My name is Alexander FufaeV and here I write about:

The Electron Shell Model: Distribution of Electrons Inside Atoms Explained

In atomic physics, the Electron Shell Model describes the distribution of electrons on the »energy shells« of an atom in the periodic table of elements. For example, it can be used to understand the electron distribution of an oxygen atom. We can see from the periodic table that an oxygen atom has 8 electrons. But how are they distributed in the atom? The shell model answers this question.

Each shell represents a certain energy. The electrons can occupy these shells and thus take on this energy. Each shell can only hold a certain number of electrons. The electrons on the outermost shell of an atom are known as valence electrons and they play a crucial role in chemical reactions.

The shells are labeled with letters:

The innermost (first) shell is called the K shell. It can hold a maximum of 2 electrons. Then it is completely filled.
The second shell is called the L-shell. It can hold a maximum of 8 electrons.
The third shell is called the M shell. It can hold a maximum of 18 electrons.
The fourth shell is called the N-shell. It can hold a maximum of 32 electrons.

Electron Shell Model with K, L, M and N Shells

How do I know the maximum number of electrons a shell can hold? There is a formula for this. Let's denote $n_{\text{max}}$ as the maximum number of electrons that a shell can hold. You can then calculate this number using the following formula:

$$ \begin{align} n_{\text{max}} ~=~ 2 n^2 \end{align} $$

Here $n$ is the shell number. For the first shell, the K shell, $ n = 1$ applies. For the second shell, the L-shell, $ n = 2 $ applies. For the third shell, the M shell, $ n = 3 $ applies. And so on.

Filling the electron shells of different atoms

If you look at the periodic table, you can read the number of electrons from the atomic number in the corner. The hydrogen atom has an atomic number of 1, which means it only has one electron. We start with the innermost shell, the K shell, and add an electron to it. As the hydrogen atom has no other electrons, it only has a half-filled K-shell.

Electron Shells of Hydrogen

The lithium atom has the atomic number 3, which means it has 3 electrons. We start with the K shell. The K-shell can hold two electrons, so we fill it with two electrons. The third electron is placed on the next shell, the L shell.

Table : Electron shell of a lithium atom
Shell	Number of electrons
K Shell	2
L Shell	1

Electron Shells of Lithium

The oxygen atom has 8 electrons. Two are on the K shell. The L shell can hold a maximum of 8 electrons, so the remaining 6 electrons are placed on it.

Table : Electron shell of an oxygen atom
Shell	Number of electrons
K Shell	2
L Shell	6

Electron Shells of Oxygen

The Aluminum atom has 13 electrons. 2 electrons are added to the K-shell. 8 electrons are added to the L-shell, then the K-shell is full. The remaining 3 electrons are added to the M shell.

Table : Electron shell of an aliminium atom
Shell	Number of electrons
K Shell	2
L Shell	8
M Shell	3

Electron Shells of Aluminium