Electron configuration is the arrangement of electrons around an atom’s nucleus in its ground state. Shells are designated by integers and contain orbitals, which are designated s, p, d, and f. Each shell can hold a maximum of 2n2 electrons. Lower energy shells and orbitals are filled before higher ones. Electron configuration is important in chemistry as it affects how elements react with each other.
Electron configuration usually refers to the arrangement of electrons around the nucleus of an atom in its ground state, the state in which all electrons in the atom exist at the lowest possible energy level. The different energy levels occupied by electrons are often referred to as shells surrounding the nucleus of the atom. Each shell is designated by an integer, starting with 1. The higher the shell number, the greater its distance from the nucleus of the atom. The electrons in each shell exist in regions called orbitals or subshells which are designated s, p, d and f.
Each electron shell can be occupied by no more than 2n2 electrons, where “n” stands for the shell number. The first shell, the one closest to the nucleus, will therefore contain only two electrons, the second eight, the third 18 and so on. Within a shell, each orbital can be occupied by no more than two electrons.
Each shell contains the same type of orbital found in the previous shell as well as a new type of orbital. The first shell contains only one s orbital, but the second shell contains one s orbital and three p orbitals; each of these p orbitals can hold two electrons, so the combined p orbitals within a shell can hold up to six electrons. The third shell has one s orbital, three p orbitals and five d orbitals. The seven f orbitals occur first in the fourth shell, which also contains one s orbital, three p orbitals, and five d orbitals. Orbitals beyond f orbitals exist but are rarely discussed.
An electron configuration graph shows the order in which orbitals are filled within shells. For example, the electron configuration for the element sodium is 1s2 2s2 2p6 3s1, which means that sodium’s 11 electrons are found in the first, second, and third electron shells. The s orbitals of the first and second shells each contain two electrons, and the p orbital of the second has six electrons. The s orbital of the third shell contains only one electron; its three p and five d orbitals are unoccupied.
When writing electron configuration notation, the superscript on the letter denoting an orbital type can never be higher than the maximum number of electrons that can occupy that orbital type. The superscripts for s, p, def will never be greater than 2, 6, 10 and 14, respectively.
Lower energy shells and orbitals are filled before those with a higher energy level. This does not mean, however, that one shell will be completely filled before electrons start occupying the next shell. A configuration graph shows that the 4s orbital will be occupied before the 3d orbitals. This happens because as the number of electrons increases, the electrons interact with each other and create conditions where the highest orbital is the lowest energy state for the next electron to occupy.
Understanding electron configuration is especially important for studying chemistry. This is because chemical reactions generally occur in the valence or outer shell electrons. The electron configuration of the valence shell provides important information about how each element reacts with the others.
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