According to the Aufbau Principle, orbitals are filled in the order of increasing energy, which is determined by the $(n+l)$ rule. For the 6th period ($n=6$), the orbitals involved are $6s$, $4f$, $5d$, and $6p$.

1. Calculate $(n+l)$ values: $ns \rightarrow 6s: 6 + 0 = 6$ $(n-2)f \rightarrow 4f: 4 + 3 = 7$ $(n-1)d \rightarrow 5d: 5 + 2 = 7$ $np \rightarrow 6p: 6 + 1 = 7$

2. Determine Order: $6s$ has the lowest energy (lowest sum). For $4f, 5d,$ and $6p$, the sums are equal (7). In such cases, the orbital with the lower $n$ value fills first.

• Order: $4f (n=4) \rightarrow 5d (n=5) \rightarrow 6p (n=6)$.

3. Final Sequence: $6s \rightarrow 4f \rightarrow 5d \rightarrow 6p$ Substituting $n=6$: $ns \rightarrow (n-2)f \rightarrow (n-1)d \rightarrow np$.