• The text in italics is copied from that url
• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
The aufbau principle, from the German Aufbauprinzip (building-up principle), also called the aufbau rule, states that in the ground state of an atom or ion, electrons fill subshells of the lowest available energy, then they fill subshells of higher energy.
Ofcourse you can claim such a principle or theory, but such theory should also be validated by means of observations.
If the physical observations are in agreement with the principle is true. But that is not all, you also have to measure the energy.
For example, the 1s subshell is filled before the 2s subshell is occupied. In this way, the electrons of an atom or ion form the most stable electron configuration possible.
Also the second sentence has to be validated by means of observations or experiment.
Electron behavior is elaborated by other principles of atomic physics,
IMO elaborated means: is described in more detail. But this should be backed up by observations. In fact you cannot have a principle which is in conflict with observations.
This sentence ends with:
such as Hund's rule and the Pauli exclusion principle.
OKay.
Hund's rule asserts that if multiple orbitals of the same energy are available, electrons will occupy different orbitals singly before any are occupied doubly.
Why is that?
Also here some observations or experiments must be performed to validate this claim.
If double occupation does occur, the Pauli exclusion principle requires that electrons that occupy the same orbital must have different spins (+1/2 and -1/2).
Why is that?
Also here some observations or experiments must be performed to validate this claim.
Passing from one element to another of the next higher atomic number, one proton and one electron are added each time to the neutral atom.
Okay.
Now a set of different rules follow:
1) The maximum number of electrons in any shell is 2n^2, where n is the principal quantum number.
This results in the following sequence 2, 8, 18, 32, for respectivily n=1, 2, 3 and 4.
This raises the issue what does this physical say.
2) The maximum number of electrons in a subshell (s, p, d, or f) is equal to 2(2l + 1) where l = 0, 1, 2, 3... Thus these subshells can have a maximum of 2, 6, 10, and 14 electrons respectively.

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Created: 2 March 2019

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