Chemistry · Periodic Classification Of Properties · NEET
They have partly filled d-orbitals with unpaired electrons. When light falls on them, an electron jumps from one d-orbital to another slightly higher d-orbital. This is called a d-d transition. The electron absorbs one colour of visible light to make this jump, and the colour we see is the leftover (complementary) light. Example: Cu2+ looks blue, Fe3+ looks yellow-brown. If the d-orbitals are empty or completely full, no d-d jump is possible, so the ion is colourless (like Zn2+ with a full 3d10).
In transition elements the (n-1)d and ns orbitals have almost the SAME energy. So the atom can lose different numbers of electrons without much extra energy. It can lose only the ns electrons, or the ns plus a few (n-1)d electrons. This gives many possible oxidation states. Example: iron shows +2 and +3; manganese shows +2, +3, +4, +6, +7. Main-group (s and p block) elements usually cannot do this because their inner electrons are held too tightly.
Two reasons. First, they show variable oxidation states, so they can easily give and take electrons during a reaction (they act like an electron bridge). Second, their partly filled d-orbitals can hold reactant molecules on the metal surface for a short time, bringing reactants close together so they react faster. Example: Fe is used in the Haber process (making ammonia), V2O5 in the Contact process, and Ni in hydrogenation of oils.
The strict definition: an element whose ATOM or a STABLE ION has a partly filled d-orbital. 'Partly filled' means d1 to d9. This is why Zn, Cd, and Hg (which have full d10 in both atom and common ions) are called d-block but NOT typical transition elements. They do not show the usual colour and variable valence strongly.
No. Only elements with partly filled d-orbitals (d1 to d9) show strong colour and variable valence. Zn (3d10 4s2), Cd, and Hg have completely filled d-orbitals, so their common ions (Zn2+, Cd2+, Hg2+) are colourless and they mostly show only ONE oxidation state (+2). That is why they are not counted as typical transition metals. Sc3+ (d0) is also colourless.
They are in groups 3 to 12, the middle block between the s-block (groups 1-2) and the p-block (groups 13-18). Their general valence configuration is (n-1)d^1-10 ns^0-2. There are four series: 3d (Sc to Zn), 4d (Y to Cd), 5d (La, Hf to Hg), and 6d.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Zn2+ has a completely filled 3d10 configuration, so no d-d electron jump is possible and it stays colourless. Cu2+ has 3d9 (one unpaired electron and a vacancy), so a d-d transition can happen and it appears blue.
In these elements the (n-1)d and ns orbitals lie very near each other in energy. Because of this small energy gap, electrons from both can take part in bonding, which is exactly why variable oxidation states appear.
Not always by itself, but for NEET: colour in an ion usually means partly filled d-orbitals, which is the mark of a typical transition element. A colourless d-block ion (like Zn2+) points to a full d10 and a non-typical transition element.
Iron (Fe) in the Haber process for making ammonia, and Vanadium pentoxide (V2O5) in the Contact process for making sulphuric acid. Both work because transition metals have variable oxidation states and free d-orbitals.
