Write one main characteristic of f-block elements or inner transition elements

Explanation

**Step-by-Step Text Solution:**

1. **Identify the f-block Elements**: The f-block elements, also known as inner transition elements, include the lanthanides and actinides. These elements are located at the bottom of the periodic table.

2. **Characteristic of f-block Elements**: One main characteristic of f-block elements is that they form colored compounds or colored ions.

3. **Reason for Color Formation**: The color in these compounds arises due to the presence of unpaired electrons in the f-orbitals. When visible light strikes these ions, the unpaired electrons can absorb specific wavelengths of light.

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