What Is a Transition Element? Definition and Why Zn, Cd, Hg Are Not
Chemistry · D And F Block Elements · NEET
A transition element is a metal that has a partly filled (incomplete) d-subshell either in its free atom OR in at least one of its common ions. Zinc, cadmium and mercury are NOT transition metals because they have a completely filled d10 arrangement in both the atom and their +2 ions. Memory hook: "Empty or full d = not transition; in-between d = transition." Zn is at the end, so its d is already FULL.
A transition element must have a partly filled d-subshell in its atom or a common ion. Cu passes through Cu2+ (3d9); Zn, Cd, Hg stay d10 in both atom and ion, so they are d-block but not transition metals.
Your doubts, answered
What is the exact IUPAC definition of a transition element?
According to IUPAC (and NCERT), a transition metal is a metal that has an incomplete (partly filled) d-subshell either in the neutral atom OR in one of its common ions. The key word is EITHER-OR. So even if the free atom looks full, if one common ion has a partly filled d, the element still counts as a transition element. For NEET, learn this line word for word.
Why are Zn, Cd and Hg not transition metals even though they are in the d-block?
Zn, Cd, Hg and Cn (group 12) have the outer configuration (n-1)d10 ns2. Their d-subshell is COMPLETELY filled (d10) in the free atom, and it stays d10 in their common +2 ions too (Zn2+ = 3d10). Since neither the atom nor the common ion has a partly filled d-subshell, they fail the definition. They ARE d-block elements, but they are NOT transition elements. This is the most common NEET trap.
Is 'd-block element' the same as 'transition element'?
No. Every transition element is in the d-block, but not every d-block element is a transition element. The d-block is groups 3 to 12 (based on position in the table). 'Transition element' is a chemical definition based on a partly filled d-subshell. Zn, Cd, Hg are d-block by position but non-transition by definition. Remember: d-block = address; transition = behaviour.
Why IS Sc still a transition element if Sc3+ has an empty d?
Scandium's atom is [Ar]3d1 4s2, so the free ATOM already has a partly filled d-subshell (3d1). The definition says the incomplete d can be in the atom OR the ion. Sc satisfies it through the atom, so it qualifies, even though Sc3+ is 3d0. Many students wrongly drop Sc; do not. Only Zn, Cd, Hg are the borderline non-transition members.
Is Cu a transition element even though it is 3d10 4s1?
Yes. Copper atom is [Ar]3d10 4s1 (d looks full), BUT its common ion Cu2+ is 3d9, which is partly filled. Since one common ion has an incomplete d-subshell, Cu is a transition element. Compare with Zn2+ (3d10, still full) which is why Zn fails and Cu passes.
⚠️ The NEET trap ✗ Zn, Cd, Hg are in the d-block, so they must be transition metals. ✓ They are d-block by position but their d-subshell is FULL (d10) in both the atom and the common +2 ion, so they are NOT transition metals. Only a partly filled d counts. 🧠 Full d10 = not transition. Empty d0 alone in the atom = not transition (that is why Sc still qualifies through its atom, but Zn2+ = 3d10 disqualifies Zn).
Real NEET questions
NEET 2018
Which one of the following ions exhibits d-d transition and paramagnetism as well?
A · MnO4^-
B · Cr2O7^2-
C · CrO4^2-
D · MnO4^2- ✓
Solution: A d-d transition and paramagnetism both need at least one electron in the d-subshell (a partly filled d, i.e. d1 or more) — the same idea behind the transition-element definition. MnO4^- (Mn is +7) and Cr2O7^2-, CrO4^2- (Cr is +6) all have a d0 metal centre, so they are diamagnetic and their colour comes from charge transfer, not d-d. In MnO4^2- (manganate) Mn is +6, i.e. d1: one unpaired d electron means it is paramagnetic AND can show a true d-d transition. So the answer is (D). This shows why a full or empty d behaves differently from a partly filled d.
NEET 2023 Phase 2
Assertion (A): Ionisation enthalpy increases along each series of the transition elements from left to right, however small variations occur. Reason (R): There is a corresponding increase in nuclear charge which accompanies the filling of electrons in the inner d-orbitals.
A · Both (A) and (R) are correct and (R) is the correct explanation of (A) ✓
B · Both (A) and (R) are correct but (R) is not the correct explanation of (A)
C · (A) is correct but (R) is not correct
D · (A) is not correct but (R) is correct
Solution: Across a transition series, electrons are added to the inner (n-1)d orbitals while nuclear charge rises. The added d electrons shield the outer ns electrons poorly, so the effective nuclear charge felt by the outer electron increases from left to right, raising ionisation enthalpy — with only small variations because of changing d-electron repulsions and stability of half/fully filled d. So both statements are true and R correctly explains A. Answer (A). Note this reasoning applies precisely because transition elements are progressively filling that inner d-subshell.
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In one line, what makes an element a transition element?
It has a partly filled d-subshell in the free atom or in at least one of its common ions.
Which d-block elements are NOT transition elements?
The group 12 elements: zinc (Zn), cadmium (Cd), mercury (Hg) and copernicium (Cn). Their d-subshell is full (d10) in both the atom and the common +2 ion.
Is Zn2+ paramagnetic or coloured?
No. Zn2+ is 3d10 (fully filled), so it has no unpaired d electrons and no d-d transition. It is diamagnetic and colourless — extra proof it behaves as a non-transition metal.
Do Zn, Cd, Hg still get studied in this chapter?
Yes. They are the end members of the 3d, 4d and 5d series, so NCERT studies their chemistry alongside the transition metals even though, by definition, they are not transition elements.
Why does this definition matter for NEET?
NEET regularly asks which element is or is not a transition metal, and why a species is coloured or paramagnetic. All of these depend on whether the d-subshell is partly filled, so this one definition unlocks many marks.
