Rutherford's -particle scattering experiment showed that almost all of an atom's mass and its entire positive charge sit in a tiny central nucleus. Nuclear size follows , where is the mass number and is the same constant for every nucleus. Since volume , and nuclear mass is also closely proportional to
Nuclei
Class 1210 previous-year questions from this chapter every option and the correct answer, free · where the marks are
1. Nuclear size, mass defect, binding energy
Exam focus: Binding energy per nucleon peaks near mass number 56 (iron) and falls on both sides — exactly why fusion of light nuclei and fission of heavy nuclei both release energy, moving toward that peak. Nuclear density comes out roughly the same for every nucleus.
Mass-Energy Equivalence, Mass Defect and Binding Energy
Physics Wallah - Alakh Pandey
Thorough coverage of mass defect, binding energy and nuclear stability; nuclear size/radius is covered in a separate, earlier video in the same series.
PhysicsNEET 2022show ▾A nucleus of mass number 189 splits into two nuclei having mass numbers 125 and 64. The ratio of the radii of the two daughter nuclei respectively is:
- A.25 : 16
- B.1 : 1
- C.4 : 5
- D.5 : 4✓
The worked solution is part of a paid pack.
PhysicsNEET 2021show ▾A nucleus with mass number 240 breaks into two fragments each of mass number 120. The binding energy per nucleon of the unfragmented nucleus is 7.6 MeV while that of the fragments is 8.5 MeV. The total gain in binding energy in the process is:
- A.0.9 MeV
- B.9.4 MeV
- C.804 MeV
- D.216 MeV✓
The worked solution is part of a paid pack.
PhysicsNEET 2020show ▾The energy equivalent of 0.5 g of a substance is:
2. Radioactivity and decay law
Exam focus: A radioactive decay constant is fixed and completely unaffected by temperature, pressure or chemical combination, unlike an ordinary reaction rate — so half-life stays exactly the same regardless of how much substance remains or what conditions it's kept in.
Law of Radioactive Decay and Half-Life
Physics Wallah - Alakh Pandey
Part 3 of the channel's radioactivity mini-series, focused specifically on the decay law/half-life math; general radioactivity basics and alpha/beta/gamma emission are in earlier parts of the same series, not included here.
PhysicsNEET 2024show ▾
In the nuclear emission stated above, the mass number and atomic number of the product , respectively, are:
- A.286, 80
- B.288, 82
- C.286, 81✓
- D.280, 81
Solution
An emission lowers by 4 and by 2: . Each emission leaves unchanged; a positron lowers by 1 and an electron raises it by 1. Net over : . So is .
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Save for spaced revision (free account)PhysicsNEET 2023show ▾The half-life of a radioactive substance is 20 minutes. In how much time does the activity of the substance drop to th of its initial value?
- A.20 minutes
- B.40 minutes
- C.60 minutes
- D.80 minutes✓
The worked solution is part of a paid pack.
PhysicsNEET 2022show ▾In the given nuclear reaction, the element X is:
PhysicsNEET 2021show ▾A radioactive nucleus undergoes spontaneous decay in the sequence
where is the atomic number of element X. The possible decay particles in the sequence are:
PhysicsNEET 2021show ▾The half-life of a radioactive nuclide is 100 hours. The fraction of the original activity that will remain after 150 hours would be:
PhysicsNEET 2019show ▾An -particle consists of:
- A.2 protons and 2 neutrons only✓
- B.2 electrons, 2 protons and 2 neutrons
- C.2 electrons and 4 protons only
- D.2 protons only
The worked solution is part of a paid pack.
3. Fission and fusion
Exam focus: Commercial nuclear power runs on fission, splitting heavy nuclei like uranium — controlled fusion, despite powering the Sun, isn't commercially viable because forcing light nuclei close enough to fuse means overcoming their Coulomb repulsion at extreme temperature and pressure.
Nuclear Fusion: Fusion in the Sun, and Fission vs Fusion
Physics Wallah - Alakh Pandey
Covers nuclear fusion, the Sun's fusion reactions, and explicitly contrasts fission vs fusion; fission itself is covered in more depth in the preceding video of the series.