Final answer:
The isotope used to define the mole is Carbon-12 (12¹²C), which states that 12 grams of Carbon-12 contains one mole of the substance. The atomic mass of Carbon-12 is 12 amu, and its molar mass is also 12 g/mol.
Step-by-step explanation:
The isotope used to define the mole by stating that 12 grams of the isotope contains one mole of the substance is Carbon-12, 12¹²C. A carbon-12 atom has a mass number of 12, which means it contains six protons and six neutrons in its nucleus, surrounded by six electrons in a neutral state. The atomic mass of carbon-12 is exactly 12 atomic mass units (amu), and by definition, the molar mass of carbon-12 is also exactly 12 grams per mole (12 g/mol). As such, one mole of any substance is the amount that contains as many entities (atoms, molecules, ions, etc.) as there are in 12 grams of carbon-12, which is approximately 6.022 x 10²³ entities, known as Avogadro's number.