• # question_answer 2) How would you justify the presence of 18 elements in the 5th period of the periodic table?

The fifth period corresponds to the filling of fifth energy shell, i.e., $n=5$. Out of $5s,\text{ }5p,\text{ }5d$ and $5f$-orbitals, the energies of $5d$ and $5f-$orbitals are higher than $6s-$orbital. Thus, $5s$ and $5p-$orbitals are filled. However, between $5s$ and $5p-$orbitals, five $4d-$orbitals are also filled as their energies lie in between these orbitals. Hence, nine orbitals are filled (one $5s$ three $5p$ and five $4d-$orbitals). Therefore, the maximum number of elements that can be accommodated in fifth period is 18 corresponding to accommodation of 18 electrons on nine orbitals.