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 Eu and Yb and occasionally for Sm and Tm.
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 offelectrons. Neither La3+nor Lu3+ion shows any colour but the rest do so. However, absorption bands are narrow, probably because of the excitation withinflevel. The lanthanoid ions otherthan the
f0type (La3+and Ce4+) and thef14type (Yb2+and Lu3+) are all paramagnetic.
The first ionisation enthalpies of the lanthanoids are around600 kJ mol–1, the second about 1200 kJ mol–1comparable with those of calcium. A detailed discussion of the variation of the third ionisation enthalpies indicates that the exchange enthalpy considerations (as in 3dorbitals of the first transition series), appear to impart a certain degree of stability to empty, half-filled and completely filled orbitalsflevel. This is indicated from the abnormally low value of the third ionisation enthalpy of lanthanum, gadolinium and lutetium.
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 forEVfor the half-reaction:
Ln3+(aq) + 3e–→Ln(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, Ln3C, Ln2C3and LnC2are formed when the metals are heated with carbon. They liberate hydrogen from dilute acids and burn in halogens to form halides. They form oxides M2O3and hydroxides 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.
Fig. 8.7 Chemical reactions of the lanthanoids
The best single use of the lanthanoids is for the production of alloy steels for plates and pipes. A well known alloy ismischmetallwhich consists of a lanthanoid metal (~ 95%) and iron (~ 5%) and traces of S, C, Ca and Al. A good deal ofmischmetallis used in Mg-based alloy to produce bullets, shell and lighter flint. Mixed oxides of lanthanoids are employed as catalysts in petroleum cracking. Some individual Ln oxides are used as phosphors in television screens and similar fluorescing surfaces.
