General Characteristics

Physical Properties

All the lanthanoids are silvery-white, soft metals, and they tarnish quickly on exposure to air. Softness is not uniform across the series, though — hardness increases with increasing atomic number, and by the time the series reaches samarium the metal is steel-hard.

Melting points mostly fall in a fairly narrow band, roughly $1000$–$1200\ \text{K}$, but samarium again stands out as an exception, melting at the much higher $1623\ \text{K}$. Structurally, the lanthanoids are typical metals and are good conductors of both heat and electricity. Density and most other physical properties change smoothly from one element to the next across the series, with europium and ytterbium as the main exceptions, and samarium and thulium occasionally behaving irregularly as well.

Colour and Magnetism of the Ions

Many trivalent lanthanoid ions are coloured, both as solids and in aqueous solution, and this colour is attributed to the presence of $f$ electrons. The two ions with no partially filled $f$ subshell — $\text{La}^{3+}$ (which is $f^{0}$) and $\text{Lu}^{3+}$ (which is $f^{14}$) — show no colour at all; every other ion in the series does. The absorption bands responsible for the colour tend to be narrow, which is put down to the fact that the electronic excitation is happening within the shielded $4f$ level rather than at the outer surface of the ion.

The same $f^{0}$/$f^{14}$ divide governs magnetism: every lanthanoid ion other than the $f^{0}$ type ($\text{La}^{3+}$ and $\text{Ce}^{4+}$) and the $f^{14}$ type ($\text{Yb}^{2+}$ and $\text{Lu}^{3+}$) is paramagnetic, since an $f^0$ or $f^{14}$ ion has no unpaired $f$ electrons left to respond to a magnetic field.

Ionisation Enthalpies

The first ionisation enthalpies of the lanthanoids sit around $600\ \text{kJ mol}^{-1}$, and the second around $1200\ \text{kJ mol}^{-1}$ — both broadly comparable to the values seen for calcium. Looking closely at how the third ionisation enthalpy varies along the series shows the same exchange-enthalpy effect already familiar from the $3d$ transition series: certain $f$ occupancies (empty, half-filled, completely filled) carry extra stability. This shows up as an abnormally low third ionisation enthalpy exactly at lanthanum, gadolinium, and lutetium — the three points in the series where losing a third electron leaves behind an especially stable $f^0$, $f^7$, or $f^{14}$ ion.

Chemical Reactivity

The lanthanoids' chemical character shifts gradually across the series. The earlier members behave reactively, in a manner fairly close to calcium, while with increasing atomic number the later members start to behave more like aluminium.

The standard electrode potentials for the reduction half-reaction

fall in a narrow band of $-2.2$ to $-2.4\ \text{V}$ for almost the whole series, with europium as the one outlier at $-2.0\ \text{V}$ — only a small variation overall, consistent with how similar these metals are to one another.

The lanthanoid metals react in a range of characteristic ways:

• They combine with hydrogen on gentle heating in the gas.
• Heated with carbon, they form carbides of formula $\text{Ln}_3\text{C}$, $\text{Ln}_2\text{C}_3$, and $\text{LnC}_2$.
• They liberate hydrogen from dilute acids.
• They burn in halogens to give the trihalides, $\text{LnX}_3$.
• They form oxides, $\text{M}_2\text{O}_3$, and hydroxides, $\text{M(OH)}_3$ — the hydroxides are genuine, definite compounds rather than simply hydrated oxides, and they are basic in character, much like the oxides and hydroxides of the alkaline earth metals.

…