What is the principal quantum number () of the outermost shell from which electrons are universally — d and f Block Elements Chemistry Question

💡 Solution & Explanation

Even though the $(n-1)d$ subshell is filled last according to the Aufbau principle (being slightly higher in energy during filling), once the atom is formed, the $(n-1)d$ orbitals drop below the $ns$ orbitals in energy. Therefore, during ionization of a $3d$ transition metal, the outermost $4s$ electrons (where $n=4$) are always removed before the inner $3d$ electrons.