# Particle number

The number of particles ( symbol :) is an extensive physical quantity of the dimension number and describes the absolute number of particles in a system . The number of particles has the unit one and is directly proportional to the amount of substance . If the information relates only to a certain substance X of a mixture , its name or chemical formula is added as an index to the symbol for the number of particles and thus obtained . ${\ displaystyle N}$ ${\ displaystyle N _ {\ rm {X}}}$

Particle numbers can no longer be determined directly for macroscopic systems. Therefore, for the sake of simplicity, the amount of substance is given in its unit of mole instead . The number of particles in one mole is also called Avogadro's constant , which has had the following exact value since the redefinition of the International System of Units in 2019 : ${\ displaystyle N}$ ${\ displaystyle N _ {\ rm {A}}}$

${\ displaystyle N _ {\ rm {A}} = 6 {,} 022 \; 140 \; 76 \ cdot 10 ^ {23} \ \ mathrm {mol} ^ {- 1}}$

and with their help you can convert the number of particles and the amount of substance into each other: ${\ displaystyle N}$${\ displaystyle n}$

${\ displaystyle N = n \ cdot N _ {\ rm {A}}}$

## Individual evidence

1. DIN EN ISO 80000-1: 2013, sizes and units - general , chap. 3.8 and 5.
2. DIN EN ISO 80000-11: 2013, sizes and units - parameters of the dimension number , chap. 0.3.
3. CODATA Recommended Values. National Institute of Standards and Technology, accessed July 30, 2019 . Value for the Avogadro constant. The value is exact.