You cannot make generalizations like this! Jim Clark Chemguide. The Order of Filling Orbitals The aufbau principle explains how electrons fill low energy orbitals closer to the nucleus before they fill higher energy ones. Figure 1: Electronic energies orbitals. When d-block first row elements form ions, the 4s electrons are lost first. We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first.
We know that the 4s electrons are lost first during ionization. The electrons lost first will come from the highest energy level, furthest from the influence of the nucleus. So the 4s orbital must have a higher energy than the 3d orbitals. Those statements are directly opposed to each other and cannot both be right.
Where is the flaw in the logic? The Solution The elements up to argon: There is no problem with these. The general pattern that we drew in the diagram above works well. Potassium and calcium: The pattern is still working here. The 4s orbital has a lower energy than the 3d, and so fills next. That entirely fits with the chemistry of potassium and calcium. The d-block elements: For reasons which are too complicated to go into at this level, once you get to scandium, the energy of the 3d orbitals becomes slightly less than that of the 4s, and that remains true across the rest of the transition series hence, Figure 1 is incorrect as drawn.
Making Sc: Putting the final electron in, to make a neutral scandium atom, needs the same sort of discussion. Summary In each of these cases we have looked at, the 3d orbitals have the lowest energy, but as we add electrons, repulsion can push some of them out into the higher energy 4s level.
If you build up the scandium atom from scratch, the last electrons to go in are the two 4s electrons. These are the electrons in the highest energy level, and so it is logical that they will be removed first when the scandium forms ions.
And that's what happens. The 4s electrons are also clearly the outermost electrons, and so will largely define the radius of the atom. The lower energy 3d orbitals are inside them, and will contribute to the screening. There is no longer any conflict between these properties and the order of orbital filling. Now look at what happens when you add the next 5 electrons. Solution Because that is the structure in which the balance of repulsions and the size of the energy gap between the 3d and 4s orbitals happens to produce the lowest energy for the system.
Conclusion The current method of teaching students to work out electronic structures is fine as long as you realize that that is all it is - a way of working out the overall electronic structures, but not the order of filling.
You can say that for potassium and calcium, the 3d orbitals have a higher energy than the 4s, and so for these elements, the 4s levels fill before than the 3d. Post by sarahartzell1A » Fri Nov 02, pm. Laurence Lavelle Skip to content. Quick links. Email Link. Is electron removed from 3d or 4s first? How do you calculate the ideal gas law constant? How do you find density in the ideal gas law?
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