I've been taught that electrons are removed first from valence shell electrons with highest energy. But In Lanthanides, for example taking configuration- [Xe]4f⁴6s², If we go to +3 Oxidation State, then the configuration is [Xe]4f³. But valence shell is 5th, so why wasn't electron removed from 5p instead of 4f?
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25p electrons have significantly lower energy than 6s and 4f. 4f orbitals are being filled after 5p and 6s. Remember that with advent of d and f orbitals, the single valence shell number does not apply much. – Poutnik Feb 15 '22 at 13:26