`=>` The f-block consists of the two series, lanthanoids (the fourteen elements following lanthanum) and actinoids (the fourteen elements following actinium).

`=>` Because lanthanum closely resembles the lanthanoids, it is usually included in any discussion of the lanthanoids for which the general symbol `color{red}(L n)` is often used.

`=>` Similarly, a discussion of the actinoids includes actinium besides the fourteen elements constituting the series.

`=>` The lanthanoids resemble one another more closely than do the members of ordinary transition elements in any series.

`=>` They have only one stable oxidation state and their chemistry provides an excellent opportunity to examine the effect of small changes in size and nuclear charge along a series of otherwise similar elements.

`=>` The chemistry of the actinoids is, on the other hand, much more complicated.

● The complication arises partly owing to the occurrence of a wide range of oxidation states in these elements and partly because their radioactivity creates special problems in their study.

The names, symbols, electronic configurations of atomic and some ionic states and atomic and ionic radii of lanthanum and lanthanoids (for which the general symbol `color{red}(L n)` is used) are given in Table 8.9.

`=>` Atoms of these elements have electronic configuration with `color{red}(6s^2)` common but with variable occupancy of `color{red}(4f)` level (Table 8.9).

`=>` However, the electronic configurations of all the tripositive ions (the most stable oxidation state of all the lanthanoids) are of the form `color{red}(4f^n)` (`color{red}(n = 1)` to `14` with increasing atomic number).

`=>` 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 `color{red}(M^(3+))` ions (Fig. 8.6).

● 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 `color{red}(4 f)` electron by another is less than one `color{red}(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.

● The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.

● The almost identical radii of `color{red}(Zr)` (`160` pm) and `color{red}(Hf)` (`159` pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation.

`=>` In the lanthanoids, `color{red}(La(III))` and `color{red}(L n(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.

`=>` Therefore, the formation of `color{red}(Ce(IV))` is favoured by its noble gas configuration, but it is a strong oxidant reverting to the common `+3` state.

● The `color{red}(E^⊖)` value for `color{red}(Ce^(4+)// Ce^(3+))` is `+ 1.74 V` which suggests that it can oxidise water. However, the reaction rate is very slow and hence `color{red}(Ce(IV))` is a good analytical reagent.

`=>` `color{red}(Pr)`, `color{red}(Nd)`, `color{red}(Tb)` and `color{red}(Dy)` also exhibit `+4` state but only in oxides, `color{red}(MO_2)`.

`=>` `color{red}(Eu^(2+))` is formed by losing the two `color{red}(s)` electrons and its `color{red}(f^7)` configuration accounts for the formation of this ion.

● However, `color{red}(Eu^(2+))` is a strong reducing agent changing to the common `+3` state.

`=>` Similarly `color{red}(Yb^(2+))` which has `color{red}(f^(14))` configuration is a reductant.

`=>` `color{red}(Tb(IV))` has half-filled `color{red}(f)`-orbitals and is an oxidant.

`=>` The behaviour of samarium is very much like europium, exhibiting both `+2` and `+3` oxidation states.

`=>` All the lanthanoids are silvery white soft metals and tarnish rapidly in air.

`=>` The hardness increases with increasing atomic number, samarium being steel hard.

`=>` Their melting points range between `1000` to `1200 K` but samarium melts at `1623 K`.

`=>` They have typical metallic structure and are good conductors of heat and electricity.

`=>` Density and other properties change smoothly except for `color{red}(Eu)` and `color{red}(Yb)` and occasionally for `color{red}(Sm)` and `color{red}(Tm)`.

`color{green}(text(Colour ))` : ● Many trivalent lanthanoid ions are coloured both in the solid state and in aqueous solutions.

● Colour of these ions may be attributed to the presence of `color{red}(f)` electrons.

● Neither `color{red}(La^(3+))` nor `color{red}(Lu^(3+))` ion shows any colour but the rest do so.

● However, absorption bands are narrow, probably because of the excitation within `f` level.

`color{green}(text(Magnetism ))` : ● The lanthanoid ions other than the `color{red}(f^0)` type `color{red}((La^(3+))` and `color{red}(Ce^(4+)))` and the `color{red}(f^( 14))` type `color{red}((Yb^(2+))` and `color{red}(Lu^(3+)))` are all paramagnetic.

● The paramagnetism rises to maximum in neodymium.

`color{green}(text(Ionisation Enthalpy ))` : ● The first ionisation enthalpies of the lanthanoids are around `600 kJ mol^(–1)`, the second about `1200 kJ mol^(–1)` comparable with those of calcium.

● The variation of the third ionisation enthalpies indicates that the exchange enthalpy considerations (as in `color{red}(3d)` orbitals of the first transition series), appear to impart a certain degree of stability to empty, half-filled and completely filled orbitals `f` level.

● This is indicated from the abnormally low value of the third ionisation enthalpy of lanthanum, gadolinium and lutetium.

`color{green}(text(Chemical Reactivity ))` : ● In their chemical behaviour, in general, the earlier members of the series are quite reactive similar to calcium but, with increasing atomic number, they behave more like aluminium.

● Values for `color{red}(E^⊖)` for the half-reaction :

`color{red}(L n^(3+) (aq) + 3e^(–) → L n(s))`

are in the range of `–2.2` to `–2.4 V` except for `Eu` for which the value is `– 2.0 V`.

● This is, of course, a small variation.

● The metals combine with hydrogen when gently heated in the gas.

● The carbides, `color{red}(L n_3C)`, `color{red}(L n_2C_3)` and `color{red}(L nC_2)` 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 `color{red}(M_2O_3)` and hydroxides `color{red}(M(OH)_3)`. The hydroxides are definite compounds, not just hydrated oxides. They are basic like alkaline earth metal oxides and hydroxides. Their general reactions are depicted in Fig. 8.7.

`color{green}(text(Uses ))` : ● The best single use of the lanthanoids is for the production of alloy steels for plates and pipes.

● A well known alloy is mischmetall which consists of a lanthanoid metal `(~ 95%)` and iron `(~ 5%)` and traces of `color{red}(S)`, `color{red}(C)`, `color{red}(Ca)` and `color{red}(Al)`.

● A good deal of mischmetall is used in `color{red}(Mg)`-based alloy to produce bullets, shell and lighter flint.

● Mixed oxides of lanthanoids are employed as catalysts in petroleum cracking.

● Some individual `color{red}(L n)` oxides are used as phosphors in television screens and similar fluorescing surfaces.