Where the d-Block Sits
The periodic table, in its long form, divides into four blocks named after the subshell that is being filled across that region: the s-block (groups 1–2), the p-block (groups 13–18), the d-block (groups 3–12), and the f-block set apart at the bottom. The d-block occupies the large middle section, flanked between the s- and p-blocks. NCERT states it precisely: the d-block contains the elements of groups 3–12 in which the d orbitals are progressively filled in each of the four long periods.
The defining electronic feature is that the d orbitals of the penultimate energy level — the $(n-1)$d orbitals — receive electrons, while the outermost $n$s orbital is already occupied. This is why d-block elements are also called the elements where the last electron enters an inner d subshell rather than the outermost p subshell. The general outer configuration is therefore $(n-1)\mathrm{d}^{1-10}\,n\mathrm{s}^{1-2}$, with familiar exceptions such as $\ce{Cr}$ ($3\mathrm{d}^5 4\mathrm{s}^1$) and $\ce{Cu}$ ($3\mathrm{d}^{10} 4\mathrm{s}^1$) driven by the stability of half- and fully-filled d sets.
Because these elements stand between the highly electropositive s-block metals and the less electropositive p-block elements, the historical name “transition” captures the idea that their properties are transitional between the two flanking blocks. They are uniformly metallic, and their chemistry — variable oxidation states, colour, magnetism, catalysis, complex formation — flows from those partly filled d orbitals.
The Block Map of the Periodic Table
A single schematic fixes the geography better than any paragraph. The figure below shows the four blocks, with the d-block highlighted as the central bridge spanning groups 3–12, and the f-block detached at the foot of the table.
The d-block (teal) is the central bridge of groups 3–12. The s-block lies to its left, the p-block to its right, and the f-block is lifted out as two separate rows below the main body.
Two structural facts follow from this map. First, the d-block only begins from the fourth period — the first two periods have no d electrons, and the third period jumps straight from $3\mathrm{s}/3\mathrm{p}$ to the $4\mathrm{s}$ filling. The $3\mathrm{d}$ orbitals first become accessible after $4\mathrm{s}$, giving Sc the honour of opening the d-block. Second, the d-block is exactly ten groups wide because a d subshell holds a maximum of ten electrons ($\mathrm{d}^{1}$ through $\mathrm{d}^{10}$).
“Penultimate” vs “outermost” shell
In d-block elements the electron being added enters the $(n-1)$d orbital — the second-outermost shell — while the outermost shell remains $n$s. Candidates who write the configuration as $n\mathrm{d}$ instead of $(n-1)\mathrm{d}$ misplace the electrons entirely. For the 3d series, the period number is 4 ($n=4$), so the d orbitals being filled are $3\mathrm{d} = (n-1)\mathrm{d}$.
Rule: d-block filling = $(n-1)\mathrm{d}$, never $n\mathrm{d}$.
The Four Transition Series
Because the d orbitals fill across four separate long periods, the d-block resolves into four horizontal rows, traditionally called the four transition series. NCERT names them explicitly, and each is anchored to a particular set of d orbitals.
| Series | Period | d orbitals filled | Elements (NCERT) |
|---|---|---|---|
| First (3d) series | 4 | 3d | Sc → Zn |
| Second (4d) series | 5 | 4d | Y → Cd |
| Third (5d) series | 6 | 5d | La and Hf → Hg |
| Fourth (6d) series | 7 | 6d | Ac and Rf → Cn |
The third and fourth series carry a small subtlety. In the 5d series, lanthanum (La, $5\mathrm{d}^1 6\mathrm{s}^2$) starts the row, then the fourteen lanthanoids intervene as inner-transition elements, and the d-block resumes at hafnium (Hf) and continues to mercury (Hg). Likewise the 6d series begins with actinium (Ac), is interrupted by the actinoids, and runs from rutherfordium (Rf) to copernicium (Cn). This interleaving is the reason the f-block is drawn as a separate panel — placing it inline would make the table impractically wide.
Four stacked rows of the d-block. The 5d and 6d rows are split because the f-block (dashed purple) is inserted after the first member, La and Ac respectively.
What “Transition Element” Means
The name has two readings, and NEET tests both. The historical reading is descriptive: these metals were called transition elements because their chemical properties stand between those of the s- and p-block elements — a transition from the most electropositive metals to the least electropositive non-metals.
The modern IUPAC reading is operational. A transition metal is defined as a metal that has an incompletely filled d subshell either in its neutral atom or in any of its common ions. The phrase “or in its ions” is the load-bearing part: an element can still qualify even if its neutral atom happens to have a complete d set, provided a common ion does not. This distinction is what separates the genuine transition metals from the d-block members that merely share their address.
IUPAC: transition metals are metals which have an incomplete d subshell either in the neutral atom or in their ions. The presence of partly filled d (or f) orbitals is what sets these elements apart from the non-transition elements.
The practical payoff is enormous. Partly filled d orbitals are the source of the entire characteristic profile of the block: multiple stable oxidation states, paramagnetism from unpaired electrons, intense colour from d–d transitions, catalytic activity, and a strong tendency to form coordination complexes. Where the d shell is full and stays full, those properties largely vanish.
Once the position is clear, the exceptions in $(n-1)\mathrm{d}^{1-10}n\mathrm{s}^{1-2}$ are next. See d-Block Electronic Configurations for Cr, Cu and the half/full-filled stability rule.
Why Zn, Cd and Hg Are Excluded
Group 12 — zinc, cadmium and mercury — is the classic exception, and NEET returns to it often. Apply the IUPAC test directly. In the ground state each has a complete $\mathrm{d}^{10}$ set:
| Element | Ground-state outer config. | Common ion | Ion config. | Transition? |
|---|---|---|---|---|
| Zn (30) | 3d¹⁰ 4s² | $\ce{Zn^2+}$ | 3d¹⁰ | No |
| Cd (48) | 4d¹⁰ 5s² | $\ce{Cd^2+}$ | 4d¹⁰ | No |
| Hg (80) | 5d¹⁰ 6s² | $\ce{Hg^2+}$ | 5d¹⁰ | No |
| Cu (29) | 3d¹⁰ 4s¹ | $\ce{Cu^2+}$ | 3d⁹ | Yes |
For Zn, Cd and Hg, the $\mathrm{d}^{10}$ shell is complete in the atom and in the common $+2$ ion, where only the two $n$s electrons are lost. No common species has an incomplete d subshell, so all three fail the test and are not regarded as transition metals. Contrast copper: although $\ce{Cu}$ atoms and $\ce{Cu+}$ both carry $\mathrm{d}^{10}$, the very common $\ce{Cu^2+}$ ion is $3\mathrm{d}^9$ — an incomplete d subshell — so copper is a transition metal.
NCERT is careful to add the caveat: although Zn, Cd and Hg are not typical transition metals, they sit at the end of the 3d, 4d and 5d series respectively, so their chemistry is studied alongside the transition metals as the natural closing members of each row.
Sc vs Zn — both partial in the atom?
A mirror trap sits at the other end of the row. Scandium ($3\mathrm{d}^1 4\mathrm{s}^2$) forms $\ce{Sc^3+}$ which is $3\mathrm{d}^0$ — also a non-partial d shell. Yet Sc is counted as a transition element because the atom has an incomplete d subshell, and the IUPAC rule reads “atom or ion.” Zn fails on both atom and ion; Sc passes on the atom. Test every common species, not just one.
Pass the IUPAC test if EITHER atom or a common ion has incomplete d.
d-Block vs Transition Metal
The two labels are almost — but not quite — interchangeable, and the difference is purely the group-12 caveat above. Every element of groups 3–12 is a d-block element by position. The transition metals are the subset of those that additionally satisfy the IUPAC incomplete-d condition.
| Term | Criterion | Includes Zn, Cd, Hg? |
|---|---|---|
| d-block element | Position: groups 3–12, d orbitals being filled | Yes |
| Transition metal | Property: incomplete d in atom or common ion | No |
In everyday NEET usage the names “transition metals” and “d-block” are used loosely as synonyms — NCERT itself says the names are “often used to refer to” the d-block. But when a question explicitly asks whether Zn or Hg is a transition metal, fall back on the strict definition: by position, yes (d-block); by property, no (not a transition metal).
d-Block vs Inner Transition (f-Block)
The chapter pairs the d-block with the f-block precisely because the two are parallel ideas one level deeper. The f-block elements are called the inner transition elements: in them the antepenultimate $4\mathrm{f}$ and $5\mathrm{f}$ orbitals are progressively filled, one shell further in than the d orbitals of the transition metals. They are placed in a separate panel at the bottom of the table.
| Feature | d-block (transition) | f-block (inner transition) |
|---|---|---|
| Orbital filled | (n−1)d | (n−2)f (4f, 5f) |
| Position in table | Groups 3–12, main body | Separate panel at bottom |
| Number of series | Four (3d, 4d, 5d, 6d) | Two (4f, 5f) |
| Members | 10 per complete series | 14 per series |
| Names | Transition metals | Lanthanoids (4f, Ce–Lu); Actinoids (5f, Th–Lr) |
So the two inner-transition series are the lanthanoids — the 4f series running Ce to Lu — and the actinoids — the 5f series running Th to Lr. The shared logic is filling an inner subshell while the outermost s orbital stays occupied; the difference is only how deep that inner subshell lies, which makes the f-block elements still more similar to one another within a series than the d-block members are.
Classifying an Element From Its Configuration
The single most common exam application of “position” is being handed an electronic configuration and asked which block it belongs to. The decision rule is the subshell receiving the differentiating electron, and the worked example below shows the exact reasoning NEET 2025 demanded.
Identify the block for each configuration: (A) [Ne]3s¹, (B) [Ar]3d³4s², (C) [Kr]4d¹⁰5s²5p⁵, (D) [Ar]3d¹⁰4s¹, (E) [Rn]5f⁰6d²7s².
(A) [Ne]3s¹ → differentiating electron in 3s → Na, s-block (main group).
(B) [Ar]3d³4s² → filling 3d → V, d-block (incomplete d → transition).
(C) [Kr]4d¹⁰5s²5p⁵ → outermost electron in 5p → I, p-block (main group).
(D) [Ar]3d¹⁰4s¹ → 3d/4s → Cu, d-block (Cu²⁺ is 3d⁹ → transition).
(E) [Rn]5f⁰6d²7s² → 6d/7s region with 5f beginning → Th, f-block (inner transition).
Main-group (s- or p-block) only: A and C.
The procedure generalises: locate the subshell of the last/differentiating electron. If it is $n$s or $n$p, the element is main-group (s- or p-block). If it is $(n-1)$d, it is d-block — then check the ion to confirm whether it is a true transition metal. If it is $(n-2)$f, it is f-block (inner transition). This one rule answers a remarkable share of the block-placement questions the exam sets.
Position of the d-block in one screen
- The d-block = groups 3–12, the central bridge between the s-block and p-block; ten groups wide because a d subshell holds ten electrons.
- Differentiating electron enters the $(n-1)$d orbital; general outer config $(n-1)\mathrm{d}^{1-10}n\mathrm{s}^{1-2}$.
- Four series: 3d (Sc–Zn), 4d (Y–Cd), 5d (La, Hf–Hg), 6d (Ac, Rf–Cn).
- IUPAC transition metal = incomplete d in the atom OR a common ion; the “or ion” clause is the key.
- Zn, Cd, Hg ($\mathrm{d}^{10}$ in atom and $+2$ ion) are d-block by position but not typical transition metals.
- The f-block (inner transition) fills 4f/5f, sits at the table's foot: lanthanoids (Ce–Lu) and actinoids (Th–Lr).