The d-Block Elements

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Introduction

• d-block elements

🡺 locate between the s-block and� p-block

🡺 known as transition elements

🡺 occur in the fourth and subsequent periods of the Periodic Table

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d-block elements

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Introduction

Transition elements are elements that contain an incomplete d sub-shell (i.e. d¹ to d⁹) in at least one of their oxidation states in compounds.

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Introduction

Cd and Zn are not transition elements because

They form compounds with only one oxidation state in which the d sub-shell are NOT incomplete.

Cd → Cd²⁺ 4d¹⁰ Zn → Zn²⁺ 3d¹⁰

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The first transition series

the first horizontal row of the d-block elements

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Characteristics of transition elements

(d-block vs s-block)

1. Physical properties vary slightly with atomic number across the series (cf. s-block and p-block elements)
2. Higher m.p./b.p./density/hardness than s-block elements of the same periods.
3. Variable oxidation states

(cf. fixed oxidation states of s-block elements)

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Characteristics of transition elements

4. Formation of coloured compounds/ions

(cf. colourless ions of s-block elements)

5. Formation of complexes

6. Catalytic properties

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The building up of electronic configurations of elements:

🡺 Aufbau principle

🡺 Pauli exclusion principle

🡺 Hund’s rule

Electronic Configurations

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• 3d and 4s sub-shells are very close to each other in energy.
• Relative energy of electrons in sub-shells depends on the effective nuclear charge they experience.
• Electrons enter 4s sub-shell first
• Electrons leave 4s sub-shell first

Electronic Configurations

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Cu

Cu²⁺

After ‘electrons’ left the atom

Relative energy levels of orbitals in atom and in ion

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• Valence electrons in the inner 3d orbitals

Electronic Configurations

• Examples:

🡺 The electronic configuration of scandium: 1s²2s²2p⁶3s²3p⁶3d¹4s²

🡺 The electronic configuration of zinc: 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²

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Electronic configurations of the first series of the d-block elements

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• A half-filled or fully-filled d sub-shell

has extra stability

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d -Block Elements as Metals

Physical properties of d-Block elements :

🡺 good conductors of heat and electricity

🡺 hard

🡺 strong

🡺 malleable and ductile

• d-Block elements are typical metals

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d -Block Elements as Metals

• Physical properties of d-Block elements:

🡺 lustrous

🡺 high melting points and boiling points

• Exceptions : Mercury

🡺 low melting point

🡺 liquid at room temperature and pressure

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d -Block Elements as Metals

• d-block elements

🡺 extremely useful as construction materials

 strong and unreactive

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d -Block Elements as Metals

🡺 used for construction and making machinery nowadays

🡺 abundant

🡺 easy to extract

• Iron

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d -Block Elements as Metals

• Iron

🡺 corrodes easily

🡺 often combined with other elements to form steel

∴ harder and higher resistance to corrosion

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d -Block Elements as Metals

• Titanium

🡺 used to make aircraft and space shuttles

🡺 expensive

Corrosion resistant, light, strong and withstand large temperature changes

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d -Block Elements as Metals

• The similar atomic radii of the transition metals facilitate

🡺 formation of substitutional alloys

🡺 the atoms of one element to replace those of another element

🡺 modify their solid structures and physical properties

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d -Block Elements as Metals

• Manganese

confers hardness & wearing resistance to its alloys� e.g. duralumin : alloy of Al with Mn/Mg/Cu

• Chromium

🡺 confers inertness to stainless steel

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Atomic Radii and Ionic Radii

• Two features can be observed:

1. The d-block elements have smaller atomic radii than the s-block elements

2. The atomic radii of the d-block elements do not show much variation across the series

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Variation in atomic radius of the first 36 elements

Atomic Radii and Ionic Radii

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25

26

(i) ↑ in nuclear charge

(ii) ↑ in shielding effect (repulsion between e^-)

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• At the beginning of the series

🡺 atomic number ↑

🡺 effective nuclear charge ↑

🡺 the electron clouds are pulled closer to the nucleus

🡺 atomic size ↓

Atomic Radii and Ionic Radii

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• In the middle of the series

🡺 the effective nuclear charge experienced by 4s electrons increases very slowly

🡺 only a slow decrease in atomic radius in this region

🡺 more electrons enter the inner� 3d sub-shell

🡺 The inner 3d electrons shield the outer 4s electrons effectively

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• At the end of the series

🡺 the screening and repulsive effects of the electrons in the 3d sub- shell become even stronger

🡺 Atomic size ↑

Atomic Radii and Ionic Radii

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• Many of the differences in physical and chemical properties between the d-block and s-block elements

🡺 explained in terms of their differences in electronic configurations and atomic radii

Comparison of Some Physical and Chemical Properties between the d-Block and s-Block Elements

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1. Density

Densities (in g cm^–3) of the s-block elements and the first series of the d-block elements at 20°C

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• d-block > s-block

 1. the atoms of the d-block elements are generally smaller in size

2. more closely packed

(fcc/hcp vs bcc in group 1)

3. higher atomic mass

1. Density

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• The densities

🡺 generally increase across the first series of the d-block elements

 1. general decrease in atomic radius across the series

2. general increase in atomic mass across the series

1. Density

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2. Ionization Enthalpy

K → Ca (sharp ↑) ; Ca → Sc (slight ↑)

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2. Ionization Enthalpy

Sc → Cu (slight ↑) ; Cu → Zn (sharp ↑)

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• The first ionization enthalpies of the�d-block elements

🡺 greater than those of the s-block elements in the same period of the Periodic Table

 1. The atoms of the d-block elements are smaller in size

2. greater effective nuclear charges

2. Ionization Enthalpy

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Sharp ↑ across periods 1, 2 and 3

Slight ↑ across the transition series

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• Going across the first transition series

🡺 the nuclear charge of the elements increases

🡺 additional electrons are added to the ‘inner’ 3d sub-shell

2. Ionization Enthalpy

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• The screening effect of the additional�3d electrons is significant

2. Ionization Enthalpy

• The effective nuclear charge experienced by the 4s electrons increases very slightly across the series

• For 2^nd, 3^rd, 4^th… ionization enthalpies,

similar gradual ↑ across the series are observed.

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Electron has to be removed from completely filled 3p subshell

3d¹⁰

d¹⁰/s²

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• The first few successive ionization enthalpies for the d-block elements

🡺 do not show dramatic changes

 4s and 3d energy levels are close to each other

2. Ionization Enthalpy

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Difficult to remove e^- from fully- or half-filled sub-shells

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3. Melting Points and Hardness

d-block >> s-block

 1. both 4s and 3d e^- are involved in the formation of metal bonds

2. d-block atoms are smaller

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3. Melting Points and Hardness

K has an exceptionally small m.p. because it has an more open b.c.c. structure.

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 Unpaired electrons are relatively more involved in the sea of electrons

Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

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3d

4s

Sc

Ti

V

1. m.p. ↑ from Sc to V due to the ↑ of unpaired d-electrons (from d¹ to d³)

Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

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2. m.p. ↓ from Fe to Zn due to the ↓ of unpaired d-electrons (from 4 to 0)

Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

3d

4s

Fe

Co

Ni

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Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

3. Cr has the highest no. of unpaired electrons but its m.p. is lower than V.

3d

4s

Cr

It is because the electrons in the half-filled d-subshell are relatively less involved in the sea of electrons.

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Sc Ti V Cr Mn Fe Co Ni Cu Zn

1541 1668 1910 1907 1246 1538 1495 1455 1084 419

4. Mn has an exceptionally low m.p. because it has the very open cubic structure.

Why is Hg a liquid at room conditions ?

All 5d and 6s electrons are paired up and the size of the atoms is much larger than that of Zn.

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• The metallic bonds of the d-block elements are stronger than those of the s-block elements

∴ much harder than the s-block elements

3. Melting Points and Hardness

• The hardness of a metal dependent on

🡺 the strength of the metallic bonds

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Mohs scale : - A measure of hardness

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn

0.5 1.5 3.0 4.5 6.1 9.0 5.0 4.5 -- -- 2.8 2.5

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• In general, the s-block elements

🡺 react vigorously with water to form metal hydroxides and hydrogen

4. Reaction with Water

• The d-block elements

🡺 react very slowly with cold water

🡺 react with steam to give metal oxides and hydrogen

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d-block compounds vs s-block compounds�A Summary : -

Ions of d-block metals have higher charge density

⇒ more polarizing

⇒ 1. more covalent in nature

2. less soluble in water

3. less basic (more acidic)

e.g. Fe(OH)₃ < Fe(OH)₂ << NaOH

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• One of the most striking properties

🡺 variable oxidation states

Variable Oxidation States

• The 3d and 4s electrons are

🡺 in similar energy levels

🡺 available for bonding

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• Elements of the first transition series

🡺 react with other elements to form compounds

🡺 form ions of roughly the same stability by losing different numbers of the 3d and 4s electrons

Variable Oxidation States

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Oxidation states of the elements of the first transition series in their oxides and chlorides

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Oxidation states of the elements of the first transition series in their compounds

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1. Scandium and zinc do not exhibit variable oxidation states

• Scandium of the oxidation state +3

🡺 the stable electronic configuration of argon (i.e. 1s²2s²2p⁶3s²3p⁶)

• Zinc of the oxidation state +2

🡺 the stable electronic configuration of [Ar] 3d¹⁰

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2. (a) All elements of the first transition series (except Sc) can show an oxidation state of +2

(b) All elements of the first transition series (except Zn) can show an oxidation state of +3

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3. Manganese has the highest oxidation state +7

E.g. MnO₄^-, Mn₂O₇

Mn⁷⁺ ions do not exist.

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The +7 state of Mn does not mean that all 3d and 4s electrons are removed from Mn to give Mn⁷⁺.

Instead, Mn forms covalent bonds with oxygen atoms by making use of its half filled orbitals

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Draw the structure of Mn₂O₇

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3. Manganese has the highest oxidation state +7

• The highest oxidation state

🡺 not be greater than the total number of the 3d and 4s electrons

 inner electrons (3s, 3p…) are not involved in covalent bond formation

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4. For elements after manganese, there is a reduction in the number of possible oxidation states

• The 3d electrons are held more firmly

 the decrease in the number of unpaired electrons

 the increase in nuclear charge

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5. The relative stability of various oxidation states is correlated with the stability of electronic configurations

• Electronic configurations with half-filled or fully-filled sub-shell has extra stability

Stability : -

Ti⁴⁺(aq) > Ti³⁺(aq)

Ar [Ar] 3d¹

Ti⁴⁺(g) < Ti³⁺(g)

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5. The relative stability of various oxidation states is correlated with the stability of electronic configurations

Stability : - Mn²⁺(aq) > Mn³⁺(aq)

[Ar] 3d⁵ [Ar] 3d⁴

Fe³⁺(aq) > Fe²⁺(aq)

[Ar] 3d⁵ [Ar] 3d⁶

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5. The relative stability of various oxidation states is correlated with the stability of electronic configurations

Stability : -

Zn²⁺(aq) > Zn⁺(aq)

[Ar] 3d¹⁰ [Ar] 3d¹⁰4s¹

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Colours of aqueous ions of vanadium of different oxidation states

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Colours of compounds or ions of manganese in different oxidation states

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(a)

Colours of compounds or ions of manganese in differernt oxidation states: (a) +2; (b) +3; (c) +4

(b)

(c)

Mn²⁺(aq)

Mn(OH)₃(aq)

MnO₂(s)

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(e)

(d)

Colours of compounds or ions of manganese in differernt oxidation states: (d) +6; (e) +7

MnO₄^2–(aq)

MnO₄^–(aq)

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Oxidizing power of Mn(VII) depends on pH of the solution

In an acidic medium (pH 0)

In an alkaline medium (pH 14)

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The reaction does not involve H⁺(aq) nor OH⁻(aq)

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MnO₂ is oxidized to MnO₄²⁻ in alkaline medium

2MnO₂ + 4OH⁻ + O₂ → 2MnO₄²⁻ + 2H₂O

Preparing MnO₄⁻ from MnO₂

1. 2MnO₂ + 4OH⁻ + O₂ → 2MnO₄²⁻ + 2H₂O

2. 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O

3. Filter the resulting mixture to remove MnO₂

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• Another striking feature of the d-block elements is the formation of complexes

Formation of Complexes

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• Most of the d-block metals

🡺 form coloured compounds

Coloured Ions

🡺 due to the presence of the incompletely filled d orbitals in the� d-block metal ions

Zn²⁺, Cu⁺(3d¹⁰), Sc³⁺, Ti⁴⁺(3d⁰)

Which aqueous transition metal ion(s) is/are not coloured ?

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Colours of some d-block metal ions in aqueous solutions

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Colours of some d-block metal ions in aqueous solutions

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Colours of some d-block metal ions in aqueous solutions

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Colours of some d-block metal ions in aqueous solutions

Co²⁺(aq)

Fe³⁺(aq)

Zn²⁺(aq)

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In gaseous state,

the five 3d orbitals are degenerate

i.e. they are of the same energy level

In the presence of ligands,

The five 3d orbitals interact with the orbitals of ligands and split into two groups of orbitals with slightly different energy levels

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The splitting of the degenerate 3d orbitals of a d-block metal ion in an octahedral complex

Interact more strongly with the orbitals of ligands

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• Criterion for d-d transition : -

presence of unpaired d electrons in the d-block metal atoms or ions

d-d transition is possible for

3d¹ to 3d⁹ arrangements

d-d transition is NOT possible for

3d⁰ and 3d¹⁰ arrangements

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3d⁹ : d-d transition is possible

↑↓↑↓↑↓

↑↓↑

Cu²⁺

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3d⁰ : d-d transition NOT possible

Sc³⁺

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Potassium dichromate

It is prepared in two steps :

​

1. First the chromite ore ( FeCr₂O₄) is fused with

​

Na₂CO₃ or K₂CO₃ in free access of air

​

​

4 FeCr₂O4 + 8 Na₂CO₃ + 7O₂ 8 Na₂CrO₄

+ 2 Fe₂O₃

​

+ 8 CO₂

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STEP : 02

(ii) The yellow soln of sodium chromate is filtered and acidified with H₂SO₄ to give a soln. from which orange sodium dichromate can be crystallized.

​

2 Na₂CrO₄ + 2H⁺ Na₂Cr₂O₇

+2Na⁺ + H₂O

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Potassium dichromate to Sodium dichromate

Sodium dichromate is more soluble

than Potassium dichromate

therefore K₂Cr₂O₇ is prepared by treating Na₂Cr₂O₇ with KCl.

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Na₂Cr₂O₇ + KCl K₂Cr₂O₇ + 2NaCl

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Chromate and Dichromate ions

CrO4^2-

Cr

O

O

O

O

2-

Chromate ion

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Dichromate ion

Cr₂O₇ ^2-

2-

O

O

O

Cr

O

Cr

O

O

O

126⁰

179 pm

163 pm

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Chemical properties

K₂Cr₂O₇ and Na₂Cr₂O₇ are strong oxidising agents :

In acidic solution its oxidising action can be represented as :

​

Cr₂O₇^2- +14H⁺ +6e^- 2Cr³⁺ + 7 H₂O

(E =1.33V)

​

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