In Wilson cloud chamber experiment, two particles were found to show equal deviations but in opposit — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Scientific Context of the Cloud Chamber Experiment In 1932, Carl David Anderson discovered the positron while studying cosmic rays using a Wilson cloud chamber. When charged particles pass through a cloud chamber in the presence of a magnetic field, they leave ionized tracks of water droplets. The trajectory of these tracks is curved due to the magnetic force acting on the moving charged particles. --- ### Step 2 - Physics of Magnetic Deflection (Lorentz Force) A charged particle of charge \(q\) and mass \(m\) moving with velocity \(\vec{v}\) in a perpendicular magnetic field \(\vec{B}\) experiences a magnetic force given by the Lorentz force equation: \[\vec{F} = q(\vec{v} \times \vec{B})\] This force acts as a centripetal force, causing the particle to move in a circular path of radius \(r\): \[\frac{m v^2}{r} = |q| v B \implies r = \frac{m v}{|q| B}\] According to the observations in the experiment: 1. **Equal deviations:** The radius of curvature \(r\) of both tracks is identical, meaning both particles have equal magnitudes of \(q/m\). 2. **Opposite directions:** The tracks curved in opposite directions, indicating opposite charge signs. --- ### Step 3 - Anderson's Nomenclature Since both particles have the same rest mass (\(m \approx 9.109 \times 10^{-31} \text{ kg}\)) and the same magnitude of charge (\(|q| \approx 1.602 \times 10^{-19} \text{ C}\)), but opposite charge signs: * The positively charged partner (\(+e\)) was named the **positron** (the "positive electron"). * The negatively charged partner (\(-e\)) was named the **negatron** (the "negative electron") to distinguish it from the positron. Therefore, "negatron" is simply a historical term for the ordinary, negatively charged **electron** (\(\ce{e^-}\)). --- ### Step 4 - Systematic Option Analysis * **Option (A) neutron:** Incorrect. A neutron (\(\ce{n^0}\)) is electrically neutral (\(q = 0\)) and would not be deflected in a cloud chamber. * **Option (B) neutrino:** Incorrect. A neutrino (\(\ce{\nu}\)) has no electric charge and produces no visible track. * **Option (C) proton:** Incorrect. Although a proton (\(\ce{p^+}\)) carries a \(+e\) charge, its mass (\(m_p \approx 1.673 \times 10^{-27} \text{ kg}\)) is approximately \(1836\) times greater than that of an electron. It would show a much smaller deviation. * **Option (D) electron:** Correct. The negatron corresponds directly to the ordinary negative electron (\(\ce{e^-}\)). \[\text{Correct Option: } \boxed{\text{D}}\]