Which electronic level would allow an atom to absorb a photon but not to emit a photon ?

Options

A. 3s
B. 2p
C. 1s
D. 3d

Explanation

To determine which electronic level allows an atom to absorb a photon but not to emit one, we need to understand the concepts of electronic energy levels and the behavior of electrons in these levels.

### Step-by-Step Solution:

1. **Understanding Electronic Levels**:
- Atoms have various electronic energy levels, which are often denoted by principal quantum numbers (n). The lowest energy level is called the ground state, while higher energy levels are called excited states.

2. **Identifying the Ground State**:
- The ground state of an atom is the lowest energy configuration of its electrons. For hydrogen, this is the 1s level, which is the closest to the nucleus and has the least energy.

3. **Photon Absorption**:
- When an atom absorbs a photon, an electron can move from a lower energy level (ground state) to a higher energy level (excited state). This process requires energy, which is provided by the photon.

4. **Photon Emission**:
- Emission occurs when an electron falls from a higher energy level back to a lower energy level, releasing energy in the form of a photon.

5. **Analyzing the 1s Level**:
- The 1s level, being the lowest energy state, can absorb a photon to move to a higher energy level (for example, 2s or 2p). However, it cannot emit a photon to drop to a lower energy level since it is already at the lowest energy state.

6. **Conclusion**:
- Therefore, the electronic level that allows an atom to absorb a photon but not to emit one is the **1s level**.

### Final Answer:
The electronic level that allows an atom to absorb a photon but not to emit a photon is **1s**.