Physics XII - Chapter 13: Nuclei
Solve Nuclei Class 12 MCQs. Practice nuclear size, binding energy, radioactivity, decay laws, fission, and fusion with important exam questions.
Quick Revision Box
- Atomic Nucleus: Central core containing protons and neutrons; diameter ~10⁻¹⁴ m.
- Mass Number (A): Total nucleons (protons + neutrons) in nucleus.
- Atomic Number (Z): Number of protons; determines chemical element.
- Neutron Number (N): N = A - Z; number of neutrons in nucleus.
- Isotopes: Same Z, different A; same element, different mass.
- Isobars: Same A, different Z; different elements with same mass number.
- Isotones: Same N, different Z; different elements with same neutron number.
- Nuclear Size: R = R₀A¹ᐟ³; R₀ ≈ 1.2 × 10⁻¹⁵ m; radius proportional to cube root of A.
- Nuclear Density: ~2.3 × 10¹⁷ kg/m³; extremely high and nearly constant for all nuclei.
- Mass Defect: Δm = [Zmₚ + (A-Z)mₙ] - M; difference between calculated and actual mass.
- Binding Energy: E_b = Δmc²; energy equivalent to mass defect.
- Binding Energy per Nucleon: E_b/A; measure of nuclear stability; peaks at iron-56.
- Nuclear Forces: Strong, short-range forces holding nucleons together; charge independent.
- Radioactivity: Spontaneous disintegration of unstable nuclei.
- Alpha Decay: Emission of α-particle (⁴He nucleus); Z decreases by 2, A by 4.
- Beta Decay: Emission of β-particle (electron or positron); Z changes by ±1.
- Gamma Decay: Emission of γ-rays; energy release without change in Z or A.
- Law of Radioactive Decay: N = N₀e^(-λt); exponential decrease with time.
- Decay Constant (λ): Probability of decay per unit time.
- Half-life (T₁/₂): T₁/₂ = 0.693/λ; time for half the nuclei to decay.
- Mean Life (τ): τ = 1/λ; average lifetime of radioactive nuclei.
- Nuclear Fission: Heavy nucleus splits into medium-mass nuclei; releases energy.
- Nuclear Fusion: Light nuclei combine to form heavier nucleus; releases energy.
- Chain Reaction: Self-sustaining process where neutrons from one fission trigger others.
- Critical Mass: Minimum mass needed to sustain chain reaction.
Basic Level Questions
Chapter Summary
Nuclei takes us deeper into the atom's heart, exploring the incredibly dense and powerful core that contains 99.9% of an atom's mass in less than a trillionth of its volume. This chapter reveals the astonishing world of the atomic nucleus - a realm where unimaginable densities, immense binding energies, and mysterious nuclear forces create both the stability of matter and the power that fuels stars.
We begin by understanding the nuclear family - protons and neutrons bound together by the strong nuclear force, one of nature's most powerful yet shortest-range interactions. The concept of mass defect reveals one of Einstein's most famous equations in action: E=mc², showing how the "missing mass" in nuclei is actually converted into the binding energy that holds nucleons together. The binding energy curve becomes our guide to nuclear stability, showing why iron sits at the peak of stability and why both lighter and heavier nuclei can release energy through fusion or fission.
The mysterious world of radioactivity unfolds, showing us how unstable nuclei seek stability through alpha, beta, and gamma emissions. The beautiful mathematics of exponential decay describes this natural process with elegant precision, allowing us to calculate half-lives that range from fractions of seconds to billions of years. Radioactive dating becomes our time machine, letting us determine the ages of archaeological artifacts and even the Earth itself.
Most dramatically, we explore the twin processes of nuclear fission and fusion - the splitting of heavy nuclei and the merging of light ones - both pathways to releasing the tremendous energy stored in atomic nuclei. From the controlled chain reactions in nuclear reactors to the stellar furnaces that power our sun, we see how humanity has learned to harness the same nuclear processes that have shaped our universe since its beginning. This chapter reminds us that the tiny atomic nucleus, though immeasurably small, contains forces of cosmic significance.