What is the order of the second ionisation enthalpy of `K,Ca` and `Ba`?

Explanation

To determine the order of the second ionization enthalpy of potassium (K), calcium (Ca), and barium (Ba), we need to understand a few key concepts:

### Step 1: Define Second Ionization Enthalpy
The second ionization enthalpy (IE2) is the energy required to remove an electron from a unipositive cation (M⁺) to form a doubly charged cation (M²⁺). For example, for potassium, the process can be represented as:
\[ \text{K}^+ \rightarrow \text{K}^{2+} + e^- \]

### Step 2: Analyze the Cation Sizes
The size of the cation plays a crucial role in determining the second ionization enthalpy. As we move down the group in the periodic table, the size of the cations increases. This is because additional electron shells are added, which increases the distance between the nucleus and the outermost electrons.
...