What is the Madelung rule?

As an expert in the field of chemistry, I will provide a comprehensive explanation of the Madelung rule, which is a fundamental principle in understanding the electron configuration of atoms. The Madelung rule, also known as the diagonal rule, is a set of guidelines that determine the order in which atomic subshells are filled with electrons. This rule is named after Erwin Madelung, a German physicist who first proposed it in 1904. It is also referred to as the Janet rule or the Klechkowsky rule in some countries, after Charles Janet or Vsevolod Klechkovsky, respectively. The rule states that electrons are added to the lowest energy orbitals first, and the order of filling is determined by the sum of the principal quantum number (n) and the azimuthal quantum number (l). The subshells are filled in the order of increasing (n + l) values. This is known as the "n + l" rule. To better understand the Madelung rule, let's take a closer look at the atomic subshells and their corresponding quantum numbers. The electron configuration of an atom is described by four quantum numbers: the principal quantum number (n), the azimuthal quantum number (l), the magnetic quantum number (m_l), and the spin quantum number (m_s). The principal quantum number (n) defines the energy level of the electron, with higher values indicating higher energy levels. The azimuthal quantum number (l) defines the shape of the orbital, with values ranging from 0 to n-1. Each value of l corresponds to a specific subshell, as follows: - l = 0 corresponds to the s subshell - l = 1 corresponds to the p subshell - l = 2 corresponds to the d subshell - l = 3 corresponds to the f subshell The Madelung rule dictates the order in which these subshells are filled. The subshells are filled in the order of increasing (n + l) values. For example, the 1s subshell is filled before the 2s subshell, which is filled before the 2p subshell, and so on. It is important to note that the Madelung rule is an approximation and does not always predict the exact order of electron filling. However, it provides a useful guideline for understanding the general trends in electron configuration. In summary, the Madelung rule is a fundamental principle in chemistry that helps us understand the order in which atomic subshells are filled with electrons. By following the "n + l" rule, we can predict the electron configuration of atoms and gain insights into their chemical properties and behavior.

The order in which subshells are filled is given by the n + ? rule, also known as the Madelung rule (after Erwin Madelung), or the Janet rule or the Klechkowsky rule (after Charles Janet or Vsevolod Klechkovsky in some, mostly French and Russian-speaking, countries), or the diagonal rule.