Ionisation enthalpies of Ce, Pr and Nd are higher than Th, Pa and U. Why?

Explanation

To understand why the ionization enthalpies of cerium (Ce), praseodymium (Pr), and neodymium (Nd) are higher than those of thorium (Th), protactinium (Pa), and uranium (U), we can break down the explanation into several key points:

### Step-by-Step Solution:

1. **Identify the Elements**:
- Cerium (Ce), Praseodymium (Pr), and Neodymium (Nd) are lanthanides, which means they belong to the f-block elements with an incomplete 4f subshell.
- Thorium (Th), Protactinium (Pa), and Uranium (U) are actinides, also f-block elements, but they have an incomplete 5f subshell.

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