Electron configuration

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A representative element from the Periodic table showing the typical position of the atomic name, atomic symbol, atomic number, atomic weight, and electron configuration.

The electrons of atoms orbit at varying distances from the nucleus, which are known as shells or energy levels. Within each shell reside one or more distinct electron orbitals. Electron shells make up the electron configuration of an atom.

The configuration of electrons primarily determines an elements chemical reactivity, and particularly the outer-shell (or "valence") electrons. Because of this, the elements in the periodic table are arranged into rows (known as groups) based on how many shells of electrons an atom possesses, and into columns (known as periods) consistent with the number of electron in outer-shell.

Electron shells and subshells

Main Article: Electron shell

Main Article: Atomic orbital

An electron shell is a group of atomic orbitals with a principal quantum number (n) of the same value. The maximum number of electrons that can be held in each shell can be calculated using the formula 2n² where n is the shell number. Therefore, the first shell can hold only 2 electrons, the second 8, then 18, 25, 32, and 50.

Electron shells are made up of one or more electron subshells, or sublevels, which have two or more orbitals with the same angular momentum quantum number (l). These orbitals are identified by the letters s, p, d, f, g, h, i, etc. Each orbital can hold a maximum of 2 electons and is distinguished by its unique shape. For example, S orbitals are circular, whereas P orbitals are dumbell shaped.

These subshells correspond to the azimuthal quantum numbers (l-values) 0, 1, 2, 3, 4, 5, 6, etc. Each shell can hold up to 2, 6, 10, 14, 18, 22 and 26 electrons respectively, or 2(2l + 1) electrons in each subshell.

Notation

Periodic Table with abbreviated electron configuration

In the ground state of an atom (the condition in which it is ordinarily found), electrons are found to occupy the lowest-energy level possible, and enter into higher levels in order of increasing energy. For example, the first electron goes into the lowest level, the second into the next lowest, and so on. This is known as the Aufbau principle.

Chemists use a standard notation to describe atomic electron configurations. In this notation, a subshell is written in the form nxe, where n is the shell number, x is the subshell label and e is the number of electrons in the subshell. The periodic table at right illustrates the notation given to each element. These notations assume knowledge of the electron configuration in the lower energy levels.

Periodic table blocks

Main Article: Periodic table blocks

Because of the importance of the outermost shell, the different regions of the periodic table are sometimes referred to as periodic table blocks, named according to the sub-shell in which the "last" electron resides, e.g. the s-block, the p-block, the d-block, etc.