[NewtoAtoms]

Hello,

Would the electron configeration of Sc2+ be

1.  1s²2s²2p⁶3s²3p⁶3d¹ following regular orbital fillings

OR

2. 1s²2s²2p⁶3s²3p⁶4s¹ because 4s orbitals are filled before 3 d orbitals.

Thank you!

[Yggdrasil]

1st point:  "Regular" orbital filling follows the Aufbau principle, so I would argue that the regular pattern of orbital filling is 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p, etc.

2nd point:  Quantum mechanics makes a lot of things complicated.  It turns out that the relative energies of the 4s and 3d orbitals depend on the charge of the atom being studied.  In neutral atoms, the energy of the 4s orbital is lower than the 3d orbital so electrons will fill the 4s orbital before they fill the 3d orbital.  The situation changes for charged species.

As I mentioned in a previous thread, the energy of the 4s and 3d orbitals are very close, so subtle effects (such as the stability gained from half-filled or fully-filled sets of orbitals) can alter the filling of the 3d and 4s orbitals.  As it turns out, the charge of the atom can have this effect.  Unlike neutral species, positively charged ions tend to fill their 3d orbitals before their 4s orbitals.

Why are the 3d orbitals lower in energy that the 4s orbitals in positively-charged species but not in neutrally-charged species?  In all atoms, the negatively-charged electrons are stabilized by electrostatic interactions with the positively-charged nucleus.  The strength of these interactions is dependent on the distance of the electron from the nucleus.  As electrons get farther away from the nucleus, the strength of the interaction gets weaker. 

In positively-charged ions, the effective strength of the nucelus' pull on the electrons is stronger than in neutral atoms.  This electrostatic interaction with the nucleus exerts a stabilizing effect on electrons, lowering their energy.  Since the strength of the interaction depends on distance, electrons that are closer experience a greater stabilization than electrons that are farther from the nucleus.  Because the principle quantum number of the 3d orbitals is lower, the electrons in the 3d orbitals are on average closer to the nucleus than electrons in 4s orbitals.  Therefore, in positively charged atoms, the increased effective nuclear charge stabilizes the 3d electrons more than 4s electrons, leading to the effect that the 3d orbitals now have a lower energy than the 4s orbital.

Summary:  For positively charged species, 3d orbitals tend to fill before 4s orbitals.  Consistent with this principle, Sc²⁺ has an electron configuration of [Ar]3d¹.

Caveat:  This is a general rule.  Rules always have exceptions because nothing in nature really ever follows very simple rules.  One such exception is Sc⁺, which has a ground state electron configuration of [Ar]4s¹3d¹.  Exceptions like these can only really be predicted and understood through detailed quantum mechanical calculations.


I hope this explanation is more helpful than confusing.