The cathode rays experiment demonstrated that — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Origin and Nature of Cathode Rays During gas discharge tube experiments under high voltage and extremely low pressure, rays are emitted from the negative electrode (the cathode) and travel toward the positive electrode (the anode). These rays are known as **cathode rays**. --- ### Step 2 - Behavior of Cathode Rays in an Electric Field To investigate the electrical nature of these rays, J.J. Thomson subjected the cathode rays to a transverse electric field. When the rays passed between two oppositely charged plates, their path deflected significantly toward the **positively charged plate**. Since electrostatic attraction occurs between opposite charges: $$\text{Deflection toward Positive Plate} \implies \text{Negative Charge on Particles}$$ This observation directly demonstrated that cathode rays are streams of fast-moving, **negatively charged particles** (electrons). --- ### Step 3 - Consistency of the Charge-to-Mass ($e/m$) Ratio Thomson measured the specific charge of cathode ray particles: $$\frac{e}{m} \approx \boxed{1.76 \times 10^{11} \text{ C/kg}}$$ This value remained constant regardless of: 1. The nature of the gas filled inside the discharge tube. 2. The material of the cathode electrode. --- ### Step 4 - Systematic Analysis of the Options * **Option (A) $\alpha$-particles are the nuclei of $\ce{He}$ atoms:** Incorrect. This was discovered through nuclear physics studies, not cathode ray experiments. * **Option (B) the $e/m$ ratio for the particles of the cathode rays varies gas to gas:** Incorrect. The $e/m$ ratio for electrons is a universal constant. (It is the $e/m$ ratio of *anode rays* that varies from gas to gas.) * **Option (C) cathode rays are streams of negatively charged particles:** Correct. The deflection of the rays toward the positive plate conclusively demonstrated this. * **Option (D) the mass of an atom is essentially all contained in its very small nucleus:** Incorrect. This was the revolutionary conclusion of Rutherford's $\alpha$-particle gold foil scattering experiment in 1911. $$\text{Correct Option: } \boxed{\text{C}}$$