Question

How would you justify the presence of $18$ elements in the $5^{th}$ period of the Periodic Table?

Answer 1

For $5^{th}$ period, principal quantum number, $n=5$ and values of azimuthal quantum no., $l=0,1,2,3$

According to Aufbau principle, filling of orbitals in sub-shells takes place in increasing order of energy of sub-shell. Since energy of $4f$ sub-shell is higher than $5s$ sub-shell, the sub-shells available for filling are $5s,4d5p$.

$\therefore \text{total no. of orbitals}=\text{1 for 5s}+\text{5 for 4d}+\text{3 for 5p}=1+5+3=9$

$\therefore$ total no. of electrons that can be accomodated in 9 orbitals $=2\times 9=18$

Hence, the no. of electrons or elements in $5^{th}$ period are 18.