# Lanthanum Electronic Configuration [duplicate]

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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