What is the basic unit of amount of substance in the metric system?

The basic unit of amount of substance in the metric system is the mole. The mole is a fundamental concept in chemistry that provides a way to quantify chemical substances. It allows chemists to count particles, such as atoms, molecules, or ions, in a given sample by relating the quantity of these particles to a macroscopic scale. One mole is defined as exactly (6.022 \times 10^{23}) entities, a value known as Avogadro's number.

This concept is crucial since it connects the microscopic world of atoms and molecules with the laboratory measurements we perform in chemistry, allowing for accurate calculations involving reactions and concentrations. Other units, such as grams and liters, are used to measure mass and volume, respectively, but they do not serve the same purpose as the mole, which specifically addresses the amount of substance.