The mole is the SI base unit for 'amount of substance,' defined as containing exactly 6.022 × 10²³ elementary entities (atoms, molecules, ions) — this number is Avogadro's number (Nₐ).

One mole of any substance has a mass equal to its molar mass (in grams) — e.g., 1 mole of carbon-12 atoms weighs exactly 12 g, since carbon's atomic mass is 12 u.

• ›Mole concept connects the microscopic world (individual atoms/molecules) to macroscopic, measurable quantities (mass in grams) — this conversion is the foundation of virtually all quantitative chemistry calculations
• ›Avogadro's number was redefined as an exact value (6.02214076 × 10²³) in the 2019 SI redefinition, making it a defining constant rather than an experimentally measured one