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Thus, an object's charge can be exactly 0 e, or exactly 1 e, −1 e, 2 e, etc., but not 1 / 2 e, or −3.8 e, etc. (There may be exceptions to this statement, depending on how "object" is defined; see below.) This is the reason for the terminology "elementary charge": it is meant to imply that it is an indivisible unit of charge.
The coulomb was originally defined, using the latter definition of the ampere, as 1 A × 1 s. [4] The 2019 redefinition of the ampere and other SI base units fixed the numerical value of the elementary charge when expressed in coulombs and therefore fixed the value of the coulomb when expressed as a multiple of the fundamental charge.
Electrons have an electric charge of −1.602 176 634 × 10 −19 coulombs, [80] which is used as a standard unit of charge for subatomic particles, and is also called the elementary charge. Within the limits of experimental accuracy, the electron charge is identical to the charge of a proton, but with the opposite sign. [ 83 ]
Charge number or valence [1] of an ion is the coefficient that, when multiplied by the elementary charge, gives the ion's charge. [2]For example, the charge on a chloride ion, , is , where e is the elementary charge.
Charge is quantized: it comes in integer multiples of individual small units called the elementary charge, e, about 1.602 × 10 −19 C, [1] which is the smallest charge that can exist freely. Particles called quarks have smaller charges, multiples of 1 / 3 e , but they are found only combined in particles that have a charge that is an ...
For phosphorus (element 15) as an example, the concise form is [Ne] 3s 2 3p 3. Here [Ne] refers to the core electrons which are the same as for the element neon (Ne), the last noble gas before phosphorus in the periodic table. The valence electrons (here 3s 2 3p 3) are written explicitly for all atoms.
Not many actual molecular applications of ′ exist; see, however, the seminal work [1] on the hydrogen molecule for an early application. In the great majority of computations of molecular wavefunctions the electronic problem is solved with the clamped nucleus Hamiltonian arising in the first step of the Born–Oppenheimer approximation .
Atoms are called "electron-deficient" when they have too few electrons as compared to their respective rules, or "hypervalent" when they have too many electrons. Since these compounds tend to be more reactive than compounds that obey their rule, electron counting is an important tool for identifying the reactivity of molecules.