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Lanthanum has electronic configuration [$\ce{Xe}$] $4\mathrm{f}^0$ $5\mathrm{d}^1$ $6\mathrm{s}^2$. Why is the $4\mathrm f$ sub-shell not filled even though we know that $4\mathrm f$ is filled before $5\mathrm d$ sub-shell?

I know that for some elements energy level of $4\mathrm f$>$5\mathrm d$. Why is this so?

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Remember that energy of electrons in orbitals do not have fixed values, but depends on occupation state of other orbitals.

So if we in a thought experiment fill orbitals to get a neutral atom, 1 electron in time, some orbitals switch the order of their energies.

It has the effect the 4s orbital is filled first, having lower energy, but after some 3d orbitals are filled, energy of filled 4s Is higher then energy of 3d.

Similar case happens between 5f and 6d orbitals for first actinoids.

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  • $\begingroup$ Sir can you elaborate it, I didn't understand. $\endgroup$ – Amandip Dutta Jul 26 '20 at 12:08

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