# Thread: Help with understanding the shells of an atom

1. So, I understand there is the S orbital, then the P orbital, then D, then F, but how do I know how many electrons are in each shell for any given atom? My chem teacher explained it, but it is difficult to understand him as he never goes back to re-explain. Also, hello, I am new here   2.

3. Originally Posted by phantomhive So, I understand there is the S orbital, then the P orbital, then D, then F, but how do I know how many electrons are in each shell for any given atom? My chem teacher explained it, but it is difficult to understand him as he never goes back to re-explain. Also, hello, I am new here For each shell (which I denote by the value of its principal quantum number, n) it goes like this:
1st shell (n=1): s only
2nd shell n=2): s, p
3rd shell (n=3): s, p, d
4th shell (n=4):s, p, d, f

and so on.

In any given shell there is only 1 s orbital, whereas p orbitals come in groups of 3, d orbitals in groups of 5 and f orbitals in groups of 7.

This pattern comes from the solutions to the wave equation, and represents the possible "standing wave" patterns that an electron (which is a quantum-mechanical "wave-particle") can have in an atom, analogous to the fiundamental (1s) and harmonics that you get when you twang a piano or violin string.

Only, with quantum theory the amplitude of the "vibration" of the "string" corresponds to the probability of finding the electron at that point in space.

Each orbital can accept 2 electrons, with opposed spin orientations, i.e. one "spin up" and one "spin down".

To work out the "electronic configuration", as it is called, for any given atom, you work by filling from the lowest energy (n=1) shell first until all the electrons are in. This is called the Aufbau Principle, named after the German word for building up. Suggest looking Aufbau Principle up on Wiki - there's a fairly good article about it.

P.S. There are a couple of wrinkles in it, in that the 4s starts to be filled before the 3d when you get to Potassium, but suggest we can talk about that separately if it proves to be necessary.  Bookmarks
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