Proton–Electron Hypothesis | Explanation, Assumptions & Failures | Nuclear Physics

Описание: Proton–Electron Theory of the nucleus explained in detail. In this video, we discuss the basic assumptions, structure of nucleus, and major failures of proton–electron theory, including contradictions related to nuclear spin, Heisenberg uncertainty principle, and beta decay. This topic is important for Class 12 CBSE Physics, BSc, MSc, and competitive exams like IIT-JEE and NEET.

The Proton–Electron Theory of the nucleus was an early model proposed to explain the composition of the atomic nucleus before the discovery of the neutron. According to this theory, the nucleus consists of protons and electrons, where electrons are assumed to exist inside the nucleus to neutralize the positive charge of protons.

🔸 Basic Assumptions of Proton–Electron Theory
1. The nucleus is made up of protons and electrons.
2. The atomic number (Z) represents the number of protons present in the nucleus.
3. To account for the nuclear charge, it was assumed that (A − Z) electrons exist inside the nucleus.
4. The mass number (A) is equal to the total number of protons in the nucleus.
5. Nuclear charge = +Ze, due to Z protons.

🔸 Explanation Using an Example

For example, in a nitrogen nucleus (¹⁴₇N):
• Number of protons = 14
• Number of electrons inside nucleus = 7
• Net nuclear charge = +7e

This model was initially successful in explaining nuclear charge but failed to explain several experimental observations.

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🔸 Failures / Limitations of Proton–Electron Theory

1️⃣ Violation of Heisenberg Uncertainty Principle

If electrons were present inside the nucleus (radius ≈ 10⁻¹⁵ m), their position would be highly confined.
According to Heisenberg’s uncertainty principle, this would result in an extremely large uncertainty in momentum, giving electrons very high kinetic energy, which is not observed experimentally.

2️⃣ Nuclear Spin Problem

The theory fails to explain the observed nuclear spin of many nuclei.
Example:
• Nitrogen (¹⁴₇N) should have half-integral spin due to odd number of electrons.
• Experimentally, it has integral spin, contradicting the theory.

3️⃣ Nuclear Statistics Contradiction

Nuclei with even mass numbers should obey Fermi–Dirac statistics due to presence of electrons.
However, experimentally, such nuclei obey Bose–Einstein statistics, which contradicts proton–electron theory.

4️⃣ Continuous β-Ray Spectrum

According to proton–electron theory, electrons already exist in the nucleus and should be emitted with fixed energy during β-decay.
But experimentally, β-particles show a continuous energy spectrum, which cannot be explained by this theory.

5️⃣ Magnetic Moment Discrepancy

The magnetic moments calculated using proton–electron theory do not match with experimental values, indicating incorrect assumptions about nuclear structure.

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🔸 Conclusion

Due to these serious contradictions, the Proton–Electron Theory was rejected. The discovery of the neutron by Chadwick in 1932 led to the modern proton–neutron model of the nucleus, which successfully explains nuclear properties.

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🔹 Exam Tip (CBSE & University)

📌 Always mention Heisenberg Uncertainty Principle, nuclear spin contradiction, and beta decay while writing answers on this topic.

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