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CBSE Class 12 · Physics

Nuclei

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Nuclei” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 8 concepts

1Atomic Masses and Composition of Nucleus

Nucleus composition: Z protons, A−Z neutrons as taught; mass number A. Atomic mass is a u-scale as framed. Composition is a count, not a Bohr orbit. One 12C as 6p 6n is enough.

Using A as the proton-count is a miss — that is Z.

Figure. A nucleus is Z protons plus N neutrons. For oxygen-16 both counts are 8, so A = 16. The mass number is that sum, not a third kind of particle.

How it works

  1. Copy Z and AThe given.
  2. Write Z protons and A−Z neutronsThe composition.
  3. Keep u as the mass-scale if askedHonest extra.

2Size of the Nucleus

Size: R = R0 A^{1/3} as taught. A nucleus is tiny next to the atom. Size is a cube-root story, not a Bohr radius.

A nucleus drawn as half the atom is a lying scale.

Figure. Nuclear radius grows as A to the one-third. The curve starts at the origin and reaches 7.2 fm at A = 216 when R0 is 1.2 fm. That is a volume pack, not a linear ruler.

How it works

  1. Copy AThe given.
  2. Form R0 A^{1/3}The size.
  3. Keep it tiny versus the atomThe caution.

3Mass-Energy and Nuclear Binding Energy

Mass–energy and binding: Δm c^2 as taught; BE is the leftover that holds the nucleus. BE per nucleon as a stability-hint if named. Binding is an energy-from-missing-mass, not a glue-slogan.

Using the whole atomic mass as Δm without a split is a miss.

Figure. Binding energy per nucleon rises sharply to helium, peaks near iron-56, then falls slowly toward uranium. Fusion pays below the peak; fission pays above it. The curve starts at the origin.

How it works

  1. Name the mass defectΔm.
  2. Form Δm c^2 as BEThe energy.
  3. Keep per-nucleon if askedThe hint.

4Nuclear Force

Nuclear force: short-range, charge-independent as taught, stronger than Coulomb inside the nucleus. Nuclear force is the hold, not gravity. One short-range sentence is enough.

A 1/r^2 gravity as the nuclear hold is a miss.

Figure. Nuclear potential is repulsive at very small r, then a short attractive well, then zero. The mid-axis is V = 0. Gravity’s long reach is not this curve.

How it works

  1. Name short-range and strong as taughtThe force.
  2. Keep Coulomb as the leftover push of protonsTwo forces.
  3. Refuse gravity as this headingA different size.

5Radioactivity

Radioactivity: a nucleus changes by α, β, or γ as taught. A decay-law N = N0 e^{−λt} if named; half-life as the time-to-half. Radioactivity is a spontaneous change, not a switch you flip.

A chemical burn as radioactivity is a miss.

Figure. Undecayed nuclei fall as N = N0 × 2 to the minus n half-lives. After one half-life the count is half, not zero. The curve starts at full count and never claims the origin.

How it works

  1. Name α, β, or γ as the modeThe decay.
  2. Keep half-life as the time-look if askedThe extra.
  3. Refuse a kitchen-change as this headingA nucleus-job.

6Nuclear Energy

Nuclear energy: fission (split) and fusion (join) as taught — BE-per-nucleon leftover as the energy-source. A reactor or a star-mention as framed, school-size, not a how-to. Energy is a BE-story, not a bomb-ad.

A chemical-fuel dump as nuclear energy is a miss.

Figure. One thermal neutron on uranium-235 can split to xenon-140 and strontium-94 plus two neutrons and energy Q. 235 + 1 = 140 + 94 + 2, so A is conserved while mass defect becomes Q.

How it works

  1. Name split or joinFission or fusion.
  2. Keep BE-per-nucleon as the whyThe leftover.
  3. Refuse a how-toSchool-size.

7A definition is a test you can run

Nucleus-word is a test: Z and A, R, BE, a decay-mode, or fission/fusion. If you only say “tiny centre”, you have a heading.

A glow-sticker is not the test.

Figure. Binding energy is the mass of the separate nucleons minus the mass of the nucleus, times c squared. The definition is that subtraction, not a memorised number.

How it works

  1. Name the count, the size, BE, or the modeThe object.
  2. Give the school sentenceThe test.
  3. Then the word has contentThe definition ran.

8Name the given before the unknown

The given is Z, A or a decay-name. The unknown is the neutron-count or the mode. Copy A=12, Z=6 before you write 12 protons.

Using Z as the neutron-count is a silent swap.

Figure. Write the given counts before the unknown. For oxygen-16, A = 16 and Z = 8 are given, so N = A − Z = 8. Naming N first is how the arithmetic starts in the wrong place.

How it works

  1. Copy Z and AThe given.
  2. Name N=A−Z or the decayThe unknown.
  3. Then writeGiven first.
For 12C, Z=6, the neutron count is
  1. 6
  2. 12
  3. 18

A−Z.

Notes

  • Mapped to the official NCERT chapter “Nuclei”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • N = A − Z
  • R = R0 A^{1/3}
  • BE = Δm c^2

Recap

Hold these pegs from the official chapter “Nuclei”. The wording is ExamMaster’s teaching, not a textbook recap.

Atomic Masses and Composition of Nucleus
Nucleus composition: Z protons, A−Z neutrons as taught; mass number A.
Size of the Nucleus
Size: R = R0 A^{1/3} as taught. A nucleus is tiny next to the atom. Size is a cube-root story, not a Bohr radius.
Mass-Energy and Nuclear Binding Energy
Mass–energy and binding: Δm c^2 as taught; BE is the leftover that holds the nucleus.
Nuclear Force
Nuclear force: short-range, charge-independent as taught, stronger than Coulomb inside the nucleus.
Radioactivity
Radioactivity: a nucleus changes by α, β, or γ as taught.
Nuclear Energy
Nuclear energy: fission (split) and fusion (join) as taught — BE-per-nucleon leftover as the energy-source.

Practise Nuclei

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  • A 4-question practice set that ends the chapter
  • 1 quick check with worked explanations
  • Timed mocks scored with the real marking scheme
  • Readiness tracked per topic, kept on your device
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