Electron shells and energy sublevels

Around the nucleus an electron does not follow a fixed path; moving at huge speed it forms a cloud-like region — in hydrogen the cloud is spherical, and the electron is most often found 0.53·10⁻¹⁰ m from the nucleus. The lower the electron’s energy, the closer it is to the nucleus. Electrons of similar energy form one energy shell (level); shells are marked n = 1, 2, 3 ... or K, L, M, N .... The maximum number of electrons in a shell is N = 2n²: K — 2, L — 8, M — 18, N — 32. The outer shell of main-group elements never holds more than 8 electrons (2 if the first shell is the outer one). For example, sodium has electrons arranged 2, 8, 1 and chlorine 2, 8, 7.