, they can be more easily shared, lost, or gained in chemical reactions compared to inner electrons. The number of valence electrons determines the chemical reactivity of an element. Elements with a full set of valence electrons, like noble gases, are generally inert, while those with one or more vacancies in their outer shell tend to engage actively in chemical reactions. In our exercise, the dramatic increase in ionization enthalpy between certain steps helps determine the number of valence electrons. The first significant jump from 3658 kJ/mol to 25024 kJ/mol, as seen in the original exercise, indicates that the initial few electrons were removed from the valence shell, revealing that there are three valence electrons.

energy required for electron removal

The energy needed to remove an electron from an atom is called ionization enthalpy or ionization energy. It's a measure of the strength with which an electron is bound to an atom. The first ionization energy refers to the energy needed to remove the first valence electron. Each subsequent electron removal requires more energy, signifying successive ionization enthalpies. Key points to consider:

• Higher ionization energy indicates that an electron is more tightly held by the nucleus and harder to remove.
• Successive ionization requires more energy due to increased effective nuclear charge as outer electrons are removed.
• A large jump in ionization enthalpy occurs when an electron is removed from a stable, inner electron shell.

The original exercise highlights these aspects by showing varying ionization energy levels and a significant jump that points to changes in electron shells.

successive ionization

Successive ionization refers to the process of removing multiple electrons from an atom one after another. With each electron removed, the remaining electrons experience a stronger effective nuclear charge, making them more difficult to remove. This concept is crucial in understanding the electronic structure of an atom. **Observations from the exercise:** 1. **Initial Electron Removal:** The first few ionization energies typically belong to valence electrons. These are lower in energy requirements because valence electrons are farther from the nucleus. 2. **Energy Increase:** As more electrons are removed, particularly beyond the valence shell, more significant jumps in energy levels occur due to the higher nuclear charge affecting the remaining electrons. 3. **Application in Exercise:** The jump occurs between the third and fourth ionization enthalpies in the exercise, marking the transition from valence to core electrons and hence determining the element's valence electrons.