General Rules for Predicting Electron Configurations
The following general rules are used when predicting the electron configurations for multi-electronA negatively charged, sub-atomic particle with charge of 1.602 x 10-19 coulombs and mass of9.109 x 1023 kilograms; electrons have both wave and particle properties; electrons occupy most of the volume of an atom but represent only a tiny fraction of an atom's mass. atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume.:
1 The Aufbauprinzip (building-up principle). The structure of an atom may be built up from that of the elementA substance containing only one kind of atom and that therefore cannot be broken down into component substances by chemical means. preceding it in the periodic system by adding one protonThe positively charged particle in an atomic nucleus; its mass is similar to the mass of a hydrogen atom. (and an appropriate number of neutrons) to the nucleusThe collection of protons and neutrons at the center of an atom that contains nearly all of the atoms's mass. and one extranuclear electron.
2 The order of filling orbitals. Each time an electron is added, it occupies the available subshell of lowest energyA system's capacity to do work.. The appropriate shell may be determined from a diagram such as Fig. 1a which arranges the subshells in order of increasing energy. Once a subshell becomes filled, the subshell of the next higher energy starts to fill.
3 The Pauli exclusion principleThe statement that no two electrons in an atom can have the same set of four quantum numbers; the principle leads to the rule that only two electrons (having opposite spin) can occupy an atomic orbital.. No more than two electrons can occupy a single orbital. When two electrons occupy the same orbital, they must be of opposite spin (an electron pair).
4 Hund’s rule. When electrons are added to a subshell where more than one orbital of the same energy is available, their spins remain parallel and they occupy different orbitals. Electron pairing does not occur until it is required by lack of another empty orbital in the subshell.
More information is found in the section on Electron Configurations