The D- And F -Block Elements

Class 12 PCB . Chemistry . Chemistry Part I . The D and F Block Elements

Chapter 4: The D- And F -Block Elements

Ch 4

Chemistry

Class 12 PCB

d-Block Elements

Definition and Position

d-block elements are those elements in which the last electron enters the d-sub-shell of the penultimate shell. They are located in the middle of the periodic table, spanning groups 3 to 12.

Penultimate Shell

The penultimate shell is the shell just inside the valence shell. If the valence shell is the nth shell, the penultimate shell is the (n-1)th shell.

Transition Elements

Transition elements are d-block elements that have incompletely filled d-orbitals in their ground state or common oxidation states. They exhibit properties transitional between s-block and p-block elements.

Series of Transition Elements

First transition series: 3d orbitals, Sc (21) to Zn (30)
Second transition series: 4d orbitals, Y (39) to Cd (48)
Third transition series: 5d orbitals, La (57) and Hf (72) to Hg (80)
Fourth transition series: 6d orbitals, Actinium (89) and Rf (104) onwards (incomplete)
The transition series are formed by filling the d-subshell of the penultimate shell.
Each series begins with the element in which the last electron enters the (n-1)d subshell and ends when that subshell is filled.
The 3d series is also called the first transition series, the 4d series the second, and the 5d series the third transition series.

General Electronic Configuration

The general valence shell electronic configuration of transition elements is (n-1)d^1-10 ns^1-2, where n is the outermost shell. In most transition metals, the ns electrons and some (n-1)d electrons take part in bonding. Because the energies of these orbitals are close, the electronic configuration may show small irregularities in some elements.

In many transition elements, one electron from the ns orbital is promoted to the (n-1)d orbital to obtain a more stable arrangement, such as a half-filled or fully filled d-subshell.

As a result, atoms of elements like Cr and Cu do not always follow the simple expected filling order exactly.

Physical Properties

All are metals, malleable and ductile except mercury, which is liquid at room temperature.
They are good conductors of heat and electricity.
They show metallic lustre and are sonorous.
Their atomic radii are smaller than those of s-block elements but larger than those of p-block elements in the same period.
Lanthanide contraction causes 4d and 5d elements to have nearly similar sizes.
Ionic radii decrease with increasing oxidation state.
Density generally increases from left to right in a period.
Ionisation enthalpy generally increases across the series, with some irregularities.
They show strong metallic bonding due to delocalised d-electrons.
They have high enthalpy of atomisation.
They exhibit variable oxidation states because the ns and (n-1)d electrons have similar energies.
They usually have lower and irregular electrode potentials.
They often show catalytic properties due to multiple oxidation states and large surface area.
Magnetic properties: diamagnetic if all electrons are paired; paramagnetic if one or more unpaired electrons are present.
They generally have high melting and boiling points, except Zn, Cd and Hg.
They tend to form complex ions.
They form coloured compounds due to d-d electronic transitions.
They form alloys because of similar atomic sizes and metallic bonding.
They form interstitial compounds with small atoms such as H, B, C and N.

Chemical Properties

Transition metals form oxides in several oxidation states, and these oxides may be basic, amphoteric or acidic depending on the oxidation state and the nature of the metal. Lower oxidation state oxides are generally basic, whereas higher oxidation state oxides tend to be acidic or amphoteric.

Important compounds of transition elements include potassium dichromate (K₂Cr₂O₇) and potassium permanganate (KMnO₄). These compounds are powerful oxidising agents and are studied for their preparation, properties and uses in analytical chemistry and industrial processes.

Solved Examples

Example 1

Given the standard electrode potentials (E°) for Cr²⁺/Cr = −0.9 V, Cr³⁺/Cr²⁺ = −0.4 V, Mn²⁺/Mn = −1.2 V, Mn³⁺/Mn²⁺ = +1.5 V, Fe²⁺/Fe = −0.4 V, Fe³⁺/Fe²⁺ = +0.8 V, comment on:

(a) Stability of Fe³⁺ compared to Cr³⁺ and Mn³⁺ in acid solution.

(b) Ease of oxidation of iron compared to chromium and manganese.

Solution:

(a) Cr³⁺ has a negative reduction potential, indicating that Cr³⁺ is relatively stable and not easily reduced to Cr²⁺. Mn³⁺ has a large positive E°, so it is easily reduced to Mn²⁺ and is less stable. Fe³⁺ has a positive but smaller E° than Mn³⁺, so its stability is intermediate. Thus, the stability order is Cr³⁺ > Fe³⁺ > Mn³⁺.

(b) The more negative the reduction potential of the M²⁺/M couple, the easier is oxidation of the metal. Since Mn²⁺/Mn has the most negative E°, manganese is most easily oxidised. Chromium comes next and iron is least easily oxidised. The order of ease of oxidation is Mn > Cr > Fe.

Example 2

Interpret the magnetic moment of the complex K₄[Mn(CN)₆] with a magnetic moment of 2.2 BM.

Solution:

Mn is in the +2 oxidation state. The magnetic moment of 2.2 BM indicates one unpaired electron. Since CN⁻ is a strong-field ligand, the complex is low spin. For an octahedral complex with one unpaired electron, the hybridisation is d²sp³ and the geometry is octahedral.

Example 1

(a) Stability of Fe³⁺ compared to Cr³⁺ and Mn³⁺ in acid solution.

(b) Ease of oxidation of iron compared to chromium and manganese.

Solution:

(a) Cr³⁺ has a negative reduction potential, indicating it is stable and not easily reduced. Mn³⁺ has a large positive E°, so it is easily reduced to Mn²⁺ and is less stable. Fe³⁺ has a positive but smaller E° than Mn³⁺, so its stability is intermediate.

(b) Mn²⁺ has the most negative reduction potential, meaning it is most easily oxidised. The order of ease of oxidation is Mn > Cr > Fe.

Example 2

Interpret the magnetic moment of the complex K₄[Mn(CN)₆] with a magnetic moment of 2.2 BM.

Solution:

Mn is in +2 oxidation state. The magnetic moment of 2.2 BM indicates one unpaired electron, suggesting a low-spin complex with d²sp³ hybridisation due to the strong field ligand CN⁻, resulting in an octahedral geometry.

Practice Set

Level 1 (Easy): Define transition elements and state their general electronic configuration.
Transition elements are elements whose atoms or at least one of whose common ions have incompletely filled d-orbitals. Their general electronic configuration is (n-1)d¹-¹⁰ns⁰-².
Level 2 (Moderate): Explain why transition metals exhibit variable oxidation states.
Transition metals exhibit variable oxidation states because the energies of the (n-1)d and ns orbitals are very close. As a result, both ns electrons and a variable number of (n-1)d electrons can take part in bonding and ion formation, giving several oxidation states.
Level 3 (Challenging): Write the balanced chemical equation for the preparation of potassium dichromate from chromite ore.
The preparation of potassium dichromate from chromite ore involves the following steps:
- Roasting chromite ore with sodium carbonate in air: 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
- Acidification of sodium chromate to sodium dichromate: 2Na₂CrO₄ + H₂SO₄ → Na₂Cr₂O₇ + Na₂SO₄ + H₂O
- Conversion of sodium dichromate to potassium dichromate: Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Answer Key

Level 1: Transition elements are d-block elements with incompletely filled d-orbitals. General electronic configuration: (n-1)d¹-¹⁰ ns¹-².
Level 2: Variable oxidation states arise because ns and (n-1)d electrons have similar energies and can be lost during reactions.
Level 3: 4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 2Fe₂O₃ + 8Na₂CrO₄ + 8CO₂

f-Block Elements

Definition and Position

f-block elements are those in which electrons fill the (n-2)f sub-shell, the third shell from the outermost. They include two series: lanthanoids and actinoids, also called inner transition elements.

General Electronic Configuration

(n-2)f¹⁻¹⁴ (n-1)d⁰⁻¹ ns², with n = 6 for lanthanoids and n = 7 for actinoids. Exceptions exist because some elements show one electron in the (n-1)d subshell instead of complete filling of the f subshell, giving extra stability to half-filled or fully filled configurations. In lanthanoids, the filling of 4f orbitals is gradual, so electrons are added mainly to the 4f subshell, with occasional promotion to 5d. In actinoids, 5f orbitals are involved and the energies of 5f, 6d and 7s orbitals are very close, so variable electronic configurations are common.

Lanthanoids

Elements from Ce (Z=58) to Lu (Z=71) filling 4f orbitals.

Physical Properties

Dense, soft, malleable, ductile metals
High melting points
Form alloys easily
Magnetic properties: La³⁺ and Lu³⁺ are diamagnetic; others paramagnetic due to unpaired electrons
Atomic and ionic radii decrease slightly across the series due to lanthanoid contraction

Chemical Properties

Actinoids are reactive metals and are less reactive towards acids in the higher oxidation states, but many are readily attacked in the metallic state. The common oxidation state is +3, but +4, +5, +6 and +7 are also found, especially in the earlier actinoids. They form complexes readily and many of their compounds are coloured because of electronic transitions. All actinoids are radioactive, and several exist only in trace amounts or as synthetic elements.

Lanthanide Contraction

Steady decrease in size across lanthanoids due to poor shielding by 4f electrons, causing similar properties and difficulty in separation.

Uses

Misch metal (alloy of cerium and other lanthanoids) used in lighter flints
Lanthanoid oxides for glass polishing
Cerium salts in dyeing and catalysis
Catalysts in hydrogenation, dehydrogenation, petroleum cracking
Pyrophoric alloys for tracer bullets and shells

Actinoids

Elements from Ac (Z=89) to Lr (Z=103) filling 5f orbitals. The series includes both naturally occurring and man-made radioactive elements. Many actinoids show variable oxidation states and form complexes more readily than lanthanoids. The early actinoids have 5f, 6d and 7s orbitals of comparable energies, so electron distribution is not regular.

Physical Properties

Dense, silvery white metals
Highly electropositive
High melting points
Atomic and ionic radii decrease across series due to actinoid contraction
Many are coloured and paramagnetic

Chemical Properties

Less reactive towards acids, common oxidation state +3, also +4 to +7. Many are radioactive.

Uses

Thorium in cancer treatment and gas mantles
Uranium in glass industry, medicines, nuclear fuel
Plutonium in atomic reactors and bombs

Differences Between Lanthanoids and Actinoids

Lanthanoids fill 4f orbitals, while actinoids fill 5f orbitals. Lanthanoids mostly show the +3 oxidation state, whereas actinoids show a wider range of oxidation states. Lanthanoids are mostly non-radioactive, while actinoids are all radioactive. Actinoids form more complexes and are more reactive than lanthanoids.

Lanthanoid contraction is smaller and less regular than actinoid contraction because 4f electrons shield the nucleus poorly, but 5f electrons also cause a marked contraction across the actinoid series. As a result, lanthanoids have very similar chemistry, while actinoids show greater variation in properties.

Other important differences are that lanthanoids generally form fewer covalent compounds and fewer complexes, whereas actinoids show greater covalent character and stronger tendency to complex formation. Lanthanoid compounds are usually less coloured than actinoid compounds, and the chemistry of lanthanoids is more uniform across the series.

Solved Examples

Example 1

Explain why actinoid contraction is greater than lanthanoid contraction and why transition metals form coloured compounds.

Level 1 (Easy): Define lanthanoids and state their general electronic configuration.
Answer: Lanthanoids are the 14 elements from cerium (Ce) to lutetium (Lu) in which the 4f subshell is progressively filled. Their general electronic configuration is [Xe] 4f¹-¹⁴ 5d⁰-¹ 6s².
Level 2 (Moderate): Describe the lanthanoid contraction and its consequences.
Answer: Lanthanoid contraction is the gradual decrease in atomic and ionic radii of lanthanoids from La³⁺ to Lu³⁺ with increasing atomic number. It occurs because 4f electrons shield the nuclear charge poorly. Its consequences include: (i) very similar properties of lanthanoids, making their separation difficult; (ii) basicity of lanthanoid hydroxides decreases from La(OH)₃ to Lu(OH)₃; (iii) radii of 4d and 5d transition elements become similar, for example Zr and Hf have nearly the same size and properties.
Level 3 (Challenging): Write the balanced chemical equation for the preparation of potassium permanganate from pyrolusite ore.
Answer: Potassium permanganate is prepared from pyrolusite ore (MnO₂) in two steps. First, MnO₂ is fused with KOH and air/oxygen to form potassium manganate:
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
Then potassium manganate is converted to potassium permanganate by oxidation or disproportionation, for example:
2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl

Answer Key

Level 1: Lanthanoids are f-block elements filling 4f orbitals, with general electronic configuration [Xe] 4f¹-¹⁴ 5d⁰-¹ 6s².
Level 2: Lanthanoid contraction is the gradual decrease in atomic and ionic radii across the lanthanoid series due to poor shielding by 4f electrons. Consequences include similar properties among lanthanoids, difficulty in separation, and similar sizes of Zr and Hf.
Level 3: 2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O (followed by electrolytic oxidation of K₂MnO₄ to KMnO₄)

Quick Reference Table

d-Block Elements
Definition: d-block elements are those in which the last electron enters the d-sub-shell of the penultimate shell.
Located in groups 3 to 12 of the periodic table.
General electronic configuration: (n-1)d¹⁻¹⁰ ns¹⁻².
Variable oxidation states arise because the ns and (n-1)d electrons have similar energies.
They are metals, generally hard, malleable and ductile; mercury is an exception and is liquid at room temperature.
They are good conductors of heat and electricity and show metallic lustre and sonority.
Atomic radii are smaller than those of s-block elements but larger than those of p-block elements in the same period.
4d and 5d elements have nearly similar sizes due to lanthanoid contraction.
Ionic radii decrease with increasing oxidation state.
Density generally increases from left to right in a period.
Ionisation enthalpy generally increases across the series, with some irregularities.
They have high enthalpy of atomisation because of strong metallic bonding due to delocalised d-electrons.
They usually have lower and irregular electrode potentials.
They often show catalytic properties because of multiple oxidation states and large surface area.
Magnetic properties: diamagnetic if all electrons are paired; paramagnetic if one or more unpaired electrons are present.
They generally have high melting and boiling points, except Zn, Cd and Hg.
They tend to form complex ions.
They form coloured compounds due to d-d electronic transitions.
They form alloys because of similar atomic sizes and metallic bonding.
They form interstitial compounds with small atoms such as H, B, C and N.
Examples: Fe, Cu and Zn.
f-Block Elements
Definition: f-block elements are those in which electrons fill the (n-2)f sub-shell, the third shell from the outermost.
They include lanthanoids and actinoids, also called inner transition elements.
General electronic configuration: (n-2)f¹⁻¹⁴ (n-1)d⁰⁻¹ ns².
Lanthanoids are the 4f series, from Ce (Z=58) to Lu (Z=71).
Lanthanoids are dense, soft, malleable and ductile metals with high melting points.
Lanthanoids readily form alloys.
Most lanthanoid ions are paramagnetic due to unpaired electrons; La³⁺ and Lu³⁺ are diamagnetic.
Lanthanoids mainly show the +3 oxidation state, with some +2 and +4 states.
Some trivalent lanthanoid ions are coloured due to f-f electronic transitions.
Lanthanoid contraction is the gradual decrease in atomic and ionic radii across the series due to poor shielding by 4f electrons.
Consequences of lanthanoid contraction include similar properties among lanthanoids and difficulty in their separation.
Actinoids are the 5f series, from Ac (Z=89) to Lr (Z=103).
Elements beyond uranium are man-made and radioactive.
Actinoids are dense, silvery white, highly electropositive metals with high melting points.
Atomic and ionic radii decrease across the actinoid series due to actinoid contraction.
Many actinoid ions are coloured and paramagnetic.
Actinoids commonly show the +3 oxidation state and also +4 to +7.
Actinoids are radioactive and are generally more reactive and form more complexes than lanthanoids.
Actinoid contraction is greater than lanthanoid contraction because 5f orbitals shield less effectively than 4f orbitals.
Examples: Ce, U and Pu.

Common Mistakes and Misconceptions

Do not write the general configuration of a transition element as if every atom is perfectly regular; some elements such as Cr and Cu show exceptions because half-filled and fully filled d-subshells are more stable.
Do not confuse d-block elements with transition elements; a d-block element may not be a transition element if its atom and common ions do not have incompletely filled d-orbitals.
Do not treat variable oxidation state as the same thing as ionisation enthalpy; variable oxidation states arise because ns and (n-1)d electrons have similar energies.
Do not assume every transition metal compound is coloured; colour is usually due to d-d transitions and compounds with d⁰ or d¹⁰ configurations are often colourless.
Do not forget that the magnetic moment depends on unpaired electrons; diamagnetic species have all electrons paired, while paramagnetic species have one or more unpaired electrons.
Do not mix up lanthanoid contraction and actinoid contraction; both are due to poor shielding, but actinoid contraction is generally greater because 5f electrons shield less effectively than 4f electrons.
Do not assume all lanthanoids show the same oxidation state; +3 is most common, but +2 and +4 also occur in some cases.
Do not assume all actinoids are equally stable; many are radioactive and show a wider range of oxidation states than lanthanoids.
Do not write unbalanced reactions for potassium dichromate or potassium permanganate preparation; the redox equations must be balanced correctly in each step.

Glossary

Transition Elements: d-block elements whose atoms or common oxidation states have incompletely filled d-orbitals
Penultimate Shell: The shell just inside the valence shell; if the valence shell is the nth shell, the penultimate shell is the (n-1)th shell
Lanthanoid Contraction: The gradual decrease in atomic and ionic radii across the lanthanoids due to poor shielding by 4f electrons
Actinoid Contraction: The gradual decrease in atomic and ionic radii across the actinoids due to poor shielding by 5f electrons
Variable Oxidation State: The ability of an element to exhibit more than one oxidation number in its compounds
Interstitial Compounds: Compounds formed when small atoms such as H, B, C or N occupy interstitial spaces in a metal lattice
Paramagnetism: Magnetic behaviour shown by species having one or more unpaired electrons
Diamagnetism: Magnetic behaviour shown by species in which all electrons are paired
Complex Ions: Ions formed by coordinate bonding between a central metal ion and ligands
Azo Compounds: Organic compounds containing the –N=N– group
Transition Metals: Transition elements that show typical metallic character and form stable ions with partially filled d-orbitals in common oxidation states
D-block Elements: Elements in which the last electron enters the d-subshell of the penultimate shell
Lanthanoids: f-block elements from Ce (Z = 58) to Lu (Z = 71) in which 4f orbitals are progressively filled
Actinoids: f-block elements from Ac (Z = 89) to Lr (Z = 103) in which 5f orbitals are progressively filled
Alloys: Homogeneous mixtures of two or more metals, or a metal with a non-metal, formed to improve properties
Magnetic Moment: A measure of the strength of magnetic behaviour of a species, often used to estimate the number of unpaired electrons
Oxidation State: The formal charge assigned to an atom in a compound or ion

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The D- And F -Block Elements

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