f - Block Elements

Deepshikha

Associate professor

Department of Chemistry

Those elements in which the last electron that gets into the f-orbital are known are called the f-block elements or Inner transition

In the f-block elements the 4f and 5f orbitals are filled. .

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f block elements are divided into two series, namely lanthanoids and actinoids. This block of elements are often referred to as inner transition metals because they provide a transition in the 6th and 7th row of the periodic table which separates the s block and the d block elements.

• There are mainly two series in the f-block that are equivalent to filling up of 4f and 5f orbitals.
• The elements include 4f series of Ce to Lu and 5f series of Th to Lw. 14 elements are there that fill up the ‘f’ orbital in each series.

Lanthanides and actinides are the two series of the f-block elements or inner transition elements. 

• Lanthanide series: The first series of elements are the lanthanides which include elements with atomic numbers from 57 to 71. These elements are non-radioactive (except for promethium). In the lanthanide series the last electron gets into the 4f orbital.

• The lanthanides belongs to III B group of the periodic table in the sixth period.
• These elements interrupt the third transition series of d- block elements in the sixth period.
• Only for the sake of convenience these elements are shown at the bottom of the periodic table.
• Their actual position is in between La (Z=57) and Hf (Z=72)together at one place.

Position of Lanthanides

Actinide series: The second series of elements are actinides which include elements with atomic numbers from 89 to 103. These elements are generally radioactive in nature. In the actinide series the last electron gets into the 5f orbital.

LANTHANIDES

OCCURRENCE

The elements in the lanthanide series are generally metals. They are good conductors of electricity and heat. As the atomic number increases the hardness of the metal also increases.

Usually, lanthanoids show +3 oxidation states but sometimes they also show up +4 and +2 oxidation states. This variation in the oxidation states is due to the fully filled, partially filled or empty f-orbital.

Oxidation state

• Lanthanides exhibit different oxidation states like +2, +3 and +4. Among these +3 is the most stable oxidation state. The elements that attain stable electronic configuration by losing 2 or 4 electrons exhibit +2 and +4 oxidation states.
• Example: Europium and ytterbium exhibits +2 and +3 oxidation states - cerium exhibits +4 oxidation state.

Oxidation state

Why Sm²⁺, Eu²⁺, and Yb²⁺ ions in solutions are good reducing agents but an aqueous solution of Ce⁴⁺ is a good oxidizing agent?

Solution

The most stable oxidation state of lanthanides is +3. Hence the ions in +2 oxidation state tend to change +3 state by loss of electron acting as reducing agents whereas those in +4 oxidation state tend to change to +3 oxidation state by gain of electron acting as a good oxidising agent in aqueous solution.

Illustrative Example

• Silvery white soft metals, tarnish in air rapidly

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• Hardness increases with increasing atomic number, samarium being steel hard.

• Good conductor of heat and electricity.

Properties

Metal combines with hydrogen when gently heated in the gas.

The carbides, Ln₃C, Ln₂C₃ and LnC₂ are formed when the metals are heated with carbon.

They liberate hydrogen from dilute acids and burn in halogens to form halides.

They form oxides and hydroxides, M₂O₃ and M(OH)₃, basic like alkaline earth metal oxides and hydroxides.

Chemical Properties

COMPLEX FORMATION BY LANTHANIDES

The lanthanides have low charge density due to their larger size in spite of having high charge (+3). Hence, they do not cause much polarization of the ligands and have a weak tendency for complex formation. This reluctance for complex formation may be attributed mainly to:

1. The unfavourable electronic configuration on the lanthanide ions.

(ii) The larger size which leads to little attraction for electron rich species

Because of the above reasons, only the high energy 5d, 6s and 6p-orbitals are available for coordination, the 4f-orbitals being screened, so that only strong (usually chelating) coordinating groups can interact. Thus, only a few complexes with unidentate ligands are formed but stable complexes are formed by Ln3+ ions with chelating ligands such as (i) oxygen containing, viz., EDTA, β-diketones, citric acid, oxalic acid, acetyl acetone, oximes, (ii) nitrogen containing, viz., ethylene diamine, NCS, etc. The Ln3+ ions do not form complexes with π-bonding ligands such as CO, NO, CNR, etc., at all.

The complex forming tendency and the stability of the complexes increases with increasing atomic number. This fact is taken as a basis to take advantage in their separation from one another. Ce(IV) complexes are relatively common, an example of high oxidation state ion seeking stabilization through complexation.

Lanthanoid contraction is a term used in chemistry to describe the decrease in ionic radii of the elements in the lanthanoid series from atomic number 58, Cerium to 71, Lutetium, which results in smaller than expected ionic radii for the subsequent elements starting with 72, Hafnium.

Lanthanide Contraction

In lanthanides there is a gradual decrease in atomic size, atomic radii and ionic radii with increase in atomic number. This regular decrease is known as lanthanide contraction.

Lanthanide contraction

In lanthanides, the nuclear charge increases by one unit at each successive element and this new electron enters the 4f-subshell. Due to the peculiar shapes of f-orbitals, there is imperfect shielding of electrons from the nuclear attraction. As a result of this the size of lanthanide atoms decreases.

Cause of lanthanide contraction

1) The atomic radii of 5d transition elements are very close to those of the corresponding 4d transition elements. Due to this the crystal structure and other properties of lanthanides are very similar.

2) There is a difficulty in separation of lanthanides due to their similar chemical properties.

Consequences

Why Zr and Hf have almost similar atomic radii?

Solution

Zr and Hf have almost similar atomic radii as a consequence of lanthanide contraction due to which their properties becomes similar.

Illustrative Example

Size of trivalent lanthanoid cation decreases with increase in atomic number. Explain.

Solution

It is due to poor shielding effect of f-electrons, valance electrons are strongly attracted towards nucleus, therefore, effective nuclear charge increases, hence ionic size decreases

Colours of these ions may be attributed to the presence of f electrons. Neither La³⁺ nor Lu³⁺ ion shows any colour but the rest do so.

Colours:

Absorption bands are narrow, probably because of the excitation within f level.

Magnetic properties

The lanthanoid ions other then the f ⁰ type (La³⁺ and Ce³⁺) and the f¹⁴ type (Yb²⁺ and Lu³⁺) are all paramagnetic. The paramagnetism rises to the maximum in neodymium.

Lanthanides have very high magnetic susceptibilities due to their large numbers of unpaired f-electrons.

The strongest known magnets contain lanthanides (eg. Nd-Fe-B, Sm-Fe-N, and Sm-Co).

1) They are used in the form of their alloys such as misch metal or pyrophoric alloys.

2) Their alloys are used in making traces bullets, cigarette and gas lighters.

3) Magnesium mixed with 3% misch metal is used in making jet engine parts.

4) The compounds of lanthanides are used in making magnetic and electronic devices. 5) Their oxides are used in glass industry.

Uses

An alloy consisting of a crude mixture of cerium, lanthanum, and other rare-earth metals obtained by electrolysis of the mixed chlorides of the metals dissolved in fused sodium chloride; used in making aluminum alloys, in some steels and irons, and in coating the cathodes of glow-type voltage regulator tubes. �

misch metal

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