# Simple Electron Configuration Question

1. Nov 26, 2013

### Patriciamsv

I have read from a few sources that an atom's third energy level can sometimes hold 8 electrons, and other times hold 18.

In atoms which possess enough electrons to exceed the the third energy level, what exactly is it that determines the amount of electrons this energy level can hold?

2. Nov 26, 2013

### Simon Bridge

The number of electrons in an energy level (the n quantum number) can hold is determined by the relationship between energy and angular momentum, and the Pauli exclusion principle. You have to model it with quantum mechanics.

The formula is $N_e=2n^2$

afaik: the n=3 state may have up to 18 electrons.
Do you have a reference for the 8 electron situation.
http://education.jlab.org/qa/electron_number.html

3. Nov 26, 2013

### Patriciamsv

Thank you for your helpful response. I have read of the 8 electron situation from GCSE and IGCSE textbooks, and as this is a quantum mechanical effect it is probable that it has been simplified for all young students.

4. Nov 26, 2013

### Simon Bridge

I suspect, then, that you will find that the GCSE/IGCSE texts are either referring to something other than the n=3 level, 8 electrons will fill up to the third subshell (=2p). or that there was a typo in the text. It is simply incorrect as you have stated it.

See, for a more complete picture:
http://en.wikipedia.org/wiki/Electron_shell