Problem 4 - A Complete Resource Book in Chemistry for JEE Main

Question

Which set has same number of unpaired electrons? (a) \(\mathrm{Fe}^{2+}, \mathrm{Mn}^{+2}\) (b) \(\mathrm{Fe}^{3+}, \mathrm{Mn}^{+2}\) (c) \(\mathrm{Cr}^{+3}, \mathrm{Ni}^{2+}\) (d) \(\mathrm{Zn}^{2+} \mathrm{Cu}^{2+}\)

Step-by-Step Solution

Answer

Set (b) FE^{3+} and Mn^{2+} have the same number of unpaired electrons, both having 5 unpaired electrons.

Step 1: Find Electron Configuration

Determine the electron configuration of each ion to find the number of electrons. For transition metals, remember to remove electrons from the outermost s orbital before removing from the d orbital.

Step 2: Find Unpaired Electrons for FE^{2+} and Mn^{2+}

- ** FE^{2+}:** - Neutral Fe is [Ar] 4s^2 3d^6. - Fe^{2+} loses two electrons: [Ar] 3d^6. - There are 4 unpaired d electrons. - ** Mn^{2+}:** - Neutral Mn is [Ar] 4s^2 3d^5. - Mn^{2+} loses two electrons: [Ar] 3d^5. - All 5 d electrons are unpaired. Conclusion: FE^{2+} and Mn^{2+} have a different number of unpaired electrons.

Step 3: Find Unpaired Electrons for FE^{3+} and Mn^{2+}

- ** FE^{3+}:** - Neutral Fe: [Ar] 4s^2 3d^6 - Fe^{3+} loses three electrons: [Ar] 3d^5. - All 5 d electrons are unpaired. - ** Mn^{2+}:** - Already calculated in Step 2 as having 5 unpaired electrons. Conclusion: FE^{3+} and Mn^{2+} have the same number of unpaired electrons.

Step 4: Find Unpaired Electrons for Cr^{3+} and Ni^{2+}

- ** Cr^{3+}:** - Neutral Cr is [Ar] 4s^1 3d^5. - Cr^{3+} loses three electrons: [Ar] 3d^3. - There are 3 unpaired d electrons. - ** Ni^{2+}:** - Neutral Ni is [Ar] 4s^2 3d^8. - Ni^{2+} loses two electrons: [Ar] 3d^8. - There are 2 unpaired d electrons. Conclusion: Cr^{3+} and Ni^{2+} have different numbers of unpaired electrons.

Step 5: Find Unpaired Electrons for Zn^{2+} and Cu^{2+}

- ** Zn^{2+}:** - Neutral Zn is [Ar] 4s^2 3d^{10}. - Zn^{2+} loses two electrons: [Ar] 3d^{10}. - There are 0 unpaired d electrons. - ** Cu^{2+}:** - Neutral Cu is [Ar] 4s^1 3d^{10}. - Cu^{2+} loses two electrons: [Ar] 3d^{9}. - There is 1 unpaired d electron. Conclusion: Zn^{2+} and Cu^{2+} have different numbers of unpaired electrons.

Key Concepts

Unpaired Electrons

Unpaired electrons are electrons in an atom’s outer shells that do not have a partner with opposite spin. In the context of electron configuration, they reside in specific orbitals and significantly influence chemical properties like magnetism. To determine the number of unpaired electrons in an ion, first write the electron configuration for the neutral atom and then adjust for the ion. Transition metals, for instance, often have unpaired electrons in their d orbitals. If a d orbital has 5 electrons, all are unpaired because each electron occupies its own orbital according to Hund's rule. Understanding whether electrons are paired or unpaired helps in predicting the chemical behavior of the ion, such as forming magnetic materials.

Transition Metals

Transition metals are elements found in the d-block of the periodic table, known for their ability to form variable oxidation states and having partially filled d orbitals. These orbitals play a crucial role in the unique behavior and properties of transition metal compounds.

• They often exhibit high melting points and densities.
• They are typically good conductors of electricity.
• Their ability to form complex ions makes them useful in various industrial chemical reactions.

Transition metals' physical and chemical properties are attributed to their d electrons. When forming ions, transition metals lose their highest energy s electrons first, followed by d electrons, making their electron configurations somewhat unique and varied.

Ion Electron Configuration

Determining the electron configuration of ions involves removing or adding electrons based on the ion's charge. For transition metals, special attention should be given to which electrons are removed. Typically, electrons are removed in the order of s then d orbitals.For example, when iron (Fe) becomes Fe⁺², it loses two electrons: the two 4s electrons first. This changes its electron configuration from \([\text{Ar}] 3d^6\) to \([\text{Ar}] 3d^6\). Understanding how ions adapt their electron configurations is essential for predicting bonding and reactivity, as these configurations affect properties like magnetic behavior and the potential to form coordination compounds.

Fe, Mn, Cr, Ni, Zn, Cu Ions

The electron configurations of Fe, Mn, Cr, Ni, Zn, and Cu ions are crucial to understanding their chemical behavior.

• Fe²⁺ () has \([\text{Ar}] 3d^6\), with 4 unpaired electrons. Fe³⁺ has \([\text{Ar}] 3d^5\), featuring 5 unpaired electrons, which means both Mn²⁺ and Fe³⁺ have the same unpaired electrons.
• Mn²⁺ is \([\text{Ar}] 3d^5\) with 5 unpaired electrons.
• Cr³⁺ is \([\text{Ar}] 3d^3\), having 3 unpaired electrons.
• Ni²⁺ has \([\text{Ar}] 3d^8\) with 2 unpaired electrons.
• Zn²⁺, with \([\text{Ar}] 3d^{10}\), has no unpaired electrons, while Cu²⁺ is \([\text{Ar}] 3d^9\), with 1 unpaired electron.

These configurations show how transitional elements might behave differently when in ionized forms, critical for their application in materials like catalysts and magnets.