One of the notable features of the transition element is the great variety of oxidation states it may show in its compounds. The elements which give the greatest number of oxidation states occur in or near the middle of the series. Manganese, for example exhibits all the oxidation states from +2 to +7. The lesser number of oxidation states at the extreme ends is either due to the loss or sharing of a few electrons (Sc, Ti) or too many d electrons of higher valence. Early in the first series scandium (II) is virtually unknown and titanium (IV) which is more stable than Ti (III) or Ti (II). On the other end, the only oxidation state of zinc is +2.
Fe shows extra stability as \(Fe^{3+}\) due to
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(d^5\)
Iron (Z = 26) has the configuration [Ar] 3d6 4s2. Working out the ions:
\( \ce{Fe^{2+}} \): remove the two 4s electrons → [Ar] 3d6
\( \ce{Fe^{3+}} \): remove one more 3d electron → [Ar] 3d5
\( \ce{Fe^{3+}} \) has a half-filled d subshell (d5), with one electron in each of the five d orbitals. This arrangement has the maximum exchange energy and is extra stable, which is why \( \ce{Fe^{2+}} \) (d6) is easily oxidised to \( \ce{Fe^{3+}} \).
Why the other options are incorrect
Option (1) d10 — Incorrect. A completely filled d subshell belongs to ions such as \( \ce{Zn^{2+}} \) and \( \ce{Cu^{+}} \); \( \ce{Fe^{3+}} \) has only five d electrons.
Option (3) s0 — Incorrect. \( \ce{Fe^{3+}} \) does have an empty 4s orbital, but so does \( \ce{Fe^{2+}} \); the empty s orbital cannot explain why \( \ce{Fe^{3+}} \) is the more stable ion.
Option (4) s2 — Incorrect. 4s2 is the configuration of the neutral iron atom; it is lost when the ion forms.
NCERT (2026-27 edition):
Trends in the M3+/M2+ Standard Electrode Potentials – “An examination of the Eo(M3+/M2+) values (Table 4.2) shows the varying trends. The low value for Sc reflects the stability of Sc3+ which has a noble gas configuration. The highest value for Zn is due to the removal of an electron from the stable d10 configuration of Zn2+. The comparatively high value for Mn shows that Mn2+(d5) is particularly stable, whereas comparatively low value for Fe shows the extra stability of Fe3+ (d5).”