Answer :

Final answer:

The 4s subshell is filled before the 3d subshell as it has a lower energy. However, during ionization, 4s electrons are removed first due to their higher energy, induced by the shielding effect of the 3d subshell.

Explanation:

In atomic structure and electron configuration, energy levels of electrons are crucial. The 4s subshell is filled before the 3d subshell in many atoms because it has a lower energy. The energy of an electron in an atom is decided predominantly by the principal quantum number (n), but, there's also a slight dependency on the azimuthal quantum number (l). After the 3p and 4s subshells are filled, the 3d subshell can start to fill as its energy becomes lower than that of 4s. The precedence of 4s over 3d is reversed after the electron configuration moves past Argon.

When it comes to ionization, however, the outermost electrons are the ones that are removed first and these are mainly from the 4s subshell. Although the 4s connects energetically before 3d, once the 3d subshell starts to fill up, it has a greater shielding effect which raises the energy of the 4s electrons. Therefore, during ionization, electrons from 4s are removed before 3d because they are further from the nucleus and more easily removed.

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