Fifth period of the periodic table have 18 elements. 5th period starts from Rb to Xe (18 elements). 5th period starts with principal quantum number n = 5 and 1 = 0, 1,2,3 and 4. When n = 5, the number of orbitals = 9.
1 for 5s
5 for 4d
3 for 5p
Total number of orbitals = 9.
Total number of electrons that can be accommodated in 9
orbitals = 9 x 2 = 18.
Hence the number of elements in 5th period is 18.
