The transition elements exhibit variable oxidation states because (n −1)d electrons can also participate in bonding as the energy difference between (n −1)d and ns orbitals is very small. The oxidation states of transition elements change in units of one whereas in p-block elements oxidation states normally differ by two units. The minimum oxidation state exhibited by a transition element is equal to the number of electrons in the ns orbital. The maximum oxidation state that can be exhibited by a transition element is equal to the total number of electrons present in both ns and (n −1)d orbitals. The elements in the middle exhibit more oxidation states, e.g., manganese exhibits from +2 to +7. The elements at the extreme ends exhibit a lesser number of oxidation states. This is because of the availability of lesser electrons to lose or to share in the earlier elements or too many d electrons due to which lesser number of unpaired electrons to share at the end. Due to anomalous electronic configuration, chromium and copper can exhibit a minimum oxidation state of +1. In the first series, the maximum oxidation state increases up to Mn and then decreases from Fe onwards. In the first series, Mn exhibits the maximum oxidation state +7. In the second series, a stable maximum oxidation state is exhibited by technetium, and an unstable maximum oxidation state +8 is exhibited by ruthenium. In the third series, stable maximum oxidation +8 is exhibited by osmium. The transition metal ions having completely filled and exactly half-filled d-sub level and those having octet in their outermost shell are stable. The stabilities of Cr³⁺ and Mn⁴⁺ ions is due to high lattice energy in solid state and high hydration energy in their aqueous solutions. Fe³⁺ ion is more stable than Fe²⁺ ion because of the stable half-filled 3d⁵ electronic configuration in Fe³⁺. In the last five elements of the 3d series, the 3d electrons are stabilized and require more energy for their removal because the 3d orbital contracts more and come nearer to the nucleus with an increase in nuclear charge. Thus in the last five elements, the +2 oxidation state becomes more stable (except Fe³⁺ ).

Transition elements exhibit variable oxidation states because they release electrons from the following orbitals

( n −1)d and ns orbitals

The correct answer is option 2. (n-1)d and ns orbitals.

Let us break down why transition elements exhibit variable oxidation states due to the release of electrons from both the \(ns\) and \((n-1)d\) orbitals:

Electron Configuration and Transition Metals

Transition Element Configuration:

Transition elements are defined by having partially filled \(d\) orbitals. Their general electron configuration is \((n-1)d^{1-10} ns^{0-2}\), where \(n\) represents the principal quantum number of the outermost shell.

Oxidation State Formation:

When transition metals form ions, they lose electrons from their outermost \(ns\) orbital first, followed by electrons from the \((n-1)d\) orbitals. Because the \(ns\) and \((n-1)d\) orbitals are close in energy, both can participate in ionization, leading to multiple possible oxidation states.

Example of Variable Oxidation States

Iron (Fe):

Electron Configuration:\( [Ar] 3d^6 4s^2\)

Common Oxidation States:

+2 (Fe²⁺): Formed by losing the two \(4s\) electrons.

+3 (Fe³⁺): Formed by losing the two \(4s\) electrons and one \(3d\) electron.

The presence of electrons in both \(4s\) and \(3d\) orbitals allows iron to exhibit multiple oxidation states.

Manganese (Mn):

Electron Configuration: \([Ar] 3d^5 4s^2\)

Common Oxidation States:

+2 (Mn²⁺): Formed by losing the two \(4s\) electrons.

+3, +4, +5, +6, +7 (MnO₄⁻): Formed by losing additional \(3d\) electrons.

The ability to lose electrons from both \(4s\) and \(3d\) orbitals allows manganese to exhibit a range of oxidation states.

Why Both Orbitals Matter

Energy Levels: The \(ns\) and \((n-1)d\) orbitals are relatively close in energy, so the electrons in both can be removed with relatively similar energy requirements.

Electron Removal: Since electrons can be removed from both orbitals, it increases the range of possible oxidation states.

Stability: The stability and energy considerations of the resulting ions can affect the range of oxidation states that a transition metal can exhibit.

Summary

Transition elements exhibit variable oxidation states because they can lose electrons from both their outermost \(ns\) and \((n-1)d\) orbitals. This flexibility in electron loss allows transition metals to adopt multiple oxidation states. Thus, the correct explanation for the variable oxidation states of transition elements is due to the ability to release electrons from both the \(ns\) and \((n-1)d\) orbitals.