How does the existence of isotopes challenge Dalton's third postulate that all atoms of a given element are identical?

• A. Isotopes are different elements with similar chemical properties.
• B. Isotopes have the same number of protons but different numbers of neutrons.
• C. Isotopes have different numbers of protons and electrons in their atoms.
• D. Isotopes have identical mass numbers for all atoms of the element.

Answer: (B)