Question

La3+ रंगहीन होता है जबकि Lu4+ नारंगी लाल होता है​

Answer 1

If an ion has no unpaired electrons, a transition cannot occur, which suggests that it doesn't absorb light in the visible spectrum and is hence colourless.

• Lanthanide contraction is the term used to describe the decrease in atomic and ionic radii in lanthanides. In ions, this gradual shrinkage of size is typical, although in atoms, it is less typical. Instead of entering the valence shell, the extra electron in the lanthanide enters the 4f shell. The order of the decreasing electron shielding effect is s>p>d>f. The 'f' orbital has the least impact due of its dispersed shape.
• We are aware that a metal ion's colour depends on its unpaired electrons. If an ion has no unpaired electrons, a transition cannot occur, which suggests that it doesn't absorb light in the visible spectrum and is hence colourless. This makes $La^{3+}$ colourless, while $La^{4+}$ is orange red.

Hence, If an ion has no unpaired electrons, a transition cannot occur, which suggests that it doesn't absorb light in the visible spectrum and is hence colourless.

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