F Block Elements - Practice Questions & MCQ

Electronic Configurations
It may be noted that atoms of these elements have electronic configuration with 6s² common but with variable occupancy of 4f level. However, the electronic configurations of all the tripositive ions (the most stable oxidation state of all the lanthanoids) are of the form 4fⁿ (n = 1 to 14 with increasing atomic number).

Atomic and Ionic Sizes
The overall decrease in atomic and ionic radii from lanthanum to lutetium (the lanthanoid contraction) is a unique feature in the chemistry of the lanthanoids. It has far reaching consequences in the chemistry of the third transition series of the elements. The decrease in atomic radii (derived from the structures of metals) is not quite regular as it is regular in M³⁺ ions. This contraction is, of course, similar to that observed in an ordinary transition series and is attributed to the same cause, the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one 4f electron by another is less than one d electron by another with the increase in nuclear charge along the series. There is fairly regular decrease in the sizes with increasing atomic number.

Oxidation States
In the lanthanoids, La(II) and Ln(III) compounds are predominant species. However, occasionally +2 and +4 ions in solution or in solid compounds are also obtained. This irregularity (as in ionisation enthalpies) arises mainly from the extra stability of empty, half-filled or filled f subshell. Thus, the formation of Ce(IV) is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common +3 state. The E^o value for Ce⁴⁺/ Ce³⁺ is + 1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent. Pr, Nd, Tb and Dy also exhibit +4 state but only in oxides, MO₂. Eu²⁺ is formed by losing the two s electrons and its f⁷ configuration accounts for the formation of this ion. However, Eu²⁺ is a strong reducing agent changing to the common +3 state. Similarly Yb²⁺ which has f¹⁴ configuration is a reductant. Tb^IV has half-filled f-orbitals and is an oxidant. The behaviour of samarium is very much like europium, exhibiting both +2 and +3 oxidation states.

General Characteristics
All the lanthanides are metals. They are soft, malleable and ductile in nature. They are not good conductors of heat and electricity. They are highly dense metals and their densities are in the range of 6.77 to 9.74 gcm^-3. The densities and atomic volumes, in general, increase with increase in atomic number. But a regular trend is not observed. They have fairly high melting points. However, no definite trend is observed.

Colour
Many of the lanthanide ions are coloured in solid-state as well as in solution. The colour is due to partially filled f-orbitals which allow f-f transitions. M³⁺ ions having 4f⁰, 4f⁷ or 4f¹⁴ configurations are colourless.

Magnetic Properties
Ions having unpaired electrons are paramagnetic while those having all the orbitals paired are diamagnetic. The lanthanide ions(M³⁺) except La³⁺(4f⁰) and Lu³⁺(4f¹⁴) are paramagnetic since they contain 1,2,....7 unpaired electrons. 

• The metals combine 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 H₂ from dilute acids and burn in presence of halogens to form halides.
• They form oxides M₂O₃ and hydroxides M(OH)₃.

The actinoids include the fourteen elements from Th to Lr. The actinoids are radioactive elements and the earlier members have relatively long half-lives, the latter ones have half-life values ranging from a day to 3 minutes for lawrencium (Z =103). The latter members could be prepared only in nanogram quantities. These facts render their study more difficult.

Electronic Configurations
All the actinoids are believed to have the electronic configuration of 7s² and variable occupancy of the 5f and 6d subshells. The fourteen electrons are formally added to 5f, though not in thorium (Z = 90) but from Pa onwards the 5f orbitals are complete at element 103. The irregularities in the electronic configurations of the actinoids, like those in the lanthanoids are related to the stabilities of the f⁰, f⁷ and f¹⁴ occupancies of the 5f orbitals. Thus, the configurations of Am and Cm are [Rn]5f⁷7s² and [Rn]5f⁷6d¹7s². Although the 5f orbitals resemble the 4f orbitals in their angular part of the wave-function, they are not as buried as 4f orbitals and hence 5f electrons can participate in bonding to a far greater extent.

Ionic Sizes
The general trend in lanthanoids is observable in the actinoids as well. There is a gradual decrease in the size of atoms or M³⁺ ions across the series. This may be referred to as the actinoid contraction (like lanthanoid contraction). The contraction is, however, greater from element to element in this series resulting from poor shielding by 5f electrons.

Oxidation States
The actinoids show in general +3 oxidation state. The elements, in the first half of the series frequently exhibit higher oxidation states. For example, the maximum oxidation state increases from +4 in Th to +5, +6 and +7 respectively in Pa, U and Np but decreases in succeeding elements. The actinoids resemble the lanthanoids in having more compounds in +3 state than in the +4 state. However, +3 and +4 ions tend to hydrolyse. Because the distribution of oxidation states among the actinoids is so uneven and so different for the former and later elements, it is unsatisfactory to review their chemistry in terms of oxidation states.