Neutron Discovery and Role - UNSOLVED PRACTICE SET

Q7. Who discovered the neutron and when? Describe the experiment that led to its discovery.

Q8. Why was the discovery of the neutron necessary? What problem in atomic structure could not be explained without neutrons?

Q9. Your teacher explains that a helium atom has 2 protons and 2 neutrons in its nucleus, but only 2 electrons outside. If protons are positively charged and electrons are negatively charged, why doesn't the nucleus fly apart due to repulsion between protons? What role do neutrons play here?

Q10. Compare a proton, neutron, and electron on the basis of: (i) charge, (ii) mass, and (iii) location in the atom.

Q11. The atomic mass of carbon is 12 u and its atomic number is 6. How many protons, neutrons, and electrons does a carbon atom have? Show your reasoning.

Q12. Why is the hydrogen-1 atom unique in not having a neutron? What does this tell us about the minimum requirements for a stable nucleus?

Q13. Describe Chadwick's experiment that led to the discovery of the neutron. Explain the observations, the conclusions drawn, and why the neutron was difficult to detect compared to the proton and electron. How did the discovery of the neutron complete the picture of atomic structure?

Q14. A student reads that the mass number of an element is the sum of protons and neutrons, while the atomic number is the number of protons. He wonders why we need to count neutrons at all if they have no charge.

(a) Explain why neutrons contribute to the mass of the atom.

(b) Explain the role of neutrons in nuclear stability.

(c) Calculate the number of neutrons in: ¹²C, ¹⁶O, ²³Na, and ¹⁹⁷Au.

(d) What would happen to an atom if all its neutrons were removed?

Q15. "The neutron is the unsung hero of the atomic nucleus — without it, the universe as we know it would not exist." Discuss this statement by explaining: (i) how neutrons prevent the nucleus from breaking apart due to proton-proton repulsion, (ii) how the neutron-proton ratio affects nuclear stability, (iii) the role of neutrons in nuclear reactions like fission, and (iv) why understanding neutrons is important for India's nuclear energy program.

Q16. Complete the following table for various atoms:

Table

Element Atomic Number (Z) Mass Number (A) Protons Neutrons Electrons

Hydrogen 1 1 ? ? ?

Carbon 6 12 ? ? ?

Oxygen 8 16 ? ? ?

Sodium 11 23 ? ? ?

Chlorine 17 35 ? ? ?

Uranium 92 238 ? ? ?

(a) Fill in all the missing values.

(b) For which element is the number of neutrons equal to the number of protons?

(c) Calculate the neutron-to-proton ratio for sodium and uranium.

(d) What trend do you observe in the neutron-to-proton ratio as the mass number increases?

Q17. In a nuclear reactor, neutrons are used to initiate and sustain the fission of uranium-235.

(a) Write the nuclear equation for the fission of U-235 when bombarded with a neutron.

(b) If one fission of U-235 releases 200 MeV of energy, calculate the energy released when 1 kg of U-235 undergoes complete fission.

(c) India's nuclear power plants (like Tarapur, Rajasthan, and Kudankulam) use this principle. If a 1000 MW reactor operates at 33% efficiency, calculate how many fissions per second are required.

(d) Why is the control of neutron speed important in a nuclear reactor?

Q18. A medical imaging technique called PET (Positron Emission Tomography) scan uses radioactive isotopes that emit positrons. When a positron meets an electron, they annihilate and produce gamma rays.

(a) Explain why the mass of the neutron is slightly greater than the mass of the proton. (Hint: Think about the quark composition)

(b) If the mass of a neutron is 1.008665 u and the mass of a proton is 1.007276 u, calculate the mass difference in kilograms.

(c) This mass difference is related to the energy released in nuclear reactions through Einstein's equation E = mc². Calculate the energy equivalent of this mass difference.

(d) Why is understanding neutron behaviour important for both nuclear medicine and nuclear power in India?