Here's why:
* Energy Levels: While the 3d orbitals are technically in the third energy level, they are slightly *higher* in energy than the 4s orbital.
* Aufbau Principle: This principle states that electrons fill orbitals in order of increasing energy. Since the 4s orbital has a lower energy than the 3d orbitals, it gets filled first.
Example:
* Potassium (K): Potassium has 19 electrons. The electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹. Notice that the 4s orbital is filled before the 3d orbitals.
Important Note: There are exceptions to this rule (like Chromium and Copper), but the general principle holds true for most elements.
