Which of the following elements contains only two electrons in the outermost shell?

Options

A. Helium
B. Beryllium
C. Beryllium
D. All of these

Explanation

To determine which element contains only two electrons in the outermost shell, we can follow these steps:

### Step 1: Understand the concept of electronic configuration
The electronic configuration of an element describes how electrons are distributed among the various orbitals and shells. The shells are labeled as K, L, M, N, etc., where K is the first shell, L is the second shell, and so on.

### Step 2: Identify the elements in question
The elements we will consider are helium (He) and beryllium (Be).

### Step 3: Write the electronic configuration for each element
- **Helium (He)**: The atomic number of helium is 2. Therefore, its electronic configuration is 1s². This means that helium has 2 electrons in the K shell, which is its outermost shell.

- **Beryllium (Be)**: The atomic number of beryllium is 4. Its electronic configuration is 1s² 2s². This means that beryllium has 2 electrons in the K shell and 2 electrons in the L shell. The outermost shell (L shell) contains 2 electrons.

### Step 4: Determine the outermost shell electrons
- For helium, the outermost shell (K shell) contains 2 electrons.
- For beryllium, the outermost shell (L shell) also contains 2 electrons.

### Step 5: Conclusion
Both helium and beryllium have 2 electrons in their outermost shells. However, since the question asks for an element that contains only two electrons in the outermost shell, the answer is:

**Answer: Helium (He)**