From the discharge tube experiment, it is concluded that — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Overview of the Gaseous Discharge Tube Experiment The gaseous discharge tube experiment involved applying a very high voltage (around $10{,}000\text{ V}$) across two electrodes sealed inside a glass tube filled with gas at extremely low pressure. Under these conditions, a stream of rays emerged from the negative electrode (cathode) and traveled toward the positive electrode (anode). These rays were named **cathode rays**. --- ### Step 2 - Understanding the Universality of Cathode Rays By testing various setups, scientists discovered two critical facts about cathode rays: 1. They bent toward the positive plate in an electric field → composed of **negatively charged particles** (electrons). 2. The $e/m$ ratio remained completely constant: $$\frac{e}{m} \approx 1.76 \times 10^{11}\text{ C/kg}$$ This constant was independent of the cathode material and the gas used. Because electrons were produced from any substance, scientists concluded that electrons are a fundamental, universal subatomic constituent — **matter contains electrons**. --- ### Step 3 - Systematic Option Analysis * **Option (A) mass of proton is fractional:** Incorrect. The mass of a proton is approximately $1.007\text{ amu}$, which is not fractional. Moreover, this is not a conclusion of the discharge tube experiment. * **Option (B) matter contains electrons:** Correct. Since cathode rays (electrons) are emitted regardless of the gas or electrode materials, this experiment conclusively proved that electrons are a universal component of all matter. * **Option (C) matter contains nucleus:** Incorrect. The existence of a tiny, dense, positively charged nucleus was discovered by Rutherford in 1911 through his $\alpha$-particle scattering experiment, not the cathode ray experiments. * **Option (D) positive rays are heavier than protons:** Incorrect. While positive ions of gases other than hydrogen are heavier than a bare proton, this is a specific detail about anode ray composition, not the primary conclusion of the discharge tube experiment. $$\text{Correct Option: } \boxed{\text{B}}$$