The Aufbau principle predicts that the 4 _s_ orbital is always filled before the 3 _d_ orbitals, but this is actually not true for most elements! From Sc on, the 3 _d_ orbitals are actually lower in energy than the 4 _s_ orbital, which means that electrons enter the 3 _d_ orbitals first. In this video, we'll discuss this in more depth and walk through all of the electron configurations for th
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