question_answer 2)

How would you justify the presence of 18 elements in the 5th period of the periodic table?

Answer:

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.