How many elements are there in the 5th period of modern periodic table ?

The modern periodic table organizes elements into horizontal rows called periods and vertical columns called groups. The 5th period is classified as a long period. It contains exactly 18 elements, beginning with the alkali metal Rubidium (atomic number 37) and concluding with the noble gas Xenon (atomic number 54) [1]. This count is determined by the filling of electron shells; specifically, the 5th period involves the filling of the 5s, 4d, and 5p orbitals. These nine orbitals can accommodate a maximum of 18 electrons, corresponding to the 18 elements in the period. In contrast, the 1st period has 2 elements, while the 2nd and 3rd periods contain 8 elements each. The 4th and 5th periods both contain 18 elements, whereas the 6th and 7th periods contain 32 elements each.

Sources

1. [1] https://simple.wikipedia.org/wiki/Period_5_element