Fusion reactions are energetically favorable for light nuclei where combining them increases binding energy per nucleon, releasing energy. For two nuclei each with mass number A fusing to form 2A, this is possible only if the product nucleus at 2A has higher binding energy per nucleon than the reactants at A. Among the options, fusion is not possible for A=36.

Binding Energy Curve Basics

The binding energy per nucleon rises sharply from light nuclei (A<20) to a peak near iron (A≈56), making fusion exothermic up to roughly A≈25-30. Beyond this, the curve flattens, so fusion of two A>30 nuclei yields no net energy gain.​

Option Analysis

• A=15: Two nuclei (e.g., around ¹⁵N or ¹⁵O) fuse to A=30. Binding energy per nucleon at A=15 is ~7.5-8 MeV, rising to ~8.3 MeV at A=30—fusion releases energy.​

• B=22: Fuses to A=44. At A=22 (~7.8 MeV/nucleon), it rises to ~8.5 MeV at A=44—possible.​

• C=29: Fuses to A=58. At A=29 (~8.2 MeV), near-peak ~8.7 MeV at A=58 (close to ⁵⁶Fe)—still possible, though marginal.​

• D=36: Fuses to A=72. At A=36 (~8.5 MeV), it drops slightly to ~8.4 MeV at A=72—endothermic, not possible.​

Correct answer: (D) 36​

Article:

In nuclear physics, particularly for CSIR NET exams, understanding when a fusion reaction is NOT possible for mass number A hinges on the binding energy curve. This fusion reaction not possible mass number A question tests knowledge of nuclear stability: two nuclei of mass A fuse if the product (2A) has higher binding energy per nucleon (BE/A), releasing energy.

Binding Energy Curve Explains Fusion Limits

Light nuclei (low A) have low BE/A (~5-7 MeV), rising to ~8.8 MeV near ⁵⁶Fe. Fusion works for A<~30 as BE/A increases; beyond, it plateaus or drops—no energy gain.​

Detailed Breakdown: Options A=15, 22, 29, 36

For A=36, fusion absorbs energy—fusion reaction not possible mass number A=36.

CSIR NET Relevance

This aligns with syllabus on nuclear reactions. Fusion powers stars up to silicon (A~28), then stops. Practice with binding energy graphs for exams.​