Problem 67 - A Complete Resource Book in Chemistry for JEE Main

Question

How many unpaired electrons are present in \(\mathrm{Ni}^{2+}\) ? (a) 8 (b) 4 (c) 2 (d) 0

Step-by-Step Solution

Answer

There are 2 unpaired electrons in \( \mathrm{Ni}^{2+} \).

Step 1: Know the Electron Configuration of Neutral Nickel

The atomic number of Nickel (Ni) is 28. For a neutral atom of Nickel, the electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^8 4s^2\). This means that Nickel has 8 electrons in the 3d subshell.

Step 2: Determine the Electron Configuration of 0 Ni^{2+} 1

To form the \(\mathrm{Ni}^{2+}\) ion, two electrons are removed from the neutral Nickel atom. The electrons are removed from the outermost shell first, which is the 4s orbital, leaving the configuration: \(3d^8\).

Step 3: Identify the Number of Unpaired Electrons in 3d Subshell

The 3d subshell can hold a maximum of 10 electrons and contains 5 orbitals. In \(3d^8\), the electron occupancy will distribute as follows: each of the 5 orbitals will first get 1 electron, making 5 electrons. Three orbitals will pair an additional electron, making 3 pairs and 2 unpaired electrons total.

Key Concepts

Unpaired Electrons

When we talk about unpaired electrons, we're focusing on those electrons in an atom or ion that do not share an orbital with another electron. Unpaired electrons are of great interest because they contribute to the magnetism of an atom, affecting its chemical and physical properties. Understanding how to identify unpaired electrons is crucial in predicting these properties.

To determine the number of unpaired electrons in an ion, we first look at its electron configuration. Electrons occupy orbitals in a way that minimizes energy, meaning they fill lower energy levels first before moving to higher ones. Within these orbitals, electrons will prefer to remain unpaired as long as possible, according to Hund's Rule, which states that every orbital in a subshell is singly occupied before any orbital is doubly occupied. For the ion \(\mathrm{Ni}^{2+}\), this results in the electron configuration \(3d^8\), where we see 2 unpaired electrons in the 3d subshell. Understanding the presence of unpaired electrons is essential for predictions in chemical reactions and material properties.

3d Subshell

The 3d subshell is an integral part of understanding transition metals and their electron configurations. It represents a key energy level within an atom's electron cloud, situated after the 3p subshell and before the 4s subshell in the current electron configuration order.

The 3d subshell consists of 5 orbitals, and can therefore accommodate up to 10 electrons in total. Its positioning and filling order can be a bit tricky to grasp because, in terms of energy, the 3d subshell actually falls just before the 4s subshell. This means that during electron filling, electrons populate the 4s orbital first, and then move into the 3d subshell. However, when forming positively charged ions like \(\mathrm{Ni}^{2+}\), electrons are removed in the opposite order—with the outermost electrons in the 4s subshell being removed first.

For \(\mathrm{Ni}^{2+}\) specifically, understanding its 3d subshell is crucial for finding how many electrons are unpaired. With its final configuration being \(3d^8\), it is evident that 2 of these electrons remain unpaired.

Transition Metals

Transition metals are a group of elements found in the d-block of the periodic table and are characterized by their partially filled d subshells. These elements, including nickel, are known for their unique set of properties, such as forming colored compounds, displaying various oxidation states, and exhibiting magnetic properties.

One of the defining features of transition metals is their ability to lose different numbers of electrons, creating ions with different charges. For example, Nickel, a transition metal, typically loses its 2 outer 4s electrons when it becomes Ni\(^{2+}\). This loss results in the configuration of \(3d^8\), which directly influences the magnetic properties due to the presence of unpaired electrons.

Transition metals play a critical role in many industrial and biological systems, thus, understanding their electron configuration, particularly in the context of unpaired electrons and subshell filling, provides valuable insight into their reactivity and role in these systems.