Final answer:
We know that an electron from an iron atom is identical to an electron from a copper atom because their charge-to-mass ratio is the same, a finding from J.J. Thomson's studies of cathode rays, which established the electron as a universal subatomic particle.
Therefore, the correct answer is: option d) We know this because the charge-to-mass ratio for the rays emitted by iron and copper cathodes is the same.
Step-by-step explanation:
The evidence of charge-to-mass ratio was measured in experiments that involved deflecting cathode rays using magnetic and electric fields.
Since the mass-to-charge ratio of these cathode rays, which are composed of electrons, was found to be independent of the metal used for the cathode, it indicates that all electrons have the same properties, regardless of the originating element.
This discovery was crucial in demonstrating electrons as fundamental, subatomic constituents of all atoms.
When English physicist J.J. Thomson studied cathode rays, he performed precise measurements that allowed him to determine that the particles emitted were negatively charged and were indeed identical, which were later known as electrons.
Thomson's experiments indicated that these electrons were much less massive than the smallest known atoms and the same from any element, showcasing the universality of electrons in all forms of matter.