d-Block elements
The d-block of the periodic table contains the elements in which the d orbitals are progressively filled in each of the four long periods. The d-block elements are also known as transition metals. Originally, the name transition metals was derived from the fact their chemical properties were transitional between those of s- and p-block elements. They show numerous oxidation states. All the transition elements exhibit typical metallic properties such as high tensile strength, ductility, malleability, thermal and electrical conductivity. Another important property of transition metals is their catalytic activity. Transition metals generally have high melting and boiling points. Many d-block elements are colored.
Which of the following pairs of metal ions is colourless:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Ti4+ and Zn2+
Most transition metal ions are coloured because of d–d transitions: an electron absorbs visible light and moves from a lower to a higher d orbital, and the complementary colour is seen. Such a transition needs a partly filled d subshell, so ions with d0 or d10 configurations are colourless, while d1 to d9 ions are usually coloured. In each ion below, the 4s electrons are removed first, then the 3d:
Ti (Z = 22) is [Ar]3d24s2; losing four electrons gives \( \ce{Ti^{4+}} \): 3d0.
Zn (Z = 30) is [Ar]3d104s2; losing two electrons gives \( \ce{Zn^{2+}} \): 3d10.
Neither ion can have a d–d transition, so both are colourless.
Why the other options are incorrect
Option (2) Cu2+ and Ni2+ — Incorrect. Cu (Z = 29) is [Ar]3d104s1, so \( \ce{Cu^{2+}} \) is 3d9 (blue); Ni (Z = 28) is [Ar]3d84s2, so \( \ce{Ni^{2+}} \) is 3d8 (green).
Option (3) Mn2+ and Fe3+ — Incorrect. Mn (Z = 25) is [Ar]3d54s2, so \( \ce{Mn^{2+}} \) is 3d5 (pink); Fe (Z = 26) is [Ar]3d64s2, so \( \ce{Fe^{3+}} \) is 3d5 (yellow).
Option (4) Cr2+ and Cr3+ — Incorrect. Cr (Z = 24) is [Ar]3d54s1, so \( \ce{Cr^{2+}} \) is 3d4 (blue) and \( \ce{Cr^{3+}} \) is 3d3 (violet).
NCERT (2026-27 edition):
Formation of Coloured Ions
