MHT-CET Chemistry · Transition and Inner Transition Elements

The d-Block: Position, d-Electron Counts and General Properties

Transition elements are the d-block elements that have a partly filled d subshell in the atom or in a common ion; they sit in groups 3 to 12 as four series of ten, their ions lose the outer s electrons before any d electrons, and the half-filled and filled d subshells explain the odd configurations of chromium and copper and the high ionisation enthalpy of zinc.

All Transition and Inner Transition Elements notes MHT-CET Chemistry strategy

Why this matters

28 PYQs, 1 HARD. Of the eighteen set from 2021 to 2025, seven place the elements — the series of Co and Mo, the groups and period of Sc to Zn, ten elements a series, the last element of the 5d series, and which element (Hg) is not a transition element; six count d electrons in an atom or ion — Ti²⁺, Ti³⁺, Co²⁺, Cr with six unpaired electrons, the half-filled Mn and Cr; five are general properties — the soft metal, the highest and lowest first ionisation enthalpy, and one HARD ionisation order. Three cards.

Concept 1 of 3: Four Transition Series in Groups 3 to 12

The d-block sits between the s-block and the p-block, in groups 3 to 12, and its properties sit between theirs too. Each row that fills a d subshell is one series of ten elements, and there are four: 3d in period 4 (Sc to Zn), 4d in period 5 (Y to Cd), 5d in period 6 (La, then Hf to Hg), and 6d in period 7 (Ac, then Rf onward). The series number is one less than the period: cobalt is in period 4, so it is a 3d element; molybdenum is in period 5, a 4d element. A transition element needs a PARTLY filled d subshell in the atom or a common ion, so zinc, cadmium and mercury, which are d¹⁰ both as atoms and as M²⁺ ions, are d-block elements but not transition elements in the strict sense.

Definition

Position: groups 3 to 12; Sc–Zn are period 4, groups 3–12.
Series (10 elements each, four series): 3d (Sc–Zn), 4d (Y–Cd), 5d (La, Hf–Hg), 6d (Ac, Rf–Cn). Last element of the 5d series: Hg.
The 5d series is La plus Hf to Hg; it does NOT run through every element from La to Hg — the fourteen lanthanoids Ce–Lu between them fill 4f.
Not transition (d¹⁰ in the atom and in the common ion): Zn, Cd, Hg. Silver counts as transition because Ag²⁺ is d⁹.
Last electron entering the $(n-1)d$ orbital marks a d-block element: Ag, not Dy or Pu (f-block).

Series	Period	Elements	Example
3d	4	Sc (21) – Zn (30)	Co, Cr, Mn, Fe
4d	5	Y (39) – Cd (48)	Mo, Ag
5d	6	La (57), Hf (72) – Hg (80)	Pt, Au, Hg The lanthanoids Ce–Lu sit inside this row but are f-block.
6d	7	Ac (89), Rf (104) onward	—

Series number = period − 1; ten elements a series.

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2025 · 19 April Shift II · Q95Easy

Example 1 · Transition and Inner Transition Elements · Position, Electronic Configuration and General Features of d-Block

Which transition series includes elements Co and Mo respectively?

Using the period number as the series number

Cobalt is in period 4 but is a 3d element; the d subshell being filled is one shell below the valence shell, (n − 1)d.

Drill 6 more on four transition series in groups 3 to 12

Concept 2 of 3: Counting d Electrons in Atoms and Ions

Write the atom as [Ar] 3dˣ 4s², with two exceptions that prefer a half-filled or filled d subshell: chromium is 3d⁵ 4s¹ and copper is 3d¹⁰ 4s¹. To make an ion, take electrons from 4s FIRST and only then from 3d. So Ti (3d² 4s²) gives Ti²⁺ 3d² and Ti³⁺ 3d¹; Co (3d⁷ 4s²) gives Co²⁺ 3d⁷. For a 3d ion the shortcut is: d electrons = atomic number − 18 − charge. Unpaired electrons then follow Hund's rule: up to five d electrons are all unpaired, and beyond five they pair one by one, so dⁿ has n unpaired for n ≤ 5 and 10 − n for n ≥ 5.

Definition

Atoms: [Ar] 3dˣ 4s² from Sc (3d¹) to Zn (3d¹⁰), except Cr 3d⁵ 4s¹ and Cu 3d¹⁰ 4s¹.
Ions: remove 4s first. $\text{Ti}^{2+}$ 3d², $\text{Ti}^{3+}$ 3d¹, $\text{Co}^{2+}$ 3d⁷ (Z = 27, +2 → 7), $\text{Mn}^{2+}$ 3d⁵, $\text{Fe}^{3+}$ 3d⁵, $\text{Cu}^{2+}$ 3d⁹.
Unpaired electrons in dⁿ: n for n ≤ 5, 10 − n for n > 5. Chromium ATOM has 6 unpaired (five 3d + one 4s).
Half-filled d: Mn (3d⁵ 4s², expected and observed) and Cr (3d⁵ 4s¹, observed). Fe, Co and Ni are not half-filled.

d electrons in a 3d ion

n_d = Z - 18 - q;\qquad \text{unpaired} = \begin{cases} n_d & n_d \le 5 \\ 10 - n_d & n_d > 5 \end{cases}

Worked example

How many d electrons and unpaired electrons does V³⁺ have (Z = 23)?

Practise this conceptself-check · 4 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 9th May Shift 1 · Q60Easy

Example 2 · Transition and Inner Transition Elements · Position, Electronic Configuration and General Features of d-Block

What is the number of electrons present in d-subshell if its atomic number is 27 and oxidation state is +2?

Removing the 3d electrons first

4s fills before 3d but empties first too. Fe²⁺ is 3d⁶, not 3d⁴ 4s²; Ti²⁺ is 3d², not 3d⁰ 4s².

Drill 5 more on counting d electrons in atoms and ions

Concept 3 of 3: Hard Metals, Soft Exceptions and First Ionisation Enthalpy

Transition metals are hard, high-melting, malleable and ductile, conduct heat and electricity well and form alloys, because unpaired d electrons join the metallic bonding. Where the d subshell is full — zinc, cadmium, mercury — the d electrons do not take part, so these metals are soft and low-melting. First ionisation enthalpy rises only slowly across the 3d series because each extra proton is largely screened by the extra 3d electron; the ends stand out: scandium is the lowest and zinc, with its stable 3d¹⁰ 4s², is by far the highest.

Definition

General properties: hard (except Zn, Cd, Hg), high melting, malleable, ductile, good conductors of heat and electricity, form alloys, coloured and paramagnetic compounds, variable oxidation states, catalysts.
First ionisation enthalpy (kJ mol⁻¹): Sc 633, Ti 658, V 650, Cr 653, Mn 717, Fe 762, Co 760, Ni 737, Cu 745, Zn 906.
Keyed orders: highest of {Ti, Sc, Zn, Ni} → Zn; lowest of {Cu, Sc, Mn, Zn} → Sc; decreasing Zn > Fe > Cr > Sc.

3d first ionisation enthalpy (ends)

\text{Sc } (633) < \ldots < \text{Fe } (762) < \text{Zn } (906)\ \text{kJ mol}^{-1}

Worked example

Arrange Mn, Sc, Zn and Ni in increasing first ionisation enthalpy.

Practise this conceptself-check · 3 quick reps

The same idea in a real exam question:

MHT-CET · 2024 · 13th May Shift 2 · Q79Hard

Example 3 · Transition and Inner Transition Elements · Position, Electronic Configuration and General Features of d-Block

Which from following series of elements is correctly arranged according to their decreasing order of ionization enthalpy (

IE_1

)

Expecting a steady rise across the series

Cr (653) is barely above Sc (633) and below Fe (762); Zn's filled shell makes it the outlier. Learn the two ends and Fe, not a smooth trend.

Drill 4 more on hard metals, soft exceptions and first ionisation enthalpy

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Counting d Electrons in Atoms and Ions
d electrons in a 3d ion
$n_d = Z - 18 - q;\qquad \text{unpaired} = \begin{cases} n_d & n_d \le 5 \\ 10 - n_d & n_d > 5 \end{cases}$
Hard Metals, Soft Exceptions and First Ionisation Enthalpy
3d first ionisation enthalpy (ends)
$\text{Sc } (633) < \ldots < \text{Fe } (762) < \text{Zn } (906)\ \text{kJ mol}^{-1}$

Reference tables (1)

Four Transition Series in Groups 3 to 124 rows

Series	Period	Elements	Example
3d	4	Sc (21) – Zn (30)	Co, Cr, Mn, Fe
4d	5	Y (39) – Cd (48)	Mo, Ag
5d	6	La (57), Hf (72) – Hg (80)	Pt, Au, Hg The lanthanoids Ce–Lu sit inside this row but are f-block.
6d	7	Ac (89), Rf (104) onward	—

Series number = period − 1; ten elements a series.

Watch out for (3)

Using the period number as the series number
→ Four Transition Series in Groups 3 to 12
Removing the 3d electrons first
→ Counting d Electrons in Atoms and Ions
Expecting a steady rise across the series
→ Hard Metals, Soft Exceptions and First Ionisation Enthalpy

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