Mini-Lesson

Nuclear & Quantum Physics

This mini-lesson covers Themes E.1 & E.3–E.5: the structure of the atom, radioactive decay and half-life, mass–energy (E = mc²) and binding energy, nuclear fission, and fusion & stars.

Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Watch for the HL flag on higher-level extensions. Press Start when you're ready.

E.1 · the atom

Structure of the atom

An atom has a tiny, dense nucleus of protons (charge +e) and neutrons, surrounded by electrons. A nuclide is written ^A_ZX, where Z = proton number and A = nucleon (mass) number.

Isotopes are nuclei of the same element (same Z) with different numbers of neutrons (different A).
Discrete emission and absorption spectra are evidence that atomic energy levels are quantised.

The strong nuclear force holds nucleons together over very short ranges, overcoming the electrostatic repulsion between the positively charged protons.

Quick check

?Two samples contain atoms with the same number of protons but different numbers of neutrons. What are they?

E.3 · radiation

Radioactive decay

Unstable nuclei emit radiation randomly and spontaneously:

Alpha (α): a helium nucleus (2p + 2n); highly ionising, stopped by paper.
Beta-minus (β⁻): a fast electron from a neutron → proton conversion; stopped by aluminium.
Gamma (γ): a high-energy photon; very penetrating, needs thick lead.

half-life T½the time for half the radioactive nuclei (or the activity) to decay

Decay is a random process — we cannot predict a single nucleus, only the statistical behaviour of many.

Calculate

#A sample starts with 80 g of a radioisotope of half-life 5.0 days. How much remains after 15 days?

g

Hint: 15 days = 3 half-lives; halve three times: 80 → 40 → 20 → 10.

Calculate

#What percentage of a radioactive sample remains after 4 half-lives?

%

Hint: fraction = (1/2)⁴ = 1/16 = 0.0625 → 6.25%.

Sort it

Which type of radiation?

Tap an item, then tap the group it belongs to.

α Alpha

β Beta-minus

γ Gamma

E.3–E.4 · mass-energy

Mass–energy & fission

Mass and energy are equivalent. A nucleus has slightly less mass than its separate nucleons — this mass defect Δm corresponds to the binding energy that holds it together:

E = mc² = Δm c²1 u of mass defect ↔ 931.5 MeV of energy

Fission: a heavy nucleus (e.g. uranium-235) absorbs a neutron and splits into two lighter nuclei plus neutrons, releasing energy and sustaining a chain reaction in a reactor.

Calculate

#Find the energy released if 3.0 × 10⁻³ kg of mass is converted to energy. Give it in units of 10¹⁴ J. (c = 3.0 × 10⁸ m s⁻¹.)

× 10¹⁴ J

Hint: E = mc² = 3.0e-3 × (3.0e8)² = 2.7e14 J → 2.7.

Calculate

#A nuclear reaction has a mass defect of 0.030 u. Find the energy released in MeV. (1 u ↔ 931.5 MeV.)

MeV

Hint: E = 0.030 × 931.5.

E.5 · stars

Fusion & stars

Fusion joins light nuclei into a heavier one, releasing energy — it powers the stars. In the Sun, hydrogen fuses to helium.

A star is in equilibrium: the inward pull of gravity is balanced by outward radiation and gas pressure from fusion.
Fusion in stars builds elements (nucleosynthesis) up to iron; heavier elements form in supernovae.

Fusion needs extreme temperature and pressure so nuclei can overcome their electrostatic repulsion — which is why it happens in stellar cores, not easily on Earth.

Quick check

?What is the fundamental difference between nuclear fusion and nuclear fission?

Match it

Match term to meaning

Tap a statement on the left, then its match on the right.

Statement

Answer

Recap

The big ideas to know

The atom: nucleus of protons + neutrons; isotopes share Z, differ in A

Decay: α (He nucleus) · β (electron) · γ (photon); random & spontaneous

Half-life: time for half the nuclei/activity to decay

Mass–energy: E = mc²; mass defect ↔ binding energy; 1 u ↔ 931.5 MeV

Fission & fusion: split heavy vs join light nuclei; fusion powers stars

That completes Nuclear & Quantum Physics for IB Diploma Physics SL. Press Finish to see your score.

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