Animated Solution for Chemistry - d and f-Block Elements: Knowing that the chemistry of lanthanoids (Ln) is dominated by its +3 oxidation state, which of the following statements is incorrect?

Lanthanides, often called the inner transition metals, are a fascinating group of elements tucked away at the bottom of the periodic table. When we study their chemistry, one thing becomes abundantly clear: their lives revolve around the $+3$ oxidation state.

Let's break down the properties of these $L{n}^{3+}$ ions and figure out which of the given statements is the odd one out.

When a lanthanide atom loses three electrons to form an $L{n}^{3+}$ ion, its electronic configuration generally becomes $[Xe]4f^n$, where $n$ ranges from $1$ to $14$.

Just like transition metals exhibit vibrant colours due to $d-d$ transitions, lanthanides have their own party trick: $f-f$ transitions. Because the $4f$ subshell is partially filled in most $L{n}^{3+}$ ions (from $4f^1$ to $4f^{13}$), there are unpaired electrons present. These electrons can absorb specific wavelengths of visible light and jump to higher energy $f$-orbitals. The transmitted or reflected light gives the compound its characteristic colour.

Therefore, the vast majority of lanthanide(III) compounds are highly coloured in both solid and aqueous states.

There are only two notable exceptions: 1. Lanthanum ($L{a}^{3+}$): It has a $4f^0$ configuration. With no $f$-electrons to excite, it is colourless. 2. Lutetium ($L{u}^{3+}$): It has a $4f^{14}$ configuration. The subshell is completely full, leaving no room for transitions, making it colourless as well.

Since most are coloured, the statement "Ln (III) compounds are generally colourless" is blatantly incorrect.

Let's quickly look at why the other statements are perfectly true:

Ionic Character: Lanthanide ions are relatively large compared to transition metals. Because of this large size and their low ionization enthalpies, they tend to form bonds that are predominantly ionic in nature.

Lanthanoid Contraction: As we move from left to right across the lanthanide series (from Cerium to Lutetium), the nuclear charge increases. However, the new electrons are added to the deep-seated $4f$ subshell, which offers very poor shielding. This results in a steady, gradual decrease in the ionic radii of $L{n}^{3+}$ ions, a phenomenon famously known as Lanthanoid Contraction.

Basic Hydroxides: Because $L{n}^{3+}$ ions are large, the charge density on the ion is relatively low. This makes the $Ln-OH$ bond weaker and more ionic, allowing it to easily release $O{H}^{-}$ ions in solution. Thus, their hydroxides are mainly basic in character. (Note: The basicity slightly decreases across the series as the size decreases due to lanthanoid contraction, but they remain basic overall).