The number of elements in the 5th period of the periodic table is

The correct option is 18.

In the periodic table, the elements are arranged in periods (horizontal rows) corresponding to the principal quantum number ($n$) of the valence shell. For the 5th period, the principal quantum number is:

$n=5$

According to the quantum mechanical model of the atom and the Aufbau principle, electrons fill the subshells in order of increasing energy. The orbitals that are filled in the 5th period correspond to the energy levels where the principal quantum number is 5 for the outer shell. The subshells filled in this period, in order of their increasing energy, are:
1. The 5s subshell (1 orbital)
2. The 4d subshell (5 orbitals)
3. The 5p subshell (3 orbitals)

To find the total number of orbitals available for filling in the 5th period, we sum the number of orbitals in these subshells:

Total orbitals = $1\text{(from 5s)}+5\text{(from 4d)}+3\text{(from 5p)}=9\text{orbitals}$

According to the Pauli exclusion principle, each orbital can accommodate a maximum of 2 electrons with opposite spins. Therefore, the maximum number of electrons that can be accommodated in these orbitals (which corresponds directly to the number of elements in that period) is:

Number of elements = $9\text{orbitals}\times 2\text{electrons/orbital}=18$

Thus, there are exactly 18 elements in the 5th period of the periodic table, starting from Rubidium (Rb, atomic number 37) and ending at Xenon (Xe, atomic number 54).