Why is the separation of lanthanide elements difficult?

Explanation

**Step-by-Step Solution:**

1. **Understanding Lanthanides**: The lanthanide series consists of 15 elements from lanthanum (La) to lutetium (Lu). These elements are known for their similar physical and chemical properties.

2. **Lanthanoid Contraction**: One of the primary reasons for the difficulty in separating lanthanides is the phenomenon known as "lanthanoid contraction." This refers to the gradual decrease in ionic and atomic radii of the lanthanide elements as the atomic number increases.

3. **Ionic and Atomic Radii**: Due to lanthanoid contraction, the differences in ionic and atomic radii among the lanthanides become very small. This means that the size of the ions does not vary significantly from one element to another, making it challenging to separate them based on size.

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