The d and f Block Elements
Those elements in which the last electron enters the (n-1)d orbital, i.e., the penultimate shell, are called d-block elements. These elements are also called transition metals as they represent a change in behavior from highly electropositive s-block elements to highly electronegative p-block elements.
The d-block contains elements from Group 3 to Group 12. It is divided into four transition series:
- 3d-series: Sc (Z = 21) to Zn (Z = 30)
- 4d-series: Y (Z = 39) to Cd (Z = 48)
- 5d-series: La (Z = 57), Hf (Z = 72) to Hg (Z = 80)
- 6d-series: Ac (Z = 89), Rf (Z = 104) to Uub (Z = 112)
Electronic Configuration:
The general outer-shell electronic configuration of d-block elements is:
\[
(n-1) d^{1-10} ns^{1-2}
\]
This shows the progressive filling of the (n-1)d orbital.
However, several exceptions occur due to the very little energy difference between the (n-1)d and ns orbitals. Moreover:
- Half-filled and completely filled orbitals are more stable compared to other configurations.
- Therefore, some elements (e.g., Cr and Cu in the 3d series) have exceptional electronic configurations.
The general electronic configuration of the first transition series (3d-series) is given below:
| Element | Symbol | Atomic Number | Electronic Configuration |
|---|---|---|---|
| Scandium | Sc | 21 | [Ar] 3d¹ 4s² |
| Titanium | Ti | 22 | [Ar] 3d² 4s² |
| Vanadium | V | 23 | [Ar] 3d³ 4s² |
| Chromium | Cr | 24 | [Ar] 3d⁵ 4s¹ (exception) |
| Manganese | Mn | 25 | [Ar] 3d⁵ 4s² |
| Iron | Fe | 26 | [Ar] 3d⁶ 4s² |
| Cobalt | Co | 27 | [Ar] 3d⁷ 4s² |
| Nickel | Ni | 28 | [Ar] 3d⁸ 4s² |
| Copper | Cu | 29 | [Ar] 3d¹⁰ 4s¹ (exception) |
| Zinc | Zn | 30 | [Ar] 3d¹⁰ 4s² |
General characteristic of d-block elements (Trends in Properties)
Physical Properties
The transition metals display typical metallic properties like malleability, ductility, high tensile strength, high thermal and electrical conductivity, metallic luster, etc.
The transition metals (with the exception of Zn, Cd, and Hg) are very hard and have low volatility. The melting and boiling points of transition metals are generally high.
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Melting Point of Transition Metals
The high melting point of transition metals is attributed to the involvement of a greater number of electrons from the (n-1)d orbitals in addition to ns electrons.
The melting points of these metals rise to a maximum at d⁵ (except Mn and Tc) and then decrease regularly with the increase in atomic number.
Enthalpy of Atomization
The transition metals have high enthalpies of atomization. The enthalpy of atomization is maximum at d⁵, which indicates that each unpaired electron is favorable for strong interatomic interaction.
Therefore, the greater the number of valence electrons, the stronger is the resultant bonding. The metals of the second and third series have greater enthalpies of atomization than the corresponding elements of the first series.
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Atomic and Ionic Radii
The atomic radii decrease across a period due to an increase in atomic number. The atomic radii of transition metals are smaller than their corresponding s-block elements.
The atomic radii of transition metals decrease with an increase in atomic number, but this decrease becomes small after midway.
For example, in the 3d-series, the order of atomic radii is:
\[
\scriptsize \text{Sc > Ti > V ≈ Cr ≈ Mn ≈ Fe ≈ Co ≈ Ni < Cu < Zn}
\]
The decrease in atomic radii is due to an increase in nuclear charge, but the addition of extra electrons to the (n-1)d orbitals increases the screening effect. Thus, after midway, there is no change in atomic radii.
Atomic and Ionic Radii
The values of atomic radii at the end of a series are a bit higher due to electron–electron repulsions.
The variation of atomic radii of transition elements in a series is shown below:
The ionic radii follow the same trend as atomic radii, i.e., it decreases across a transition series.
In the case of ions of the same atom, the ionic radii decrease with an increase in the charge of ions.
For example:
\[
\scriptsize \text{Fe}^{3+} < \text{Fe}^{2+}, \; \text{Ni}^{3+} < \text{Ni}^{2+}, \; \text{Mn}^{3+} < \text{Mn}^{2+}
\]
Ionization Energy
The ionization energy increases across a series due to an increase in nuclear charge, but the increase is not regular.
For example, in the case of the 3d series, ionization energy values of Sc, Ti, V, and Cr are nearly close to one another.
Similarly, ionization energy values of Fe, Co, Ni, and Cu are almost similar. This is due to an increase in the screening effect, which neutralizes the increasing nuclear charge, so ionization energy increases quite slowly.
The 2nd and 3rd ionization energies are greater than the 1st ionization energy.
In each series, the elements of Group 12 (Zn, Cd, Hg, etc.) have the highest value of ionization energy due to the extra stability associated with their filled (n-1)d and ns orbitals.
The 2nd ionization energy of Cu and Cr is sufficiently higher because an electron is to be removed from a completely filled subshell, i.e., 3d¹⁰ in the case of Cu and a half-filled subshell, i.e., 3d⁵ in the case of Cr. Similarly, the 3rd ionization energy of Mn is very high because the 3rd electron is to be removed from a stable half-filled 3d subshell, i.e., 3d⁵.
Oxidation State:
The transition metals exhibit a large number of oxidation states due to the involvement of (n-1) d and ns electrons. The common oxidation state of the 3d series is +2 (except for Scandium, where it is +3). The oxidation states of the 3d series are shown below:
| Element | Electronic Configuration | Oxidation State |
|---|---|---|
| Scandium (Sc) | [Ar] 3d¹ 4s² | +2, +3 |
| Titanium (Ti) | [Ar] 3d² 4s² | +2, +3, +4 |
| Vanadium (V) | [Ar] 3d³ 4s² | +2, +3, +4, +5 |
| Chromium (Cr) | [Ar] 3d⁵ 4s¹ | +1, +2, +3, +4, +5, +6 |
| Manganese (Mn) | [Ar] 3d⁵ 4s² | +2, +3, +4, +5, +6, +7 |
| Iron (Fe) | [Ar] 3d⁶ 4s² | +2, +3, +4, +6 |
| Cobalt (Co) | [Ar] 3d⁷ 4s² | +2, +3, +4 |
| Nickel (Ni) | [Ar] 3d⁸ 4s² | +2, +3, +4 |
| Copper (Cu) | [Ar] 3d¹⁰ 4s¹ | +1, +2 |
| Zinc (Zn) | [Ar] 3d¹⁰ 4s² | +2 |
- In the 3d series, the highest oxidation state is exhibited by Manganese (Mn) as +7, while the highest oxidation state for any transition metal is +8, shown by Ruthenium (Ru) and Osmium (Os).
- Elements in or near the middle of the series exhibit the greatest number of oxidation states.
- Elements at the extremes show fewer oxidation states, due to the limited number of electrons available for bonding.
The stability of transition metals also depends on their oxidation states. For example:
- Mn²⁺ ([Ar] 3d⁵) is more stable than Mn³⁺ ([Ar] 3d⁴).
In the d-block, higher oxidation states are more stable for heavier elements.
Some transition elements can form compounds in the zero oxidation state, such as: Nickel in [Ni(CO)₄] and Iron in [Fe(CO)₅].
Standard Electrode Potential (E°):
- The standard electrode potential of all transition metals is lower than that of hydrogen (except for Copper (Cu) and Mercury (Hg)).
- A lower electrode potential indicates a more stable oxidation state.
- Metals with a negative value of electrode potential act as better reducing agents.
- Most transition metals (except Cu, Pt, Ag, Hg, Au) have negative values of electrode potential.
- The negative value of electrode potential decreases across a series due to increase in ionization energy.
Magnetic Properties:
- The d-block elements are either paramagnetic (attracted by magnetic fields) or diamagnetic (repelled by magnetic fields). The paramagnetic behavior arises due to the presence of one or more unpaired electrons.
- The magnetic character is expressed in terms of the magnetic moment \(( \mu ) \), which is given by:
\[
\mu = \sqrt{n(n+2)} \, \text{BM}
\]
where:
- BM is Bohr Magneton, equal to \( \frac{eh}{4\pi mc} \)
- ( n ) = number of unpaired electrons.
- The greater the number of unpaired electrons, the higher the value of the magnetic moment, and consequently, the greater the paramagnetic character.
- Transition metals and ions with a d⁵ configuration possess maximum magnetic moment.
- Transition metals strongly attracted by magnetic fields are called ferromagnetic, e.g., Iron (Fe), Cobalt (Co), Nickel (Ni), etc.
Formation of Interstitial Compounds:
- Small non-metallic atoms such as H, B, C, N, etc., can occupy the interstitial spaces within transition metals to form interstitial compounds.
- These compounds are non-stoichiometric in nature, for example:
- \( \text{FeO}_{0.95} \)
- \( \text{TiH}_{1.7} \)
- \( \text{VH}_{0.56} \).
- These compounds do not follow the common rules of valency. Their bonds are neither completely ionic nor covalent, and their formula does not correspond to any normal oxidation state of the metal.
Properties of Interstitial Compounds:
- They have high melting points, higher than those of pure metals.
- They are very hard; for instance, some metal borides are as hard as diamonds.
- They retain metallic conductivity.
- They are chemically inert.
- Example: Steel is an interstitial compound of Iron (Fe) and Carbon (C).
Complex Formation:
The d-block elements have a remarkable ability to form complex compounds due to the following reasons:
- Small size.
- High nuclear charge.
- Presence of vacant d-orbitals.
These complexes are commonly called coordination compounds. Examples include:
- ([Cu(NH₃)₄]SO₄),
- (Na₃[Fe(CN)₆]),
- ([Cr(NH₃)₆]Cl₃).
The transition metals form a large number of complex compounds due to their small size, high ionic charge, and the availability of d-orbitals for bond formation.
Alloy Formation:
Alloys are homogeneous mixtures of two or more elements, in which at least one element is a metal. Transition elements form a large number of alloys because they have nearly the same atomic size, allowing the atoms of one metal to substitute the atoms of another.
| Alloy | Composition | Use |
|---|---|---|
| Brass | Cu + Zn | In making utensils |
| Bronze | Cu (90%) + Sn (10%) | In making statues |
| Gun Metal | Cu + Sn + Zn | In automobiles for making gears and bearings |
| Stainless Steel | Steel + Cr + Ni | In making household utensils |
| Cupro Nickel | Ni + Cu | In making silver coins |
| Alnico | Steel + Al + Ni + Co | In making permanent magnets |
| Dental Alloy | Ag + Sn + Hg | In filling tooth cavities |
| Monel Metal | Ni + Cu + Fe (traces) | Resistant to corrosion |
| Bell Metal | Cu (80%) + Sn (20%) | In making bells |
| Coinage Silver | Ag + Cu + Ni | In making coins |
| Invar | Steel + Ni | Heat resistant |
Colour Formation:
Most compounds of transition metals and their ions are coloured. This colour arises due to the excitation of electrons from d-orbitals of lower energy to d-orbitals of higher energy (i.e., d-d transition).
The energy required for d-d excitation lies in the visible region. Transition metals absorb radiations from the visible spectrum and exhibit complementary colours.
- Transition metal ions with completely filled d-orbitals (3d¹⁰, 4d¹⁰, or 5d¹⁰), such as \( \text{Zn}^{2+} \), \( \text{Cd}^{2+} \), \( \text{Hg}^{2+} \), \( \text{Cu}^+ \), \( \text{Ag}^+ \), and \( \text{Au}^+ \), are colourless.
Examples of colours of specific ions:
- \( \text{Fe}^{2+} \) (3d⁶) – green
- \( \text{Fe}^{3+} \) (3d⁵) – yellow
- \( \text{Co}^{2+} \) (3d⁷) – pink
- \( \text{Cu}^{2+} \) (3d⁹) – blue
- \( \text{Ti}^{3+} \) (3d¹) – purple
- \( \text{Cr}^{3+} \) (3d³) – violet
- \( \text{Mn}^{2+} \) (3d⁵) – light pink
Catalytic Property:
Transition metals and their compounds act as catalysts in various chemical reactions. The catalytic properties are attributed to the utilization of (n-1)d orbitals or the formation of interstitial compounds.
Examples of catalytic uses:
- Platinum (Pt) – used in the manufacture of \( \text{H}_2\text{SO}_4 \) (sulfuric acid).
- Iron/Molybdenum (Fe/Mo) – used in the manufacture of \( \text{NH}_3 \) (ammonia) in Haber’s process.
- Nickel (Ni) – used in the hydrogenation of oils.
- Vanadium Pentoxide \(( \text{V}_2\text{O}_5 ) \) – used in the oxidation of \( \text{SO}_2 \) into \( \text{SO}_3 \).
- Manganese Dioxide \(( \text{MnO}_2 ) \) – used in the decomposition of \( \text{KClO}_3 \) to prepare oxygen.
Important Compounds of d-Block
Some of the important compounds of d-block elements are Potassium Dichromate and Potassium permanganate.
Potassium Dichromate \(( \text{K}_2\text{Cr}_2\text{O}_7 ) \)
Potassium dichromate is an important compound of chromium. It is prepared from chromite ore (ferrochrome or chrome iron, \( \text{FeCr}_2\text{O}_4 \)) via the following steps:
- Formation of Sodium Chromate: \[
\tiny 4\text{FeCr}_2\text{O}_4 + 8\text{Na}_2\text{CO}_3 + 7\text{O}_2 \longrightarrow 8\text{Na}_2\text{CrO}_4 + 2\text{Fe}_2\text{O}_3 + 8\text{CO}_2
\] - Conversion to Sodium Dichromate: \[
\tiny 2\text{Na}_2\text{CrO}_4 + \text{H}_2\text{SO}_4 \longrightarrow \text{Na}_2\text{Cr}_2\text{O}_7 + \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}
\] - Formation of Potassium Dichromate: \[
\scriptsize \text{Na}_2\text{Cr}_2\text{O}_7 + 2\text{KCl} \longrightarrow \text{K}_2\text{Cr}_2\text{O}_7 + 2\text{NaCl}
\]
Properties of \( \text{K}_2\text{Cr}_2\text{O}_7 \) (Potassium Dichromate):
- Colour and Appearance:
It is an orange-red crystalline compound. - Solubility:
It is moderately soluble in cold water but freely soluble in hot water. - Thermal Decomposition:
On strong heating, it decomposes to release oxygen:
\[
\scriptsize 2\text{K}_2\text{Cr}_2\text{O}_7 \longrightarrow 2\text{K}_2\text{CrO}_4 + \text{Cr}_2\text{O}_3 + \frac{3}{2}\text{O}_2
\] - Reaction with Alkali:
When an alkali is added to the orange-red solution of \( \text{K}_2\text{Cr}_2\text{O}_7 \), a yellow solution of \( \text{K}_2\text{CrO}_4 \) is obtained:
\[
\scriptsize \text{K}_2\text{Cr}_2\text{O}_7 + 2\text{KOH} \longrightarrow 2\text{K}_2\text{CrO}_4 + \text{H}_2\text{O}
\]
- \( \text{Cr}_2\text{O}_7^{2-} \) (Orange) → \( \text{CrO}_4^{2-} \) (Yellow)
- Reaction with Acid:
On acidifying the yellow-coloured solution of \( \text{K}_2\text{CrO}_4 \), it changes back to orange-red \( \text{K}_2\text{Cr}_2\text{O}_7 \):
\[
\scriptsize 2\text{K}_2\text{CrO}_4 + \text{H}_2\text{SO}_4 \longrightarrow \text{K}_2\text{Cr}_2\text{O}_7 + \text{K}_2\text{SO}_4 + \text{H}_2\text{O}
\] - Oxidizing Agent:
\( \text{K}_2\text{Cr}_2\text{O}_7 \) acts as a powerful oxidizing agent in the presence of dilute sulfuric acid:
\[
\tiny \text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 \longrightarrow \text{K}_2\text{SO}_4 + \text{Cr}_2(\text{SO}_4)_3 + 4\text{H}_2\text{O} + 3[\text{O}]
\]
Oxidizing Reactions:
- Liberation of Iodine \(( \text{I}_2 ) \) from Potassium Iodide \(( \text{KI} ) \):
\[
\tiny \text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 + 6\text{KI} \longrightarrow 4\text{K}_2\text{SO}_4 + \text{Cr}_2(\text{SO}_4)_3 + 7\text{H}_2\text{O} + 3\text{I}_2
\] - Oxidation of Hydrogen Sulfide \(( \text{H}_2\text{S} ) \) to Sulfur \(( \text{S} ) \):
\[
\tiny \text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 + 3\text{H}_2\text{S} \longrightarrow text{K}_2\text{SO}_4 + \text{Cr}_2(\text{SO}_4)_3 + 7\text{H}_2\text{O} + 3\text{S}
\] - Oxidation of Sulfur Dioxide \(( \text{SO}_2 ) \) to Sulfuric Acid \(( \text{H}_2\text{SO}_4 ) \):
\[
\tiny \text{K}_2\text{Cr}_2\text{O}_7 + 4\text{H}_2\text{SO}_4 + 3\text{SO}_2 + 3\text{H}_2\text{O} \longrightarrow \text{K}_2\text{SO}_4 + \text{Cr}_2(\text{SO}_4)_3 + 3\text{H}_2\text{SO}_4 + 4\text{H}_2\text{O}
\]
Uses of \( \text{K}_2\text{Cr}_2\text{O}_7 \) (Potassium Dichromate):
(1) It is used in volumetric analysis for the estimation of \( \text{Fe} \) and \( \text{I}_2 \) in redox titrations.
(2) It is used in leather tanning, calico printing, and dyeing.
(3) It acts as a strong oxidizing agent.
(4) Its acidified solution provides a test to determine whether a person is drunk. The orange-red \( \text{K}_2\text{Cr}_2\text{O}_7 \) turns green due to the formation of \( \text{Cr}_2(\text{SO}_4)_3 \): \[
\tiny 3\text{C}_2\text{H}_5\text{OH} + 2\text{K}_2\text{Cr}_2\text{O}_7 + 8\text{H}_2\text{SO}_4 \longrightarrow 2\text{Cr}_2(\text{SO}_4)_3 + 3\text{CH}_3\text{COOH} + 2\text{K}_2\text{SO}_4 + 11\text{H}_2\text{O}
\] (Here, ethanol oxidizes to acetic acid while potassium dichromate reduces to chromium sulfate.)
(5) It is used in photography to harden gelatin films.
Potassium Permanganate \(( \text{KMnO}_4 ) \)
Potassium permanganate is an important compound of manganese. It is prepared from pyrolusite ore \(( \text{MnO}_2 ) \) through the following processes:
Preparation:
- Fusion with Potassium Hydroxide and Oxygen:
Pyrolusite \(( \text{MnO}_2 ) \) is fused with potassium hydroxide \(( \text{KOH} ) \) or potassium carbonate \(( \text{K}_2\text{CO}_3 ) \) in the presence of atmospheric oxygen \(( \text{O}_2 ) \) to form potassium manganate \(( \text{K}_2\text{MnO}_4 ) \):
\[
\scriptsize 2\text{MnO}_2 + 4\text{KOH} + \text{O}_2 \longrightarrow 2\text{K}_2\text{MnO}_4 + 2\text{H}_2\text{O}
\] - Oxidation to Potassium Permanganate:
The potassium manganate is oxidized to potassium permanganate using chlorine \(( \text{Cl}_2 ) \), ozone \(( \text{O}_3 ) \), or carbon dioxide \(( \text{CO}_2 ) \):
\[
\scriptsize 2\text{K}_2\text{MnO}_4 + \text{Cl}_2 \longrightarrow 2\text{KMnO}_4 + 2\text{KCl}
\] - Electrolysis of Potassium Manganate:
Potassium manganate can also be electrolyzed to produce potassium permanganate:
\[
\scriptsize \text{K}_2\text{MnO}_4 \longleftrightarrow 2\text{K}^+ + \text{MnO}_4^-
\]
Reactions at Electrodes During Electrolysis:
- At Anode:
\[
\text{MnO}_4^{2-} \longrightarrow \text{MnO}_4^- + \text{e}^-
\] - At Cathode:
\[
2\text{H}^+ + 2\text{e}^- \longrightarrow \text{H}_2
\]
Properties of \( \text{KMnO}_4 \) (Potassium Permanganate):
- Colour and Appearance:
It is a purple-coloured crystalline compound. - Solubility:
It is fairly soluble in hot water but sparingly soluble in cold water. - Decomposition on Heating or Reaction with Alkali:
When heated alone or treated with alkali, it decomposes to release oxygen:
\[
\scriptsize 2\text{KMnO}_4 \xrightarrow{\Delta} \text{K}_2\text{MnO}_4 + \text{MnO}_2 + \text{O}_2
\]
\[
\scriptsize 4\text{KMnO}_4 + \text{KOH} \longrightarrow 4\text{K}_2\text{MnO}_4 + 2\text{H}_2\text{O} + \text{O}_2
\] - Reaction with Cold Concentrated Sulfuric Acid:
On treatment with cold concentrated \( \text{H}_2\text{SO}_4 \), it forms manganese heptoxide \(( \text{Mn}_2\text{O}_7 ) \):
\[
\scriptsize 2\text{KMnO}_4 + 2\text{H}_2\text{SO}_4 \longrightarrow \text{Mn}_2\text{O}_7 + 2\text{KHSO}_4 + \text{H}_2\text{O}
\] - Oxidizing Agent:
\( \text{KMnO}_4 \) acts as a powerful oxidizing agent under various conditions:
(a) In Acidic Medium:
- Oxidation of Iron \(( \text{Fe}^{2+} ) \):
Green \( \text{Fe}^{2+} \) is converted to yellow \( \text{Fe}^{3+} \):
\[
\scriptsize 5\text{Fe}^{2+} + \text{MnO}_4^- + 8\text{H}^+ \longrightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} + 5\text{Fe}^{3+}
\] - Liberation of Iodine \(( \text{I}_2 ) \) from Potassium Iodide \(( \text{KI} ) \):
\[
\scriptsize 10\text{I}^- + 2\text{MnO}_4^- + 16\text{H}^+ \longrightarrow 2\text{Mn}^{2+} + 8\text{H}_2\text{O} + 5\text{I}_2
\]
(b) In Neutral or Faintly Alkaline Medium:
- Oxidation of Iodide \(( \text{I}^- ) \) to Iodate \(( \text{IO}_3^- )\):
\[
\scriptsize 2\text{MnO}_4^- + \text{H}_2\text{O} + \text{I}^- \longrightarrow 2\text{MnO}_2 + 2\text{OH}^- + \text{IO}_3^-
\] - Oxidation of Thiosulfate \(( \text{S}_2\text{O}_3^{2-} ) \) to Sulfate \(( \text{SO}_4^{2-} ) \):
\[
\scriptsize 8\text{MnO}_4^- + 3\text{S}_2\text{O}_3^{2-} + \text{H}_2\text{O} \longrightarrow 8\text{MnO}_2 + 6\text{SO}_4^{2-} + 2\text{OH}^-
\]
Uses of \( \text{KMnO}_4 \) (Potassium Permanganate):
- Used in volumetric estimation of \( \text{H}_2\text{O}_2 \), ferrous salts, oxalates, etc.
- Acts as a disinfectant.
- Used as a germicide.
- Acts as Baeyer’s reagent for detecting unsaturation in compounds.
- Functions as a strong oxidizing agent.
- Used in purifying water.