Final answer:
The amount of substance that has the same number of particles as in 12g of carbon-12 is called a mole and is defined as having 6.022 × 1023 particles, known as Avogadro's number.
Step-by-step explanation:
The amount of a substance that contains as many particles as there are atoms in exactly 12g of carbon-12 is known as a mole. A mole is defined in chemistry as containing exactly 6.022 × 1023 particles, which is a value referred to as Avogadro's number. This definition is crucial because it provides a direct link between the mass of an element or compound and the number of its constituent particles. When you have 12g of carbon-12, you have one mole of carbon atoms, and similarly, the molar mass of any substance in grams per mole is numerically equivalent to its atomic or formula weight in amu (atomic mass units).