Which of the following is incorrect statement? — Atomic Structure Chemistry Question
Question
Which of the following is incorrect statement?
💡 Solution & Explanation
### Step 1 - Origin of Cathode Rays Cathode rays are streams of fast-moving electrons produced under high voltage in a gaseous discharge tube. The strong electric field at the negative electrode (cathode) accelerates electrons which are pulled out from the metal surface of the cathode. Thus, statement **(a)** is factually **correct**. --- ### Step 2 - Propagation of Cathode Rays In the absence of external electric or magnetic fields, cathode rays travel in straight lines. This was demonstrated by placing a solid object in the path of the rays, which cast a sharp shadow on the opposite glass wall. Thus, statement **(b)** is factually **correct**. --- ### Step 3 - Mass Comparison Between Anode and Cathode Rays * Cathode rays consist of electrons: $m_e \approx 9.109 \times 10^{-31}\text{ kg}$ * Anode rays consist of positively charged gaseous ions. The lightest possible particle in anode rays is a proton ($\ce{H^+}$): $m_p \approx 1.673 \times 10^{-27}\text{ kg}$ * Ratio: $\frac{m_p}{m_e} \approx 1836$ Since even the lightest positive ion is approximately $1836$ times heavier than an electron, anode rays are significantly **heavier** than cathode rays. Thus, statement **(c)** is factually **correct**. --- ### Step 4 - Origin of Anode Rays Unlike cathode rays, anode rays are **NOT** emitted from the metal surface of the positive electrode (the anode). Instead, they are produced in the gas space between the electrodes when high-speed electrons from the cathode collide with and ionize neutral gas atoms: $$\ce{M(g) + e^- -> M^+(g) + 2e^-}$$ These positive ions then fly toward the cathode and pass through holes (canals) in it, forming anode rays. Therefore, they originate from ionization of gas in the inter-electrode space, **not** from the anode itself. Thus, statement **(d)** is factually **incorrect**. --- ### Step 5 - Conclusion Since statement (d) is the only incorrect statement: $$\text{Correct Option: } \boxed{\text{D}}$$