# Why are "actinides" and "lanthanides" not placed into Groups?

$\text{f-element metals}$ do not follow the Group numbers......
......they are a class of metals all by themselves, and they have a peculiar chemistry, which is marked by extreme Lewis-acidity. They commonly form ${M}^{3 +}$ ions. Because the Group 3 metals, scandium, and yttrium, also commonly form ${M}^{3 +}$ ions, their chemistry is often lumped together with that of the lanthanides.