Why does 4s fill before 3d?
Understand the Problem
The question is asking why, in terms of electron configuration, the 4s orbital is filled with electrons before the 3d orbital. This relates to the principles of atomic structure and energy levels in quantum mechanics.
Answer
The 4s sublevel is filled before the 3d sublevel because it has a lower energy level.
According to the Aufbau principle, the 4s sublevel is filled before the 3d sublevel because the 4s orbital is lower in energy.
Answer for screen readers
According to the Aufbau principle, the 4s sublevel is filled before the 3d sublevel because the 4s orbital is lower in energy.
More Information
The Aufbau principle states that electrons occupy the lowest energy orbital available, which is why the 4s orbital, being lower in energy, fills before the 3d orbital.
Tips
A common mistake is to assume that the order of filling is strictly based on numerical sequence (n values), ignoring the energy levels.
