In atoms, the motion state of electrons is expressed by wave function, which is also called atomic orbit. Each orbit has a sub-layer orbit. For example, the first layer has only one sublayer, which is a 1s orbit and can accommodate two electrons; The second layer has two sublayers, namely 2s and 2p orbitals, while 2p orbitals have three degenerate orbitals, each of which can hold two electrons, so the second layer always holds eight electrons. The third layer has three sublayers, which are 3s, 3p, 3d and D degenerate orbitals, so it can accommodate 18 electrons. The rules are complicated and the details are troublesome. If you are a high school student, you don't need to know so much, it will be confusing. If you have the energy to study, you can look for inorganic chemistry textbooks in this university.