The specific charge of cathode rays — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Definition of Specific Charge The **specific charge** is the charge-to-mass ratio of a particle: $$\text{Specific Charge} = \frac{e}{m}$$ For cathode rays (electrons): $\frac{e}{m_e} \approx 1.76 \times 10^{11}\text{ C/kg}$ ### Step 2 - J.J. Thomson's Experiment Showed Invariance Thomson (1897) measured $e/m_e$ of cathode rays under varying conditions: 1. Different gases (H₂, He, CO₂, air) inside the discharge tube 2. Different electrode materials (iron, copper, platinum) 3. Different potential differences across the electrodes In ALL cases, $e/m_e$ remained constant at $\approx 1.76 \times 10^{11}$ C/kg. This proved cathode rays consist of identical, fundamental particles (electrons) present in all matter. ### Step 3 - Analysis of Options * **Option (A) Incorrect:** $e/m$ is independent of gas type. (Note: Anode/canal rays DO depend on gas type — different gases give different $e/m$) * **Option (B) Incorrect:** $e/m$ is independent of electrode material — all metals emit identical electrons. * **Option (C) Incorrect:** Potential difference changes electron speed/KE but not their $e$ or $m$. * **Option (D) Correct:** $e/m$ of all electrons is the same constant $\approx 1.76 \times 10^{11}$ C/kg regardless of experimental conditions. $$\text{Correct Option: } \boxed{\text{D}}$$