Electron shells

A very important component in how atoms behave is dictated by how their electron shells look. An electron shell could be said to be a shell on different heights above the nuclei, where a specific number of electrons may be placed.

The electron shell names

The electron shells are counted from the nuclei and outward. The innermost shell is called the K-shell, the next one is called the L-shell, thereafter comes the M-shell. It continues in the same manner until you reach the Q-shell, which is the outermost possible shell on any atom.

The outermost shell which contains electrons is called the valence shell. The electrons of this shell provide the atom with its particular characteristics.

The amount of electrons per shell

The amount of e- (electrons) which may maximally fit inside a particular shell can be calculated by this simple formula: \( \mathrm{ Amount \; of\; e^- = 2 \cdot n^2}\)
where n is the number of the electron shell. The K-shell, which is the innermost shell is number 1. Through the formula we conclude that a maximum of two electrons may fit in the K-shell. In the same manner, we can conclude that the L-shell contains a maximum of 8 electrons (n = 2), and the M-shell may contain 18 electrons (n = 3).

How electrons are placed into shells

Electrons are generally placed in the innermost shells before shells further out. This happens due to electrons having lower energy the closer they are to the nuclei.

Explanation of a term
When we use the term "how electrons are placed", we refer to a theoretical scenario where we would remove all electrons from the atom, and put them back one by one.

Electron configuration

An electron configuration (in this case based on the Bohr atomic model) describes in which shells the electrons of an atom are placed in. Below you can see an example:

20 p+ 2 8 8 2

The element Ca (calcium) contains 20 protons, and therefore has 20 electrons. First we fill the K-shell with 2 electrons. The K-shell is then full. The L-shell is filled with 8 electrons (and is thereafter full). The M-shell is filled with 8 electrons (but is not full!), and the N-shell (the outermost shell) contains 2 electrons.

A new example can be seen below:

30 p+ 2 8 18 2

Zn (zink) contains 30 electrons. We first fill the K- and L-shell completely. The M-shell is also completely filled. The N-shell (the outermost electron shell) contains 2 electrons.

Guidelines for how to fill electron shells

You can use the following points (based on Hund's rule of maximum multiplicity), from the top and down to get an idea of how the electron shells of an atom is filled:

(1) The K-shell is filled to 2 (full)
(2) The L-shell is filled to 8 (full)
(3) The M-shell is filled to 8 (not full)

This covers all atoms up until 18 electrons. Thereafter, the rules become less straightforward how the electron shells are filled.

(4) The N-shell is filled to approximately 2 electrons (not full)
(5) The M-shell is filled to 18 elektroner (thereafter full)
(6) The N-shell is filled to 8 elektroner (not full)

We will not go further in this article, but it continues in the same manner, where outer electron shells are slightly filled, and thereafter inner shells are filled.

Since the Bohr atomic model is a simplification of reality, it is hard to motivate why electron shell M and forward are filled in the way the are. This is better explained by orbitals, which is not included in this level of chemistry.

Read more about how the outermost electrons affect the properties of the atom in the article about valence electrons.


By adding energy, it is possible to excite an electron. The electron then jumps from its normal electron shell to an electron shell further away from the nuclei. Since the electron contains more energy than it normally does, it will eventually fall back into the electron shell with lower energy, emitting energy in the process.

Excitation can be described by energy levels:

Excitation of the electron in a hydrogen atom. Incoming energy (Eex) excites the electron, which jumps from the K-shell to the L-shell. The excited electron is however not stable, and will eventually fall back to the K-shell, emitting energy (Eem) in the process.

Excitation of electrons is the reason why metals produce colors in a flame test, where metals are placed in a flame.

The next article in the series is about valence electrons.

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