Nuclei
Class 12 · Physics
4 ideas in Class 12 Physics, explained in plain words, with games and quizzes to practise.
Nucleus: Composition and Size
Part of: Nucleus and Binding Energy
Nuclei pack Z protons and N neutrons, with mass number A as their sum. Isotopes share Z, isobars share A, and isotones share N. Radius grows as cube root of A, so every nucleus has nearly the same enormous density.
Why it matters. Dating rocks, fuelling reactors and reading spectra all start from A, Z, N. Mixing up the three families mislabels every nuclide.
Formulas
- Nucleons: A = Z + N
- Radius: R = 1.2 x A^(1/3) fm
- Nuclear density nearly same for all nuclei
Key words
- Mass number
- : Total nucleons A, equal to protons plus neutrons.
- Isotopes
- : Nuclides with equal protons but different neutrons.
- Isobars
- : Nuclides with equal mass number but different protons.
- Fermi
- : Length unit 10^-15 m for nuclear sizes.
Try it in two minutes
- Split A into Z protons and N neutrons.
- Take cube root of A times 1.2 fm for radius.
- Compare two nuclides by shared Z, A or N.
Try at home. Sort carbon-12, carbon-14 and nitrogen-14 into isotope or isobar pairs.
Mass-Energy and Binding Energy
Part of: Nucleus and Binding Energy
A bound nucleus weighs less than its separate nucleons; the missing mass times c squared is the binding energy. Per-nucleon binding peaks near iron, so both splitting heavy nuclei and fusing light ones release energy by climbing toward iron.
Why it matters. Reactors and stars both cash in mass difference. Forgetting the square, or crediting heavier products, reverses every energy answer.
Formulas
- Defect: dM = (Z mp + N mn) - M
- Binding: BE = dM x c^2
- 1 u = 931.5 MeV
Key words
- Mass defect
- : Missing mass between free nucleons and the bound nucleus.
- Binding energy
- : Energy equal to mass defect, holding the nucleus together.
- Atomic mass unit
- : Mass unit equal to 931.5 MeV of energy.
Try it in two minutes
- Add free nucleon masses and subtract the nucleus mass.
- Multiply the defect in u by 931.5 for MeV.
- Divide by A and compare with iron's peak.
Try at home. Rank helium, iron and uranium by stability from the curve.
Nuclear Force and Radioactivity
Part of: Radioactivity, Fission and Fusion
The nuclear force binds nucleons with short-range attraction that ignores charge but turns repulsive at the closest range. Unstable nuclei decay by alpha (helium nucleus), beta (electron from neutron change) or gamma (photon), halving each half-life with decay constant ln 2 over half-life.
Why it matters. Medicine, power and dating all use decay timing and radiation types. Mixing up which particle changes Z or A ruins every equation.
Formulas
- Survivors: N = N0 x (1/2)^(t/T)
- Decay constant: lambda = 0.693 / T
- Alpha: A falls 4, Z falls 2
Key words
- Half-life
- : Time for half of a radioactive sample to decay.
- Decay constant
- : Fraction decaying per unit time, equal to ln 2 over half-life.
- Becquerel
- : Unit of activity, one decay per second.
- Alpha particle
- : Helium nucleus ejected in heavy-nucleus decay.
Try it in two minutes
- Name the emitted particle for each decay.
- Track how Z and A shift in each case.
- Halve the sample once per half-life.
Try at home. Balance one alpha and one beta decay equation on paper.
Fission and Fusion Energy
Part of: Radioactivity, Fission and Fusion
Fission splits heavy nuclei with about 200 MeV per event, sustained by a neutron chain reaction tamed with moderators and control rods. Fusion joins light nuclei, releasing about 25.7 MeV per helium made, but needs stellar heat to beat Coulomb repulsion.
Why it matters. Power stations and the Sun run on these two paths. Confusing which input goes where, or the size of each release, breaks energy policy reasoning.
Formulas
- Fission Q about 200 MeV per uranium nucleus
- Sun's fusion Q about 25.7 MeV per helium
- Energy from lost mass: E = dM x c^2
Key words
- Fission
- : Splitting of a heavy nucleus into lighter ones with energy release.
- Fusion
- : Joining of light nuclei into a heavier one with energy release.
- Chain reaction
- : Neutrons from each fission triggering further fissions.
- Moderator
- : Material slowing neutrons to sustain fission efficiently.
Try it in two minutes
- Find the mass lost between sides of the reaction.
- Multiply by 931.5 MeV per u.
- Name the controller: rods absorb, moderator slows.
Try at home. Explain why fusion power on Earth is harder than fission power.
What your child will practise
- Fission and Fusion Energy: Desert Quick Fire (Quick Fire)
- Fission and Fusion Energy: Ocean Puzzle (Clue Puzzle)
- Mass-Energy and Binding Energy: Jungle Puzzle (Clue Puzzle)
- Nuclear Force and Radioactivity: Ocean Quick Fire (Quick Fire)
- Nucleus: Composition and Size: Space Quest (Flip and Match)
- Quiz: Fission and Fusion Energy (8 questions)
- Quiz: Mass-Energy and Binding Energy (8 questions)
- Quiz: Nuclear Force and Radioactivity (8 questions)
- Quiz: Nucleus: Composition and Size (8 questions)