Why do alkali metal tarnishes in air?

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Alkali Metals**: Alkali metals are located in Group 1 of the periodic table and include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). These metals have unique properties due to their electronic configuration.

2. **Atomic Size and Valence Electrons**: Alkali metals have the largest atomic size in their respective periods. This means that the outermost electron (the valence electron) is located further from the nucleus, making it less tightly held.

3. **Low Ionization Energy**: The valence electron in alkali metals is held loosely due to the large atomic size. As a result, these metals have low ionization energy, which means they can easily lose their outermost electron.

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