class 12 Chemistry D Block Transition elements notes

 

The d-Block Transition Elements (Sections 4.1 – 4.2)

This study guide provides a comprehensive overview of the transition elements within the d-block, focusing on their definitions, periodic positioning, and electronic configurations as outlined in the initial sections of the chemistry curriculum.

1. Introduction and IUPAC Definition of Transition Elements

The d-block of the periodic table consists of elements in Groups 3 to 12. These elements are characterised by the progressive filling of the d-orbitals across four long periods.

Definitions

• d-Block Elements: Elements in which the d-orbitals are being filled. These are located in the large middle section of the periodic table.
• Transition Metals: Often used interchangeably with d-block elements, though specific criteria apply under modern definitions.
• Inner Transition Metals: Elements of the f-block (lanthanoids and actinoids) where 4f and 5f orbitals are progressively filled.

The IUPAC Definition

According to the International Union of Pure and Applied Chemistry (IUPAC), transition metals are defined as metals which have an incomplete d-subshell either in their neutral atom state or in their common ions.

• Historical Context: The name "transition" originally referred to the fact that their chemical properties were transitional between the highly reactive s-block (metals) and the p-block (non-metals) elements.
• The Group 12 Exception (Zn, Cd, Hg): Zinc (Zn), Cadmium (Cd), and Mercury (Hg) have a full d^{10} configuration in their ground state as well as in their common oxidation states. Consequently, they are not strictly regarded as transition metals according to the IUPAC definition.
• Study Inclusion: Despite not being transition metals by definition, Group 12 elements are studied alongside the d-block elements because they are the "end members" of the transition series and share certain chemical associations.

2. Position in the Periodic Table (Section 4.1)

The d-block occupies the central region of the periodic table, flanked between the s-block (Groups 1 and 2) and the p-block (Groups 13 to 18).

The Four Transition Series

The transition elements are organised into four horizontal series corresponding to the filling of the 3d, 4d, 5d, and 6d orbitals:

Series	Orbital Filled	Range of Elements
First Series	3d	Scandium (Sc) to Zinc (Zn)
Second Series	4d	Yttrium (Y) to Cadmium (Cd)
Third Series	5d	Lanthanum (La) and Hafnium (Hf) to Mercury (Hg)
Fourth Series	6d	Actinium (Ac) and Rutherfordium (Rf) to Copernicium (Cn)

Note: The 5d series is interrupted by the lanthanoids, and the 6d series is interrupted by the actinoids.

3. Electronic Configurations of the d-Block Elements (Section 4.2)

The electronic configuration of these elements is defined by the filling of the penultimate energy level (n-1)d orbitals.

General Configuration

The general electronic configuration for the outer orbitals of d-block elements is: (n-1)d^{1–10} ns^{1–2}

• (n-1)d: Represents the inner d-orbitals containing 1 to 10 electrons.
• ns: Represents the outermost s-orbital containing 1 or 2 electrons.

The Case of Palladium (Pd)

Palladium is a notable exception to the general configuration. Its electronic configuration is: 4d^{10} 5s^0 (Z = 46)

Stability of Half-Filled and Fully-Filled Orbitals

Energy gaps between the (n-1)d and ns orbitals are very small. Because half-filled (d^5) and completely filled (d^{10}) sets of orbitals possess extra stability, certain elements exhibit anomalous configurations to achieve these states:

1. Chromium (Cr, Z = 24)

• Expected: 3d^4 4s^2
• Actual: 3d^5 4s^1
• Reason: Achieving a half-filled d^5 subshell provides greater stability. The energy gap between 3d and 4s is small enough to allow this electron distribution.

2. Copper (Cu, Z = 29)

• Expected: 3d^9 4s^2
• Actual: 3d^{10} 4s^1
• Reason: Achieving a completely filled d^{10} subshell provides maximum stability.

Configurations of the First Transition Series (3d)

Element	Symbol	Atomic Number (Z)	Configuration (Outer Orbitals)
Scandium	Sc	21	3d^1 4s^2
Titanium	Ti	22	3d^2 4s^2
Vanadium	V	23	3d^3 4s^2
Chromium	Cr	24	3d^5 4s^1
Manganese	Mn	25	3d^5 4s^2
Iron	Fe	26	3d^6 4s^2
Cobalt	Co	27	3d^7 4s^2
Nickel	Ni	28	3d^8 4s^2
Copper	Cu	29	3d^{10} 4s^1
Zinc	Zn	30	3d^{10} 4s^2

4. Important Questions for CBSE/State Board Exams

Question 1: Scandium (Z = 21) is a transition element, but Zinc (Z = 30) is not. Why? Answer: According to the IUPAC definition, a transition element must have an incompletely filled d-subshell. Scandium in its ground state has a 3d^1 configuration (incompletely filled), making it a transition element. Zinc, however, has a completely filled 3d^{10} configuration in its ground state as well as in its common oxidation state (Zn^{2+}), so it is not regarded as a transition element.

Question 2: Why is Silver (Z = 47) classified as a transition element despite having a fully filled 4d^{10} subshell in its ground state? Answer: Although Silver has a 4d^{10} 5s^1 configuration in its ground state, it is classified as a transition element because it can form ions (such as Ag^{2+}) that possess an incomplete d-subshell (4d^9). The IUPAC definition includes elements that have incomplete d-subshells in their common ions.

Question 3: Explain why Group 12 elements have the general configuration (n-1)d^{10} ns^2 and are not regarded as transition metals. Answer: Elements in Group 12 (Zn, Cd, Hg, Cn) have their d-orbitals completely filled in their ground state and in their common oxidation states. Because they lack the "incomplete d-subshell" required by the IUPAC definition, they are classified as d-block elements but not transition metals.

5. JEE & NEET Corner (High-Yield Conceptual Questions)

Q1. Which element in the 4d series has a unique configuration where the outermost s-orbital is empty?

• Answer: Palladium (Pd). Its configuration is 4d^{10} 5s^0. This is an exception caused by the very small energy difference between the 4d and 5s orbitals, allowing the 5s electrons to fully occupy the 4d subshell for stability.

Q2. What is the fundamental reason for the anomalous electronic configurations of Chromium and Copper?

• Answer: There are two primary factors:
  1. The relative stability of half-filled (d^5) and completely filled (d^{10}) orbitals.
  2. The very small energy gap between the (n-1)d and ns orbitals, which prevents a large energy penalty when an electron enters the d-orbital instead of the s-orbital.

Q3. Based on the provided definition, are all d-block elements transition elements?

• Answer: No. While all transition elements belong to the d-block, not all d-block elements are transition elements. Specifically, Group 12 elements (Zn, Cd, Hg) are d-block elements but fail the transition metal criteria of having an incomplete d-subshell in their atomic or ionic states.

Q4. Why do transition elements and their compounds require separate study from non-transition elements?

• Answer: The presence of partly filled d-orbitals confers unique characteristic properties on these elements, such as the display of a variety of oxidation states, formation of coloured ions, catalytic properties, paramagnetic behaviour, and the ability to form complex ions with various ligands. These properties distinguish them significantly from s- and p-block elements.

Physical Properties and Atomic Trends of d-Block Elements

1. Introduction to the Transition Series (d-Block)

The d-block elements, situated in the central heart of the periodic table (Groups 3–12), represent a critical domain where the penultimate d-orbitals are progressively filled. For the purpose of competitive examinations, you must strictly adhere to the IUPAC definition: a transition metal is defined as an element which possesses an incomplete d-subshell in either its neutral atomic state or in its common ionic states.

Key Concept: The Group 12 Distinction Note that Zinc, Cadmium, and Mercury (Group 12) have a full d^{10} configuration in their ground state as well as in their common oxidation states. Consequently, they are not regarded as transition metals, though their chemistry is studied alongside the d-block as end members of the series.

The four transition series are summarised below:

Transition Series	d-Orbital Filled	Starting Element	Ending Element	Atomic Number (Z) Range
First Series (3d)	3d	Scandium (Sc)	Zinc (Zn)	21–30
Second Series (4d)	4d	Yttrium (Y)	Cadmium (Cd)	39–48
Third Series (5d)	5d	Lanthanum (La), Hafnium (Hf)	Mercury (Hg)	57, 72–80
Fourth Series (6d)	6d	Actinium (Ac), Rutherfordium (Rf)	Copernicium (Cn)	89, 104–112

The electronic configuration of these elements dictates their physical behaviour, as the d-orbitals protrude to the periphery of the atom more than s- or p-orbitals, making them highly susceptible to their surroundings.

2. Fundamental Physical and Metallic Properties

The general electronic configuration of the d-block is (n-1)d^{1–10}ns^{1–2}. Memorise the critical exception: Palladium (Pd), which exhibits a 4d^{10}5s^0 configuration. This variability arises from the negligible energy difference between the (n-1)d and ns orbitals.

Transition metals exhibit a high density of delocalised electrons, which facilitates strong metallic bonding and results in the following characteristic properties:

• High Tensile Strength: Ability to withstand significant structural stress.
• Ductility and Malleability: Capacity for being drawn into wires or hammered into sheets.
• Thermal and Electrical Conductivity: Efficient transport of heat and charge due to mobile electrons.
• Metallic Lustre: Characteristic surface shine.

Lattice Structures of the Transition Elements

Transition metals are remarkably hard and exhibit low volatility. Most adopt high-symmetry lattices, though several elements transition to different structures at elevated temperatures.

Element Symbol	Lattice Structure (Normal Temp)	High-Temperature Transition/Notes
Sc, Ti, Y, Zr	hcp	Transitions to bcc at high temperatures.
V, Nb, Ta	bcc	Highly stable body-centred cubic structures.
Cr, Mo, W	bcc	Group 6 elements maintain bcc.
Mn	X	Anomalous: A typical metal structure.
Fe	bcc	Transitions to hcp and ccp at high temperatures.
Co, Ni	ccp	Transitions to hcp at high temperatures.
Cu, Ag, Au	ccp	Standard cubic close-packed.
Zn, Cd, Hg	X	Anomalous: Group 12 lacks standard symmetry.

The prevalence of these structures, particularly in the 4d and 5d series, underpins the extreme thermal stability and mechanical hardness of these elements.

3. Thermal Trends: Melting Points and Anomalous Behaviours

Melting point trends provide a direct window into the strength of interatomic metallic bonding. High melting points signify that both ns and (n-1)d electrons are participating in the lattice-stabilising "electron sea."

Generally, melting points rise across a series to a maximum at approximately the d^5 configuration, where the number of unpaired electrons available for bonding is highest. However, the "Mn-Tc Mystery" reveals a sharp dip in the middle of the 3d and 4d series.

The Logic of the Dip: Manganese (Mn) and Technetium (Tc) possess a stable d^5 (half-filled) configuration. In these specific cases, the electrons are held more firmly by the nucleus or are arranged in a lattice symmetry that is less favourable for maximum interatomic sharing. This results in weaker interatomic interaction compared to their neighbours, leading to the observed anomalous dip in melting points.

4. Analysis of Enthalpies of Atomisation

Enthalpy of atomisation (\Delta_aH^\circ) is the energy required to separate one mole of a metal into its constituent gaseous atoms. This is a fundamental metric for predicting the "noble" character of a metal.

Key Concept: The Noble Character Link High \Delta_aH^\circ = Strong Interatomic Bonding = Low Chemical Reactivity. Metals with exceptionally high enthalpies of atomisation (and thus high boiling points) tend to be more noble and less reactive.

The maxima for \Delta_aH^\circ occurs in the middle of each series because one unpaired electron per d-orbital is particularly favourable for strong interatomic interactions.

Strategic Note: Heavy Metal Bonding Metals of the second (4d) and third (5d) series have significantly higher enthalpies of atomisation than those of the first (3d) series. You must conclude from this that metal-metal bonding is much more frequent and robust in heavy transition metal compounds than in 3d elements.

5. Variation in Atomic and Ionic Sizes

Atomic and ionic radii in the d-block are governed by the competition between increasing nuclear charge and the imperfect shielding provided by inner electrons.

Horizontal Trends (Across a Period)

There is a progressive, albeit small, decrease in radii across each series. As nuclear charge increases by one unit with each element, the new electron enters a d-orbital. Because d-electrons provide imperfect shielding, they do not fully offset the increased nuclear attraction. Consequently, the cloud is pulled inward.

Vertical Trends and Shielding Efficiency

While size increases from the 3d to 4d series due to additional shells, a "stagnation point" occurs between the 4d and 5d series.

Key Concept: The Shielding Hierarchy Shielding effectiveness follows the order: s > p > d > f. Because 4f and 5d electrons provide very poor shielding, the increasing nuclear charge has a disproportionately strong effect on the outer electrons in the heavier series.

6. Lanthanoid Contraction and Its Consequences

The most critical exception to vertical size trends is the Lanthanoid Contraction. This refers to the regular decrease in the size of the 4f subshell as the nuclear charge increases.

The Root Cause

Before the 5d series begins, the 14 lanthanoid elements involve the filling of 4f orbitals. The shielding of one 4f electron by another is even less effective than that between d-electrons. This "extremely poor shielding" allows the nuclear charge to exert a massive inward pull on the entire electronic cloud.

Three Exam-Critical Consequences

1. Similarity in Radii: The radii of the 5d series are virtually the same as the 4d series (e.g., Zr and Hf).
2. Natural Co-occurrence: Due to identical sizes, these pairs (like Zr/Hf) are found together in nature.
3. Separation Difficulty: Their nearly identical physical and chemical properties make separation through standard chemical means extremely difficult.

Density Implications

From Titanium (Z=22) to Copper (Z=29), there is a significant increase in density. This is caused by the simultaneous decrease in metallic radius and the sharp increase in atomic mass.

7. Exam-Ready Assessment

Part A: Board-Style Conceptual Questions

1. According to IUPAC, why is Zinc excluded from the transition metal classification?
2. Account for the sharp anomalous dip in the melting point of Manganese (Mn) in the 3d series.
3. Explain why 4d and 5d series elements often show more frequent metal-metal bonding than 3d elements.
4. Describe the impact of Lanthanoid Contraction on the atomic radii of Zirconium (Zr) and Hafnium (Hf).

Part B: JEE/NEET-Style Multiple Choice Questions

1. The ground state electronic configuration of Palladium (Z=46) is:
   • (a) [Kr] 4d^8 5s^2
   • (b) [Kr] 4d^9 5s^1
   • (c) [Kr] 4d^{10} 5s^0
   • (d) [Kr] 4d^{10} 5s^2
2. Which subshell provides the least effective shielding of the outer electrons from nuclear charge?
   • (a) 4s
   • (b) 4p
   • (c) 3d
   • (d) 4f
3. Chromium (Z=24) adopts a 3d^5 4s^1 configuration rather than 3d^4 4s^2 primarily because:
   • (a) 4s orbitals have higher energy than 3d.
   • (b) Half-filled sets of orbitals (d^5) are relatively more stable.
   • (c) The nuclear charge is too low to fill 4s.
   • (d) It reduces inter-electronic repulsion in the 4s orbital.
4. The density increase from Titanium to Copper is most significantly attributed to:
   • (a) Increase in atomic radius.
   • (b) Constant atomic mass.
   • (c) Decrease in metallic radius and increase in atomic mass.
   • (d) Filling of the 4f subshell.
5. Which element in the 3d series does NOT adopt a bcc, hcp, or ccp lattice at normal temperatures?
   • (a) Titanium
   • (b) Vanadium
   • (c) Manganese
   • (d) Iron
6. High enthalpy of atomisation in transition metals typically correlates with:
   • (a) High volatility and low melting points.
   • (b) Noble character and high boiling points.
   • (c) Increased chemical reactivity.
   • (d) Completly filled d-subshells.

8. Rigorous Answer Key & Explanations

Part A: Answers

1. Chemical Logic: IUPAC requires an incomplete d-subshell. Zinc has a full d^{10} configuration in its neutral state and its +2 ion, thus it is not a transition metal.
2. Chemical Logic: Mn has a stable d^5 configuration; its specific electronic arrangement/lattice is less favourable for the strong interatomic interactions required for high melting points.
3. Chemical Logic: Elements of the 4d and 5d series possess higher enthalpies of atomisation than 3d elements, indicating stronger interatomic forces that facilitate metal-metal bonds.
4. Chemical Logic: Due to Lanthanoid Contraction (poor 4f shielding), the radii of the 5d series (Hf) are virtually identical to the 4d series (Zr).

Part B: Answers

1. (c) Logic: Pd is the unique d^{10}s^0 exception noted in the NCERT source context.
2. (d) Logic: Shielding effectiveness follows s > p > d > f. The 4f subshell provides the poorest shielding.
3. (b) Logic: NCERT Section 4.2 states that half-filled (d^5) and completely filled (d^{10}) orbitals offer extra stability.
4. (c) Logic: Table 4.2 shows density increases as mass rises while volume (radius) decreases.
5. (c) Logic: Table 4.1 identifies Manganese as having a typical metal structure 'X' rather than the standard bcc/hcp/ccp.
6. (b) Logic: High enthalpy of atomisation implies strong bonding, which leads to higher boiling points and "noble" (low reactivity) character.