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⁺ ).

Maximum number of oxidation states of the transition elements is derived from the following configuration

ns and (n −1)d electrons

The correct answer is option 4. ns and (n-1)d electrons.

Let us break down why the maximum number of oxidation states of transition elements is derived from both the \( ns \) and \( (n-1)d \) electrons:

Electron Configuration of Transition Elements:

Transition elements typically have the general electron configuration \((n-1)d^{1-10} ns^{0-2}\). This means that in addition to the outermost \( ns \) electrons, there are electrons in the \( (n-1)d \) subshell.

Oxidation States:

Transition elements can lose electrons from both the \( ns \) and \( (n-1)d \) orbitals when forming positive ions. The \( ns \) electrons are usually the first to be removed when an atom is ionized, followed by the \( (n-1)d \) electrons.

Variability of Oxidation States:

Because both \( ns \) and \( (n-1)d \) electrons can be involved in bonding and ion formation, transition elements can exhibit a wide range of oxidation states. For example, manganese (Mn) has the configuration \([Ar] 3d^5 4s^2\). It can lose the two \( 4s \) electrons and all five \( 3d \) electrons, resulting in oxidation states ranging from +2 to +7. Similarly, iron (Fe) with the configuration \([Ar] 3d^6 4s^2\) can exhibit oxidation states of +2 and +3, among others, by losing different numbers of \( 4s \) and \( 3d \) electrons.

Maximum Oxidation States:

The ability to lose both \( ns \) and \( (n-1)d \) electrons increases the maximum possible oxidation state. For example, ruthenium (Ru) and osmium (Os) can exhibit oxidation states up to +8.

Examples:

Manganese (Mn): Electron configuration \([Ar] 3d^5 4s^2\). Oxidation states: +2 (losing 4s electrons), +3, +4, +5, +6, +7 (losing 4s and various numbers of 3d electrons).

Iron (Fe): Electron configuration \([Ar] 3d^6 4s^2\). Common oxidation states: +2 (losing 4s electrons), +3 (losing 4s and one 3d electron).

In summary, the wide range of oxidation states observed in transition elements is due to the combined availability of both \( ns \) and \( (n-1)d \) electrons for ionization. This ability to lose multiple electrons from both orbitals allows for the variety of oxidation states characteristic of transition metals.

The correct answer is ns and (n − 1)d electrons.