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building up principle

Posted: Fri Oct 18, 2019 8:53 pm
Can someone please reexplain Aufbau's principle?

Re: building up principle

Posted: Fri Oct 18, 2019 9:00 pm
The Aufbau principle basically says that electrons fill the lowest energy levels before they move onto the next energy level.
Notice how every time we move down the period table (and into a new energy level) or even across the periodic table (and into a new orbital), the previous energy levels / orbitals are full. This shows how electrons first fill the lowest energy level and build up to the next, higher energy level.

Re: building up principle

Posted: Fri Oct 18, 2019 9:02 pm
It basically means that electrons fill orbitals in the lowest energies levels first before taking up higher energies. This is because atoms always want to be in the lowest energy state as possible.

Re: building up principle

Posted: Sat Oct 19, 2019 12:06 pm
Also, in orbitals where different configurations are present (ie. px, py, and pz for the p orbital), electrons will move to occupy each configuration and having parallel spin before pairing up within an orbital to maintain a lower energy state

Re: building up principle

Posted: Sat Oct 19, 2019 12:11 pm
Dr. Lavelle mentioned two exceptions in class to the Aufbau principle: Chromium and Copper. Could someone please explain what those exceptions are and why they are there?

Re: building up principle

Posted: Sat Oct 19, 2019 12:21 pm
Naji Sarsam 4G wrote:Dr. Lavelle mentioned two exceptions in class to the Aufbau principle: Chromium and Copper. Could someone please explain what those exceptions are and why they are there?

These exceptions are that these elements do not follow the normal filling order for typical electron configurations. This occurs because a completely full or half full d sub-level is more stable than a partially filled d sub-level.

You'd expect Cr to have an electron configuration of [Ar] 4s^2 3d^4 and Cu to have [Ar] 4s^2 3d^9, but they actually are [Ar] 4s^1 3d^5 and [Ar] 4s^1 3d^10.

In Chromium, an electron from the 4s orbital is excited and moves to produce a half full 3d orbital.
In Copper, an electron from the 4s orbital moves to produce a completely full 3d orbital.