Which of the following is not a fundamental particle? — Atomic Structure Chemistry Question

💡 Solution & Explanation

### Step 1 - Fundamental Subatomic Particles The three classical fundamental subatomic particles that constitute atoms are: * **Electron** ($\ce{e^-}$): mass $\approx 9.1 \times 10^{-31}$ kg, charge $= -1.6 \times 10^{-19}$ C * **Proton** ($\ce{p^+}$): mass $\approx 1.672 \times 10^{-27}$ kg, charge $= +1.6 \times 10^{-19}$ C * **Neutron** ($\ce{n^0}$): mass $\approx 1.675 \times 10^{-27}$ kg, charge $= 0$ These are material building blocks with definite rest mass and definite charge. ### Step 2 - What are X-rays? X-rays are **electromagnetic radiation** — high-energy, short-wavelength photons: * No rest mass ($m_0 = 0$) * No electric charge * Travel at speed of light: $c \approx 3 \times 10^8$ m/s * Energy: $E = h\nu$ Being electromagnetic waves, NOT material particles, X-rays are NOT a fundamental particle. ### Step 3 - Evaluation of Options * **Option (A) Electron:** Fundamental subatomic particle. Incorrect answer. * **Option (B) Proton:** Fundamental subatomic particle. Incorrect answer. * **Option (C) Neutron:** Fundamental subatomic particle. Incorrect answer. * **Option (D) X-rays:** Electromagnetic radiation, not a particle. **Correct answer.** $$\text{Correct Option: } \boxed{\text{D}}$$