What electron sublevel starts to fill after completion of the 5 s sublevel? \[ \]

After the completion of the **5s** sublevel, the next electron sublevel to begin filling is the **4d** sublevel. ### Explanation: The order in which electron sublevels are filled is guided by the **Aufbau principle**, which states that electrons occupy orbitals starting with the lowest energy levels before filling higher ones. The sequence is determined by the sum of the principal quantum number (**n**) and the azimuthal quantum number (**l**). Sublevels with lower \( n + l \) values are filled first. If two sublevels have the same \( n + l \) value, the one with the lower \( n \) fills first. Here's a simplified order of filling for reference: 1. **1s** 2. **2s** 3. **2p** 4. **3s** 5. **3p** 6. **4s** 7. **3d** 8. **4p** 9. **5s** 10. **4d** ← **Follows after 5s** Once the **5s** sublevel is fully occupied with electrons, the **4d** sublevel starts to fill next. ### Visual Representation: ``` Order of Filling: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → ... ``` So, the **4d** sublevel is the one that begins to fill after the **5s** sublevel is complete. **Answer:** After 5 s is filled, electrons begin to occupy the four‑d (4d) sublevel.