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

(c) 2

Step 1: Determine the Electron Configuration of Neutral Nickel (Ni)

Neutral \(\mathrm{Ni}\) has an atomic number of 28. This means it has 28 electrons. The electron configuration for \(\mathrm{Ni}\) is \(1s^2 \,2s^2 \,2p^6 \,3s^2 \,3p^6 \,3d^8 \,4s^2\).

Step 2: Remove Electrons for \\({\mathrm{Ni}}^{2+}\\) Ion

When neutral \(\mathrm{Ni}\) loses 2 electrons to become \(\mathrm{Ni}^{2+}\), these electrons are removed from the outermost shell. Therefore, we remove the \(4s^2\) electrons, resulting in a new configuration: \(1s^2 \,2s^2 \,2p^6 \,3s^2 \,3p^6 \,3d^8\).

Step 3: Identify the Number of Unpaired Electrons

Examine the \(3d^8\) subshell. In the case of \(3d^8\), we fill up the 5 orbitals in the pattern established by Hund's rule: one electron per orbital before pairing any. This results in 2 unpaired electrons.

Key Concepts

Electron Configuration

Electron configuration is a way of organizing electrons around an atom's nucleus. It follows a specific order based on the energy levels and sublevels. Every atom aims to arrange its electrons in the most stable configuration possible.
For nickel (Ni) with an atomic number of 28, its electron configuration is written as follows:

• **1s^2**: The first energy level which can hold a maximum of 2 electrons.
• **2s^2 2p^6**: The second energy level can hold up to 8 electrons.
  • 2 in its s subshell and 6 in its p subshell.
• **3s^2 3p^6**: Similarly, the third energy level's s and p subshells can collectively hold another 8 electrons.
• **3d^8**: The d subshell in the third energy level can accommodate up to 10 electrons, but nickel fills only 8 of these.
• **4s^2**: Finally, the 4s subshell holds 2 electrons in its outermost energy level.

This whole configuration presents the most stable arrangement for the neutral nickel atom.

Ni2+ Ion

When nickel forms a \(\mathrm{Ni}^{2+}\) ion, it loses 2 electrons. This transformation occurs to achieve a stable electronic structure, typically resembling the nearest noble gas configuration. Electrons are removed from the outermost shell first, which, in the case of nickel, is the 4s subshell.
Thus, the electron configuration for \(\mathrm{Ni}^{2+}\) is adjusted by removing the 2 electrons from the 4s subshell, resulting in:

• \(1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 3d^8\)

This configuration signifies that the \(\mathrm{Ni}^{2+}\) ion now relies primarily on the electrons in its 3d subshell for its interactions.

3d Subshell

The 3d subshell is an important player when discussing transition metals like nickel. It belongs to the third principal energy level and houses up to 10 electrons across 5 orbitals. These orbitals are oriented in different spatial directions. The 3d subshell in neutral nickel fills 8 out of these 10 slots.
In the case of \(\mathrm{Ni}^{2+}\), the electron configuration results in the 3d subshell being written as \(3d^8\). This means there are 8 electrons distributed among the 5 orbitals.
According to Hund's rule, each orbital in a subshell gets one electron before any starts pairing up, which helps determine the number of unpaired electrons.

Hund's Rule

Hund's Rule is a key principle in determining how electrons fill orbitals within a subshell. It states that every orbital in a subshell gets one electron before any orbital gets a second. Essentially, electrons prefer to occupy their own orbital due to repulsion forces inherent among electrons.
When applying Hund's Rule to the \(3d^8\) configuration of \(\mathrm{Ni}^{2+}\), it ensures that:

• Five orbitals in the 3d subshell initially get one electron each.
• Three of these orbitals will receive an additional electron and pair up, aligning their spins to minimize repulsion.

This results in 2 unpaired electrons left in the 3d orbitals. Hund's Rule helps chemists predict magnetic properties and behaviors of atoms and ions by understanding their electronic structure.