Name the element which has one shell only which is completely filled with electrons. Twice as many electrons in second shell as in first shell. A total of three shells, with 3 electrons in valence shell.

Explanation

To solve the question, we need to identify three elements based on the given criteria. Let's break down the requirements step by step:

### Step 1: Identify the first element
- **Requirement**: The element has one shell only, which is completely filled with electrons.
- **Analysis**: The first shell (K-shell) can hold a maximum of 2 electrons. The only element with 2 electrons in the K-shell is Helium (He).
- **Conclusion**: The first element is **Helium**.

### Step 2: Identify the second element
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