When lithium vapours were filled in the discharge tube for anode rays experiment, the anode rays wer — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Origin and Nature of Anode Rays In a gaseous discharge tube experiment, when fast-moving cathode rays (electrons) collide with neutral gas atoms, they knock out one or more electrons: \[\ce{M (g) + e^- -> M^+ (g) + 2e^-}\] Since lithium vapor is filled in the discharge tube, the anode rays are composed of singly ionized lithium cations: \[\ce{Li^+}\] --- ### Step 2 - Determine the Number of Protons and Neutrons For the lithium ions, we are given: * Mass number, \(A = 7\) * Atomic number, \(Z = 3\) 1. **Number of protons (\(N_p\)):** \[N_p = Z = 3\] 2. **Number of neutrons (\(N_n\)):** \[N_n = A - Z = 7 - 3 = 4 \text{ neutrons}\] --- ### Step 3 - Determine the Number of Electrons A neutral lithium atom (\(\ce{Li}\)) contains \(3\) protons and \(3\) electrons. The particles of the anode ray are monovalent lithium cations (\(\ce{Li^+}\)) with a net charge of \(q = +1\), meaning each atom has lost exactly \(1\) electron: \[\text{Number of electrons } (N_e) = Z - q = 3 - 1 = 2 \text{ electrons}\] Thus, each individual particle of the anode ray contains **3 protons, 4 neutrons, and 2 electrons**. --- ### Step 4 - Systematic Analysis of the Options * **Option (A) 1 proton only:** Incorrect. This represents a hydrogen ion (\(\ce{H^+}\)). * **Option (B) 3 protons and 4 neutrons only:** Incorrect. This represents a bare lithium nucleus (\(\ce{Li^{3+}}\)), completely stripped of all electrons. * **Option (C) 3 protons, 4 neutrons and 2 electrons:** Correct. This corresponds exactly to the subatomic composition of \(\ce{Li^+}\). * **Option (D) 3 protons, 3 neutrons and 3 electrons:** Incorrect. This represents a neutral lithium-6 atom (\(\ce{^{6}_{3}Li}\)). \[\text{Correct Option: } \boxed{\text{C}}\]