Explain why sodium is less reactive than potassium.

Explanation

### Step-by-Step Solution:

1. **Understanding Reactivity in Alkali Metals**:
- Reactivity in alkali metals is primarily determined by how easily they can lose their outermost electron. The easier it is to lose this electron, the more reactive the metal is.

2. **Defining Ionization Energy**:
- Ionization energy is the energy required to remove an electron from an atom. A lower ionization energy means that an electron can be removed more easily, leading to higher reactivity.

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