Hello candidate,
The principal for filling up the orbitals is based on the n + l rule, and n refers to the principal quantum number, and l refers to the azimuthal quantum number.
The azimuthal quantum number for s, p, d, and f are zero, one, two, and three respectively. As per (n + l) rule in 6th period, order of orbitals filling is 6s, 4f, 5d, 6p.
Hope that this information about helpful for you.
Good luck!!
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