Chemistry The Actinoids

Topics Covered :

● The Actinoids
● Electronic Configurations
● Ionic Sizes
● Oxidation States
● Comparison with Lanthanoids
● Applications of d- and f-block Elements

The Actinoids :

`=>` The actinoids include the fourteen elements from `color{red}(Th)` to `color{red}(Lr)`.

`=>` The names, symbols and some properties of these elements are given in Table 8.10.

`=>` 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 make their study more difficult.

Electronic Configurations of actinoids :

`=>` All the actinoids are believed to have the electronic configuration of `color{red}(7s^2)` and variable occupancy of the `color{red}(5f)` and `color{red}(6d)` subshells.

`=>` The fourteen electrons are formally added to `color{red}(5f)`, though not in thorium (`color{red}(Z = 90)`) but from `color{red}(Pa)` onwards the `color{red}(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 `color{red}(f^0, f^7)` and `color{red}(f^(14))` occupancies of the `color{red}(5f)` orbitals.

`=>` Thus, the configurations of `color{red}(Am)` and `color{red}(Cm)` are `color{red}([Rn] 5f^7 7s^2)` and `color{red}([Rn] 5f^7 6d^1 7s^2)`.

`=>` Although the `color{red}(5f)` orbitals resemble the `color{red}(4f)` orbitals in their angular part of the wave-function, they are not as buried as `color{red}(4f)` orbitals and hence `color{red}(5f)` electrons can participate in bonding to a far greater extent

Ionic Sizes of actinoids :

`=>` The general trend in lanthanoids is observable in the actinoids as well.

`=>` There is a gradual decrease in the size of atoms or `color{red}(M^(3+))` 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 `color{red}(5f)` electrons.

Oxidation States of actinoids :

`=>` There is a greater range of oxidation states, which is in part attributed to the fact that the `color{red}(5f)`, `color{red}(6d)` and `color{red}(7s)` levels are of comparable energies.

`=>` The known oxidation states of actinoids are listed in Table 8.11.

`=>` 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 `color{red}(Pa)`, `color{red}(U)` and `color{red}(Np)` but decreases in succeeding elements (Table 8.11).

`=>` 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 earlier and latter elements, it is unsatisfactory to review their chemistry in terms of oxidation states.

General Characteristics and Comparison with Lanthanoids :

`color{green}(text(Structural Variability ))` : The actinoid metals are all silvery in appearance but display a variety of structures.

● The structural variability is obtained due to irregularities in metallic radii which are far greater than in lanthanoids.

`color{green}(text(Reactivity ))` : The actinoids are highly reactive metals, especially when finely divided.

● Example : The action of boiling water on them gives a mixture of oxide and hydride and combination with most non metals takes place at moderate temperatures.

● Hydrochloric acid attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers; alkalies have no action.

`color{green}(text(Magnetic Properties ))` : The magnetic properties of the actinoids are more complex than those of the lanthanoids. Although the variation in the magnetic susceptibility of the actinoids with the number of unpaired `color{red}(5 f)` electrons is roughly parallel to the corresponding results for the lanthanoids, the latter have higher values.

`color{green}(text(Ionisation Enthalpy ))` : It is evident from the behaviour of the actinoids that the ionisation enthalpies of the early actinoids, though not accurately known, but are lower than for the early lanthanoids.

● This is quite reasonable since it is to be expected that when `color{red}(5f)` orbitals are beginning to be occupied, they will penetrate less into the inner core of electrons.

● The `color{red}(5f)` electrons, will therefore, be more effectively shielded from the nuclear charge than the `color{red}(4f)` electrons of the corresponding lanthanoids.

● Because the outer electrons are less firmly held, they are available for bonding in the actinoids.

`=>`A comparison of the actinoids with the lanthanoids reveals that behaviour similar to that of the lanthanoids is not evident until the second half of the actinoid series.

`=>` However, even the early actinoids resemble the lanthanoids in showing close similarities with each other and in gradual variation in properties which do not entail change in oxidation state.

`=>` The lanthanoid and actinoid contractions, have extended effects on the sizes, and therefore, the properties of the elements succeeding them in their respective periods.

`=>` The lanthanoid contraction is more important because the chemistry of elements succeeding the actinoids are much less known at the present time.
Q 3081501427

Name a member of the lanthanoid series which is well known to exhibit +4 oxidation state.


Cerium (Z = 58)

Some Applications of d- and f-Block Elements :

`=>` Iron and steels are the most important construction materials. Their production is based on the reduction of iron oxides, the removal of impurities and the addition of carbon and alloying metals such as `color{red}(Cr)`, `color{red}(Mn)` and `color{red}(Ni)`.

`=>` Some compounds are manufactured for special purposes such as `color{red}(TiO)` for the pigment industry and `color{red}(MnO_2)` for use in dry battery cells.

● The battery industry also requires `color{red}(Zn)` and `color{red}(Ni//Cd)`.

● The elements of Group `11` are still worthy of being called the coinage metals, although `color{red}(Ag)` and `color{red}(Au)` are restricted to collection items and the contemporary UK ‘copper’ coins are copper-coated steel.

● The ‘silver’ UK coins are a `color{red}(Cu//Ni)` alloy.

`=>` Many of the metals and/or their compounds are essential catalysts in the chemical industry.

● `color{red}(V_2O_5)` catalyses the oxidation of `color{red}(SO_2)` in the manufacture of sulphuric acid.

● `color{red}(TiCl_4)` with `color{red}(Al(CH_3)_3)` forms the basis of the Ziegler catalysts used to manufacture polyethylene (polythene).

● Iron catalysts are used in the Haber process for the production of ammonia from `color{red}(N_2//H_2)` mixtures.

● Nickel catalysts enable the hydrogenation of fats to proceed.

● In the Wacker process the oxidation of ethyne to ethanal is catalysed by `color{red}(PdCl_2)`.

● Nickel complexes are useful in the polymerisation of alkynes and other organic compounds such as benzene.

`=>` The photographic industry relies on the special light-sensitive properties of `color{red}(AgBr)`.