The symbol σ is a Greek letter, which is expressed in English as Sigma (capital σ, lowercase σ,) and transliterated in Chinese as Sigma, which is the18th Greek letter. Sigma is a statistical unit, which is used to measure the standard error of a total. In chemistry, it is also used to represent a valence bond, σ bond.

Both the с of Cyrillic alphabet and the S of Latin alphabet are derived from Sigma. Capital σ stands for the sum sign in mathematics and is a common symbol in mathematics. It is mainly used to find the sum of multiple items, which is expressed by ∑ (also called the sum sign). The symbol "σ" was first used by Euler (1707- 1783) in 1755. This symbol comes from the Greek prefix (addition), and "σ" is the capital of σ.

The use of σ

(1) * * * δ bond among valence bonds (δ bond)

The valence bond formed by the overlapping of two D orbitals is called delta bond, which can be abbreviated as "face to face".

The δ bond has only two nodes (the plane with zero electron cloud density). From the bond axis, there is no difference between the orbital symmetry of δ bond and the orbital symmetry of D orbit, and the Greek letter δ also comes from D orbit.

δ bond often appears in organometallic compounds, especially those formed by ruthenium, molybdenum and rhenium. The so-called "quadruple bond" usually refers to a σ bond, two π bonds and a δ bond.

② means charged: δ-means negatively charged and δ+means positively charged.

(3) Names of two functions in mathematics:

Kroneck delta function

dirac delta function