Which of the following is used to represent Avogadro's number?  

N_A

The correct answer is option 1. N_A.

Avogadro's number, represented by the symbol \(N_A\), is a fundamental constant in chemistry that defines the number of atoms, molecules, or particles in one mole of a substance. It is named after Amedeo Avogadro, an Italian scientist who first proposed the concept in the early 19th century.

Avogadro's number is approximately \(6.022 \times 10^{23}\) entities per mole. This means that one mole of any substance contains Avogadro's number of constituent particles, whether they are atoms, molecules, ions, or other entities.

Avogadro's number is a crucial concept in stoichiometry, the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It allows chemists to perform calculations involving the mass, number of particles, and volume of substances involved in chemical reactions.

In summary, Avogadro's number is a fundamental constant used to relate the macroscopic properties of substances to the microscopic level of individual particles, providing a bridge between the atomic and macroscopic worlds in chemistry.