Nuclei in 10 minutes: split the big, join the small
The whole chapter in one sitting: what a nucleus is made of, how big and how dense it is, where the missing mass goes, why uranium splits and why the sun shines. Five small experiments, one formula sheet, the top NEET traps and 8 quick questions.
NCERT Class 12 Physics Part II · Ch 13 Nuclei · §13.1–§13.6 · pp. 306–318 · about 10 min
Voice
About 6 minutes of teacher talk. The animations start by themselves as the teacher explains them.
The nucleus is tiny but holds almost all the mass. It weighs less than its parts, and that missing mass is binding energy. Heavy nuclei release energy by splitting (fission), light nuclei by joining (fusion).
Stage 1 · Why this chapter matters (1 min)
A sunrise, a power station and one tiny centre
Level
Every morning sunlight reaches your terrace from 1.5 × 10¹¹ m away, and every night a city runs on power from a nuclear plant. Both come from the same place: the nucleus, a centre about 10⁻¹⁵ m across that holds more than 99.9% of an atom's mass. This chapter explains what it is made of, how much it weighs, and how a tiny loss of mass becomes a huge amount of energy.
1–2questions from Nuclei in a usual NEET paper
4–8marks (+4 each, −1 for a wrong answer)
≈ 1 minper question: mostly one-line calculations
Teacher says
"This is one of the shortest chapters and one of the most scoring. Learn four formulas and the binding energy curve, and these marks are yours."
Stage 2 · Big idea 1 · Lesson 1
Inside the nucleus: protons, neutrons, Z, N and A
Level
Build a nucleus with the two sliders. Watch its symbol, its mass in atomic mass units (u) and its size change.
Takeaway
Nucleons = protons + neutrons: A = Z + N. Written as ᴬZX, e.g. ²³⁵₉₂U has 143 neutrons.
1 u = 1/12 of the mass of a ¹²C atom = 1.660539 × 10⁻²⁷ kg. Isotopes (same Z) explain fractional atomic masses.
The neutron (Chadwick, 1932) is neutral, with mass 1.00866 u, slightly more than a proton (1.00727 u).
The missing mass: binding energy and the famous curve
Level
A nucleus weighs less than its separate protons and neutrons. Drag along the curve (or use the slider) to read the binding energy per nucleon Ebn for any A.
Takeaway
Mass defect ΔM = [Zmp + (A − Z)mn] − M; binding energy Eb = ΔM c², with 1 u = 931.5 MeV/c².
Ebn ≈ 8 MeV for 30 < A < 170, maximum ≈ 8.75 MeV at A = 56, about 7.6 MeV for uranium.
Moving towards the peak releases energy: heavy nuclei split, light nuclei join.
Fusion in the sun, and balancing any nuclear reaction
Level
Pick a reaction and find the unknown particle X with the two sliders. Both balances must level: mass number on one, charge on the other.
Takeaway
Fusion: light nuclei join. They repel (Coulomb barrier), so it needs very high temperature: thermonuclear fusion. The sun's core is about 1.5 × 10⁷ K.
In the sun, 4 ¹H + 2e⁻ → ⁴He + 2ν + 6γ releases 26.7 MeV (the proton–proton cycle).
In every nuclear reaction the mass number A and the total charge are conserved. The number of protons alone is not: in ¹H + ¹H → ²H + e⁺ + ν a proton becomes a neutron.
"Three moves solve almost every numerical: cube root for size, times 931.5 for energy, and balance the top and bottom numbers for any reaction."
Stage 8 · Top traps in NEET
Where students lose these marks
Level
The trap
The fix
Mixing up isobars and isotones
isotoPe = same Proton number, isobAr = same A, isotoNe = same Neutron number.
"Twice the mass number, twice the radius"
R ∝ A^(1/3): to double R, A must become 8 times.
Heavier nucleus means denser nucleus
Nuclear density is the same for all A (ratio 1 : 1).
Mass defect with the wrong sign
ΔM = (mass of parts) − (mass of nucleus) > 0. The nucleus is lighter.
Multiplying kg by 931.5
931.5 MeV goes with mass in u. In kg use E = mc² and 1 MeV = 1.6 × 10⁻¹³ J.
Uranium has the largest Eb, so it is the most tightly bound
Tightness of binding is measured by Ebn (per nucleon). The peak is near A = 56 (iron).
"Protons and neutrons are each conserved"
Only A and total charge are conserved. In β⁺ or β⁻ processes a proton and a neutron change into each other.
Fusion needs heat to "melt" the nuclei
The high temperature gives nuclei enough kinetic energy to overcome their Coulomb repulsion.
Nuclear force depends on charge
It is charge independent and short range; only the Coulomb force acts between protons alone.
Teacher says
"Read the question twice: is it asking total binding energy or per nucleon, radius or volume, mass in u or in kilogram? Most wrong answers in this chapter are one of these swaps."
Stage 9 · Quick check
8 MCQs across the whole chapter
Level
Take R₀ = 1.2 fm and 1 u = 931.5 MeV. Write the formula first, then click.
Score: 0 / 8
Stage 10 · Your path
Five lessons, then the Chapter Challenge
Each lesson has its own cartoon movie, five experiments, worked problems and 12 MCQs. About an hour each.