ACS Inorganic Chemistry Practice Test

When a 3d series metal is ionized, the process typically involves the removal of electrons from the outermost orbitals first. For transition metals in the 3d series, the 4s orbital is filled before the 3d orbitals during the building-up process (Aufbau principle), but it is also higher in energy than the 3d orbitals when the metal is in its ionized state.

As a result, when ionization occurs, the first electron removed is from the 4s orbital, even though the 3d orbitals contain electrons. This is due to the fact that the 4s electrons are considered to be the valence electrons in these metals, making them the first to be lost when forming cations.

In contrast, if electrons were to be removed from the 3d orbital instead, it would not accurately reflect the behavior of transition metals, as the 4s electrons are at a higher energy level and are more readily removed compared to the more stable 3d electrons. Thus, the correct answer highlights the order of electron removal that is characteristic of these metal ions.