Important Key Points and Questions & Answer of d- and f block element

Elements in which the last electron enters any one of the five d-orbitals of their respective penultimate shell are known as transition elements or d-block elements.

Their general electronic configuration is (n – 1)d^{1 – 10}ns^{0 – 2}.

Transition series : d-block consists of four transition series,

1^st Transition series or 3d series 21Sc – 30Zn

2^nd Transition series or 4d series 39Y – 48Cd

3^rd Transition series or 5d series 57La, 72Hf – 80Hg

4^th Transition series or 6d series 89Ac, 104Rf –112Cn

General characteristics :

Melting and boiling points High due to strong metallic bonding

Enthalpies of atomisation High due to strong interatomic interactions

Ionisation enthalpies Generally increases from left to right in a series

Oxidation states Variable due to participation of ns and (n – 1)d electrons

Atomic radii Decrease from left to right but become constant when pairing of electrons takes place

Complex formation Form complexes due to high nuclear charge and small size and availability of empty d-orbitals to accept lone pair of electrons donated by ligands.

Coloured compounds Form coloured compounds due to d-d transitions

Magnetic properties Transition metal ions and their compounds are paramagnetic due to presence of unpaired electrons in the (n – 1)d-orbitals and it is calculated by using the formula,

Catalytic behaviour Due to variable oxidation states and ability to form complexes

Interstitial compounds Due to empty spaces in their lattices, small atoms can be easily accommodated Alloy formation Due to similar atomic sizes

INNER TRANSITION ELEMENTS (f-BLOCK ELEMENTS)

Lanthanoids : Last electron enters one of the 4f-orbitals. Cerium (at. no. 58) to lutetium (at. no. 71).

Actinoids : Last electron enters one of the 5f-orbitals. Thorium (at. no. 90) to lawrencium (at. no. 103).

General electronic configuration : (n – 2)f ^{1 – 14} (n – 1)d^{0 – 1} ns²

General characteristics of lanthanoids :

Atomic and ionic radii Decrease on going from La to Lu.

Oxidation states Most common oxidation state of lanthanoids is +3. Some elements exhibit +2 and +4 oxidation states due to extra stability of empty, half-filled or fully filled f-subshell, e.g., Ce⁴⁺ acts as an oxidising agent and gets reduced to Ce³⁺, Eu²⁺, Yb²⁺ act as strong reducing agents and get oxidised to Eu³⁺ and Yb³⁺.

Action of air All the lanthanoids are silvery white soft metals and tarnish readily in moist air. They burn in oxygen of air and form oxides (Ln₂O₃ type).

Coloured ions They form coloured trivalent metal ions due to f-f transitions of unpaired electrons. La³⁺ and Lu³⁺ are colourless ions due to empty (4f⁰) or fully (4f ¹⁴) orbitals.

Magnetic properties La³⁺, Lu³⁺ are diamagnetic while trivalent ions of the rest of lanthanoids are paramagnetic.

Reducing agents They readily lose electrons and are good reducing agents.

Electropositive character Highly electropositive because of low ionisation energies.

Alloy formation They form alloys easily with other metals especially iron.

Tendency to form complexes: Lanthanoids do not have much tendency to form complexes due to low charge density because of their large size. The tendency to form complexes and their stability increases with increasing atomic number.

Lanthanoid contraction : In lanthanoid series, with increasing atomic number, there is progressive decrease in atomic/ionic radii (M³⁺ ions) from La³⁺ to Lu³⁺.

Reason : Due to addition of new electrons into f-subshell and imperfect shielding of one electron by another in the f-orbitals, there is greater effect of increased nuclear charge than screening effect hence contraction in size occurs.

Uses of lanthanoids : Used in making mischmetal, an alloy of a lanthanoid metal (~ 95%) with iron (~ 5%) and traces of S, C, Ca and Al. It is used to make tracer bullets, shells and lighter flints.

General characteristics of actinoids :

Ionic radii Like lanthanoids, ionic radii decrease across the series. Actinoid contraction is greater due to poor sheilding effect of the 5f-electrons. Further, 5f-orbitals extend in space beyond 6s and 6p-orbitals whereas 4f-orbitals are buried deep inside the atom.

Oxidation states Like lanthanoids, most common oxidation state is +3. They also show oxidation state of +4, +5, +6 and +7, e.g., in Th, Pa, U and Np respectively. They show a large number of oxidation states because of very small energy gap between 5f, 6d and 7s subshells.

Action of air, alkalies and acids Like lanthanoids they are also silvery white metals, tarnish rapidly in air forming oxide coating and are not attacked by alkalies and are less reactive towards acids.

Coloured ions Coloured due to f-f transition except Ac³⁺(5f ⁰), Cm³⁺(5f ⁷) and Th⁴⁺(5f ⁰) which are colourless.

Magnetic properties They are strongly paramagnetic.

Density All actinoids except thorium and americium have high densities.

Melting and boiling points High melting and boiling points however there is no regular trend with rise in atomic number.

Ionisation energy They have low ionisation energies than lanthanoides.

Reducing agents All actinoids are strong reducing agents.

Electropositive character Highly electropositive metals.

Differences between lanthanoids and actinoids :

Account for the following :

Zn, Cd, Hg are considered as d-block elements but not as transition elements. OR Zn is not considered as a transition element.

Zn, Cd, Hg are considered as d-block elements but not as transition elements because they do not have partly filled d-orbitals in their atomic state or their common oxidation states (i.e., Zn²⁺, Cd²⁺, Hg²⁺).

What are the transition elements? Write two characteristics of the transition elements.

Elements which have incompletely filled d-orbitals in their ground state or in any one of their oxidation states are called transition elements.

Characteristics of transition elements :

(i) They show variable oxidation states.

(ii) They exhibit catalytic properties.

Copper(I) compounds are white whereas copper(II) compounds are coloured.

Cu(I) compounds have completely filled d-orbitals and there are no vacant d-orbitals for promotion of electrons whereas in Cu(II) compounds have one unpaired electron which is responsible for colour formation.

Transition metals form coloured compounds?

Due to presence of vacant d-orbitals and d-d transitions, compounds of the transition metals are generally coloured.

When an electron from a lower energy d-orbital is excited to a higher energy d-orbital, the energy of excitation corresponds to the frequency which generally lies in the visible region. The colour observed corresponds to the complementary colour of the light absorbed. The frequency of the light absorbed is determined by the nature of the ligand.

Why do transition elements show variable oxidation states?

Transition elements can use their ns and (n – 1)d orbital electrons for bond formation therefore, they show variable oxidation states. For example, Sc has ns2(n – 1) d1 electronic configuration.

It utilizes two electrons from its ns subshell then its oxidation state = +2. When it utilizes both the electrons then its oxidation state = +3.

Transition metals and their compounds generally exhibit a paramagnetic behaviour.

Transition metals and most of their compounds contain unpaired electrons in the (n – 1)d orbitals hence show paramagnetic behaviour.

Give reason and select one atom/ion which will exhibit asked property :

(i) Sc³⁺ or Cr³⁺ (exhibit diamagnetic behaviour)

(ii) Cr or Cu (high melting and boiling point)

(i) Sc³⁺ has 3d⁰ outer electronic configuration, therefore it is diamagnetic in nature whereas Cr³⁺ has 3d³ outer electronic configuration. So, it is paramagnetic due to presence of unpaired electrons.

(ii) In a particular series, the metallic strength increases upto middle with increasing number of unpaired electrons, i.e., upto d⁵ configuration. After Cr, the number of unpaired electrons goes on decreasing. Accordingly, the m.pt and b.pt . decrease after middle (Cr) because of increasing pairing of electrons.

Give reasons for the following :

(i) Transition metals form alloys.

(ii) Mn₂O₃ is basic whereas Mn₂O₇ is acidic.

(i) Transition metals form alloys because they have similar atomic radii.

(ii) Basic nature of oxides decreases and acidic nature increases with increase in oxidation state of the metal. Oxidation state of Mn in Mn₂O₃ is +3 while in Mn₂O₇ is +7.

Zn, Cd and Hg are soft metals.

In Zn, Cd and Hg, all the electrons in d-subshell are paired. Hence, the metallic bonds are weak. That is why they are soft metals with low melting and boiling points.

Mn shows the highest oxidation state of +7 with oxygen but with fluorine it shows the highest oxidation state of +4.

Manganese can form pπ – dπ bond with oxygen by utilising 2p-orbital of oxygen and 3d-orbital of manganese due to which it can show highest oxidation state of +7. While with fluorine it cannot form such pπ – dπ bond thus, it can show a maximum of +4 oxidation state.

Give reasons for the following :

(i) Transition metals form alloys.

(ii) Mn₂O₃ is basic whereas Mn₂O₇ is acidic.

(i) Transition metals form alloys because they have similar atomic radii.

(ii) Basic nature of oxides decreases and acidic nature increases with increase in oxidation state of the metal. Oxidation state of Mn in Mn₂O₃ is +3 while in Mn₂O₇ is +7.

Mn²⁺ is more stable than Fe²⁺ towards oxidation to +3 state.

Electronic configuration of Mn²⁺ is 3d⁵ which is half filled and hence stable. Therefore, third ionization enthalpy is very high, i.e., 3rd electron cannot be lost easily. In case of Fe²⁺, electronic configuration is 3d⁶. Hence, it can lose one electron easily to give the stable configuration 3d⁵.

The enthalpy of atomization is lowest for Zn in 3d series of the transition elements.

Zinc (Z = 30) has completely filled d-orbital (3d¹⁰), so d-orbitals do not take part in interatomic bonding. Hence, metallic bonding is weak. This is why it has very low enthalpy of atomisation (126 kJ mol^–1).

Sc (21) is a transition element but Ca (20) is not.

Sc(21) is a transition element but Ca(20) is not because Sc has incompletely filled 3d orbitals.

Transition metals form large number of complex compounds.

Transition metals form a large number of complex compounds due to following reasons :

– Comparatively smaller size of metal ions.

– High ionic charges.

– Availability of d-orbitals for bond formation.

The lowest oxide of transition metal is basic whereas the highest oxide is amphoteric or acidic.

Lowest oxidation compounds of transition metals are basic due to their ability to get oxidised to higher oxidation states. Whereas, the higher oxidation state of metal and compounds gets reduced to lower ones and hence are acidic in nature.

e.g., MnO is basic whereas Mn₂O₇ is acidic.

d-block elements exhibit more oxidation states than f-block elements.

All transition elements except the first and the last member in each series show a large number of variable oxidation states. This is because difference of energy in the (n – 1)d and ns orbitals is very little.

Hence, electrons from both the energy levels can be used for bond formation.

What is lanthanoid contraction? What are its two consequences?

Lanthanoid contraction : The steady decrease in the atomic and ionic radii of lanthanoid elements with increase in atomic number is called lanthanoid contraction. It is caused due to imperfect shielding of nuclear charge by 4f-electrons.

Consequences of lanthanoid contraction :

(i) The basic strength of oxides and hydroxides of lanthanoids decrease with increasing atomic Number

(ii) Atomic and ionic sizes of 4d transition series elements and 5d series elements are similar. e.g.,atomic radii of zirconium(Zr) is same as that of hafnium Hf.

Actinoid contraction is greater than lanthanoid contraction?

The actinoid contraction is more than lanthanoid contraction because 5f-electrons are more poorly shielded than 4f-electrons.