A mole is defined as the number of atoms in exactly 12 grams of which isotope?

• A. oxygen-16
• B. carbon-14
• C. hydrogen-1
• D. carbon-12

Answer: (D)

Verified step by step guidance

1. Understand the definition of a mole: A mole is the amount of substance that contains exactly the same number of entities (atoms, molecules, ions, etc.) as there are atoms in 12 grams of a specific isotope.
2. Identify the isotope used to define the mole: The mole is based on the number of atoms in exactly 12 grams of carbon-12, which is the isotope of carbon with 6 protons and 6 neutrons.
3. Recognize that carbon-12 is the standard for the mole, not carbon-14, oxygen-16, or hydrogen-1, even though these are common isotopes of their respective elements.
4. Recall that the number of atoms in 12 grams of carbon-12 is Avogadro's number, approximately \$6.022 \times 10^{23}$ atoms, which defines the mole.
5. Conclude that the mole is defined by the number of atoms in exactly 12 grams of carbon-12.