Choose the transition element which exhibits the least number of vari…

The correct answer is Option (1) → Sc

Transition elements show variable oxidation states because the (n−1)d and ns electrons have very similar energies, so different numbers of them can be lost or shared. The variety is greatest in the middle of the 3d series and least at the two ends, where there are either too few electrons to lose or too few unpaired d electrons to share.

Scandium (Z = 21) is [Ar]3d¹4s². Losing all three outer electrons gives \( \ce{Sc^{3+}} \) with the stable argon configuration, [Ar]. Losing fewer electrons gives no such stability, and scandium(II) is virtually unknown. So scandium shows only the +3 oxidation state, the least number of oxidation states among the transition elements. (Zinc also shows only +2, but with its completely filled 3d¹⁰ set it is not regarded as a transition element.)

Why the other options are incorrect

• Option (2) Cr — Incorrect. Chromium (Z = 24) is [Ar]3d⁵4s¹, with six outer electrons, and shows +2 to +6 (five states; +3 and +6 are the common ones).

• Option (3) Mn — Incorrect. Manganese (Z = 25) is [Ar]3d⁵4s², in the middle of the series, and shows every state from +2 to +7, the largest number in the 3d series.

• Option (4) Fe — Incorrect. Iron (Z = 26) is [Ar]3d⁶4s² and shows +2, +3, +4 and +6 (+2 and +3 are the common ones).

NCERT (2026-27 edition):
Oxidation States – “… 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 stems from either too few electrons to lose or share (Sc, Ti) or too many d electrons (hence fewer orbitals available in which to share electrons with others) for higher valence (Cu, Zn). Thus, early in the series scandium(II) is virtually unknown and titanium (IV) is more stable than Ti(III) or Ti(II). At the other end, the only oxidation state of zinc is +2 (no d electrons are involved).”

Table 4.3: Oxidation States of the first row Transition Metal (the most common ones are in bold types)

Note: The same/similar question has been asked in different years by NTA as per the details given below: