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Order to Follow When Removing Electrons
Vaishaalie_6729
#1 Posted : Thursday, June 17, 2021 5:25:39 AM
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According to the Aufbau Principle, electrons prefer to occupy the lowest available energy level. (Order of lowest to highest energy levels: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, etc.)

In the Classroom Companion, it says that the electron configuration for:

Co = [Ar] 4s^2 3d^7
Co2+ = [Ar] 3d^7

Electrons are removed from the highest energy level first. I am confused as to why two electrons were removed from the s subshell instead of the d subshell. According to the Aufbau principle, 4s is lower in energy than 3d. Also if two electrons were removed from the d-subshell, it would create the configuration: 4s^2 3d^5, where the d-subshell is half full (wouldn't we want that).

Thank you!
INSTR_Katrina_128
#2 Posted : Thursday, June 17, 2021 4:49:35 PM
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Hi Vaishaalie,

This one is very tricky!

You're right. When FILLING up the orbitals, the s orbital is at a lower energy level, and therefore gets filled up first.

However, when the orbitals are FILLED, the s orbitals is actually at a higher energy level (we're just supposed to "know this").

Therefore, it loses its electrons first and allows cobalt to become a 2+ ion.
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