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4s before 3d

Posted: Mon Oct 21, 2019 8:33 pm
by Rebecca Epner 4A
Can someone explain exactly why in some instances 4s come before 3d? And in what cases does this occur?

Re: 4s before 3d

Posted: Mon Oct 21, 2019 8:35 pm
by GFolk_1D
Hi here's a diagram of Aufbau's Principle, hope this helps clarify!

Re: 4s before 3d

Posted: Mon Oct 21, 2019 10:11 pm
by Abigail Sanders 1E
4s will come before 3d as it has a slightly lower electrostatic energy. This allows the subshell to be filled before 3d. Once 4s is full, then 3d has the lower energy so it fills with electrons.

Re: 4s before 3d

Posted: Mon Oct 21, 2019 10:16 pm
by Khushboo_3D
The reason for this is the difference in energies. According to Aufbau's principle, the 4s shell has a lower energy than the 3d shell and hence it gets filled first.

Re: 4s before 3d

Posted: Mon Oct 21, 2019 11:52 pm
by 805394719
The 4s orbital has lower energy than the 3d orbital when it is unoccupied by electrons. Because electrons always fill the lower energy level first, 4s orbital will be filled before 3d. After 4s orbital has been occupied by electrons, the 3d orbital will be filled which will shield the 4s electrons from the electrostatic interaction of the nucleus. When 3d orbital is filled, the 4s orbital has a higher energy so when an electron is pulled from the valence shell, the 4s orbital will lose an electron first, before 3d orbital does. Thus, when ionizing an element by removing an electron, the 4s orbital will lose electrons first.