Answer :
Final answer:
The 4s orbital is filled before the 3d orbital because it has lower energy when empty. Upon filling, the energy changes, making the 3d orbitals lower in energy for transition metals. In transition metal ions, the 4s electrons are always lost first, reflecting the energy shift.
Explanation:
If 3d orbitals are lower in energy than 4s for transition metals, the question arises as to why you still have to fill in the 4s orbitals first. The answer lies in the fact that the 4s orbital is filled before the 3d orbital due to the energies of the orbitals when they are empty. This order of filling is based on the Aufbau Principle, which predicts the sequence electrons fill atomic orbitals.
As electrons are added to an atom, they naturally seek the lowest available energy states. Initially, when orbitals are empty, the 4s orbital has a lower energy compared to the 3d orbital. Consequently, the 4s orbital fills with electrons before the 3d. However, once these orbitals have electrons, the energy levels shift, making 3d orbitals lower in energy than the 4s orbital for transition metals.
During electron filling in larger atoms, after the 3p subshell is filled, as with Argon (Ar), for potassium (K) the next electron enters the 4s subshell because it represents a lesser overall energy state. Nevertheless, in transition metal ions, these 4s electrons are removed first during ionization, as they are the outermost and become higher in energy once the 3d orbitals are populated.
In summary, the energy levels of orbitals are not static and can change as electrons are added. This leads to the observed filling sequence where the 4s orbital is filled before the 3d orbital in the neutral atom but is the first to be ionized from transition metals, reaffirming the 3d orbital's lower energy in the filled state.
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