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!!