Which British physicist discovered the electron?

• A. Rutherford
• B. Bohr
• C. Planck
• D. J. J. Thomson

Answer: (D)

The key idea is identifying who first showed that atoms contain smaller charged particles. J. J. Thomson, a British physicist, used cathode ray tube experiments to reveal that the rays could be deflected by electric and magnetic fields in a way that indicated the particles causing the rays were negatively charged and far lighter than atoms. From the measured charge-to-mass ratio, he concluded these particles—electrons—are fundamental components of atoms, existing inside atoms before their overall structure was known. This discovery led to the early atomic model ideas, including the plum pudding concept with electrons embedded in a positively charged substance. Rutherford later clarified that most of an atom’s mass is in a tiny nucleus, but his work didn’t discover the electron. Planck and Bohr contributed important quantum ideas about energy and atomic structure, not the initial discovery of the electron.