# Why actinides and lanthanides series are kept in the bottom of the periodic table? Reasons? Thank you.

The $\text{lanthanides}$ and $\text{actinides}$ each show progressive filling of the $f - \text{orbitals}$. We know that the filling of the $d - \text{orbitals}$ gives rise to peculiar chemistry in the transition series, given the geometric properties of the $d - \text{orbitals}$. The $f - \text{orbitals}$ are multi-lobed, and they are very poorly shielded from the nuclear charge.
And thus $f - \text{orbital}$ electrons do not tend to participate in bonding because they are too tightly held by the nuclear charge. The result is that the lanthanides tend to lose the 2 or 3 valence electrons in the s shell, and form a series of ${M}^{3 +}$ cations (or more rarely ${M}^{2 +}$ cations). Lanthanides and actinides thus tend to have a particular chemistry which is exclusive to themselves. Most lanthanide chemists also study the chemistry of scandium and yttrium, which as ${M}^{3 +}$ and ${M}^{2 +}$, very potent Lewis acids, have a parallel and complementary chemistry.