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, atomic number 57) to lutetium (Lu, atomic number 71). These elements are known for their similar properties.

2. **Lanthanide Contraction**: The primary reason for the difficulty in separating lanthanide elements is the phenomenon known as "lanthanide contraction." This refers to the gradual decrease in ionic and atomic radii of the lanthanide elements as one moves from lanthanum to lutetium.

3. **Effect of Electron Configuration**: As we progress through the lanthanide series, electrons are added to the 4f subshell. The effective nuclear charge increases due to the addition of protons in the nucleus, which pulls the electrons closer to the nucleus, resulting in a smaller atomic radius.

...