Which of the following will exhibit only +3 oxidation state?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Ac
The actinoids show in general the +3 oxidation state. Because the 5f, 6d and 7s levels have comparable energies, the elements in the first half of the series also show higher oxidation states. Actinium (5f06d17s2) loses its three outer electrons and has no 5f electrons to use, so it exhibits only +3. Thorium (6d27s2) is +4, neptunium goes up to +7, and curium shows +4 besides +3.
NCERT (2026-27 edition):
Oxidation States – “There is a greater range of oxidation states, which is in part attributed to the fact that the 5f, 6d and 7s levels are of comparable energies. The known oxidation states of actinoids are listed in Table 4.11. The actinoids show in general +3 oxidation state. The elements, in the first half of the series frequently exhibit higher oxidation states. For example, the maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np but decreases in succeeding elements (Table 4.11).”
Oxidation States of Actinium and Actinoids (Table 4.11)
| Ac | Th | Pa | U | Np | Pu | Am | Cm | Bk | Cf | Es | Fm | Md | No | Lr |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 3 | 4 | 3, 4, 5 | 3, 4, 5, 6 | 3, 4, 5, 6, 7 | 3, 4, 5, 6, 7 | 3, 4, 5, 6 | 3, 4 | 3, 4 | 3 | 3 | 3 | 3 | 3 | 3 |