Question

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

Answer 1

`"f-element metals"` do not follow the Group numbers......

Explanation:

......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.