 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 eight 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 two electrons. Then it is completely filled. The second shell is called the L-shell. It can hold a maximum of eight 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. How do I know the maximum number of electrons a shell can hold? There is a formula for this. Let's call the maximum number of electrons that a shell can hold as N-max. You can then calculate this number using the following formula. N-max is equal to 2N squared, where N is the shell number. For the first shell, the K-shell, N equals 1. For the second shell, the L-shell, N equals 2. For the third shell, the M-shell, N equals 3, and so on. Let's take a look at four examples. First, the hydrogen atom. 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 one. This 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. The lithium atom has the atomic number three. This means it has three 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 added to the next shell, the L-shell. The oxygen atom has eight electrons, two are on the K-shell. The L-shell can hold a maximum of eight electrons, so the remaining six electrons are added to it. The aluminum atom has 13 electrons. Two electrons are added to the K-shell. Eight electrons are added to the L-shell, then the K-shell is full. The remaining three electrons are added to the M-shell.