Here's why:
* Energy Levels: Electrons always try to occupy the lowest energy level available. While the 3d orbitals are higher in energy than the 3p orbitals, the 4s orbital is *lower* in energy than the 3d orbitals.
* Filling Order: The filling order of orbitals follows the Aufbau principle and Hund's rule. This means that the 4s orbital is filled before the 3d orbitals.
Important Note: While the 4s orbital is lower in energy, there are exceptions to this rule, especially in transition metals. However, for a simple excitation from 3p, the electron will most likely go to the 4s orbital.
