Question

For first-row transition metals, why do the 4s orbitals fill up before the 3d orbitals? And why are electrons lost from 4s orbitals before 3d orbitals?

Answer 1

For scandium through zinc, the `4s` orbitals fill AFTER the `3d` orbitals, AND the `4s` electrons are lost before the `3d` electrons (last in, first out). See here for an explanation that does not depend on "half-filled subshells" for stability.

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See how the `3d` orbitals are lower in energy than the `4s` for the first-row transition metals here (Appendix B.9):

All the Aufbau Principle predicts is that electron orbitals are filled from lower energy to higher energy... whatever order that may entail.

The `4s` orbitals are higher in energy for these transition metals, so naturally they tend to fill LAST (especially for late transition metals, where `V_(3d) "<<" V_(4s)`), not first.

Hence, it makes sense that the `4s` electron is predicted to be ionized first (barring `3d-4s` interactions), at least for the first ionization.