Question

Why are there 14 Lanthanides and Actinides?

Answer 1

1. The modern periodic table consists of $118$electrons, that are arranged in $18$ groups and $7$periods.
2. The vertical column represents the groups and the horizontal rows represent the periods.
3. We know that the modern periodic table consists of four different blocks namely s, p, d, and f.
4. The f-block contains Lanthanoids and Actinoids.
5. The first period of this element corresponds to being filled in $4\mathrm{f}$orbital whereas the second period corresponds to be filled in $5\mathrm{f}$.
6. The general electronic configuration of Lanthanoid is $(\mathrm{n}-2){\mathrm{f}}^{1-14}(\mathrm{n}-1){\mathrm{d}}^{0-1}{\mathrm{ns}}^2$.
7. The general electronic configuration of Actinoid is $\mathrm{Rn}{\left[5\mathrm{f}\right]}^{0-14}6{\mathrm{d}}^{0-2}7{\mathrm{s}}^2$.
8. As only 14 electrons are filled up in the f-subshell then there are a total of 14 Lanthanoids and 14 Actinoids placed at bottom of the periodic table.
9. The electronic configuration depends upon the atomic number so they are placed accordingly.