Which elements are called inner transition elements ? Name the different inner transition series. Why are they so named ?

Explanation

### Step-by-Step Text Solution:

1. **Definition of Inner Transition Elements**:
Inner transition elements are the elements that are found in the f-block of the periodic table. They are characterized by the filling of f orbitals.

2. **Identification of Inner Transition Series**:
There are two series of inner transition elements:
- The **4F series**, which is known as the **lanthanides** or **lanthanoids**.
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