Science, Tech, Math › Science Principal Energy Level Definition Share Flipboard Email Print STEVE HORRELL / SPL / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 14, 2019 In chemistry, the principal energy level of an electron refers to the shell or orbital in which the electron is located relative to the atom's nucleus. This level is denoted by the principal quantum number n. The first element in a period of the periodic table introduces a new principal energy level. Energy Levels and the Atomic Model The concept of energy levels is one part of the atomic model that is based on a mathematical analysis of atomic spectra. Each electron in an atom has an energy signature that is determined by its relationship with other negatively charged electrons in the atom and the positively charged atomic nucleus. An electron can change energy levels, but only by steps or quanta, not continuous increments. The energy of an energy level increases the further out from the nucleus it is. The lower the number of a principal energy level, the closer together the electrons are to each other and to the nucleus of the atom. During chemical reactions, it's more difficult to remove an electron from a lower energy level than from a higher one. Rules of Principal Energy Levels A principal energy level may contain up to 2n2 electrons, with n being the number of each level. The first energy level can contain 2(1)2 or two electrons; the second can contain up to 2(2)2 or eight electrons; the third can contain up to 2(3)2 or 18 electrons, and so on. The first principal energy level has one sublevel that contains one orbital, called the s orbital. The s orbital can contain a maximum of two electrons. The next principal energy level contains one s orbital and three p orbitals. The set of three p orbitals can hold up to six electrons. Thus, the second principal energy level can hold up to eight electrons, two in the s orbital and six in the p orbital. The third principal energy level has one s orbital, three p orbitals, and five d orbitals, which can each hold up to 10 electrons. This allows for a maximum of 18 electrons. The fourth and higher levels have an f sublevel in addition to the s, p, and d orbitals. The f sublevel contains seven f orbitals, which can each hold up to 14 electrons. The total number of electrons in the fourth principal energy level is 32. Electron Notation The notation used to indicate the type of energy level and the number of electrons in that level has a coefficient for the number of the principal energy level, a letter for the sublevel, and a superscript for the number of electrons located in that sublevel. For example, the notation 4p3 indicates the fourth principal energy level, the p sublevel, and the presence of three electrons in the p sublevel. Writing out the number of electrons in all the energy levels and sublevels of an atom produces the electron configuration of the atom.