Final answer:
Dalton's postulates that atoms are indivisible and that atoms of the same element are identical were disproved by Thomson's discovery of electrons and Rutherford's discovery of the nucleus and subatomic particles. Isotopes are a clear example of atoms of the same element having different masses. Correct option is a)
Step-by-step explanation:
Of Dalton's Atomic Theory postulates, Thomson and Rutherford's experiments showed that atoms are divisible, contradicting Dalton's idea that atoms are indivisible and indestructible. J.J. Thomson's cathode ray tube experiment led to the discovery of electrons, proving that atoms can be split into smaller subatomic particles. Ernest Rutherford's gold foil experiment further disproved Dalton's postulate by revealing a small, positively charged nucleus surrounded by electrons, indicating that the atom is not a solid, indivisible sphere as Dalton had presumed.
Additionally, isotopes contradict the Dalton's postulate that atoms of the same element are identical. It is now understood, based on later scientific evidence, that while atoms of the same element have the same chemical properties, they can have different masses, as seen in isotopes such as those of hydrogen or chlorine.
These discoveries were fundamental in moving from Dalton's Atomic Theory to the modern understanding of atomic structure, with subatomic particles such as protons, neutrons, and electrons, and the knowledge that atoms of the same element can have different masses.