Free Sample | Nuclear Space

⚛️ 5.1.1 The Nuclear Model of the Atom

Core

Structure of the Atom

Particle	Location	Relative Charge	Relative Mass
Proton	Nucleus	+1	1
Neutron	Nucleus	0 (neutral)	1
Electron	Orbiting nucleus	–1	1/1836 ≈ 0 (negligible)

Proton charge = +1, neutron charge = 0, electron charge = −1. These three values are guaranteed marks every year on Paper 1/2.

Describe the structure of an atom in terms of a nucleus and electrons. State the relative charges of protons, neutrons and electrons. FREE MARKS

[4]

Reveal Answer

StructureAn atom has a small, positively charged nucleus at its centre, surrounded by negatively charged electrons orbiting at relatively large distances.

NucleusThe nucleus contains protons and neutrons (collectively called nucleons).

ChargesProton: +1 | Neutron: 0 | Electron: −1

Cambridge expects "small, positively charged nucleus." The word "small" is important — the atom is mostly empty space. Rutherford's gold foil experiment proved this.

Supplement

Rutherford's Alpha Scattering Experiment

Describe how the scattering of alpha particles by a thin gold foil provides evidence for the nuclear model of the atom. Give THREE pieces of evidence. Supplement High-Frequency Paper 4

[3]

Reveal Answer

Evidence 1Most alpha particles passed straight through → atom is mostly empty space (the nucleus is very small compared to the atom).

Evidence 2A small fraction were deflected at large angles → the nucleus contains most of the atom's mass (concentrated in a very small volume).

Evidence 3A very few bounced almost straight back → the nucleus is positively charged (repels the positive alpha particles).

Three separate ideas earn three separate marks: empty space, most mass in nucleus, positive nucleus. Don't combine them — write each on its own line.

🔬 5.1.2 The Nucleus — Nuclide Notation

Core

Nuclide Notation (MUST KNOW)

^A_ZX

A = nucleon number (mass number) = protons + neutrons | Z = proton number (atomic number) | X = chemical symbol

Number of neutrons = A − Z

Example: ¹²₆C has 6 protons, 6 neutrons, 6 electrons (neutral atom). Always calculate neutrons = A − Z.

For the nuclide ²³⁸₉₂U, state: (a) the proton number, (b) the nucleon number, (c) the number of neutrons, (d) the number of electrons in a neutral atom. Core FREE MARKS

[4]

Reveal Answer

(a)Proton number Z = 92

(b)Nucleon number A = 238

(c)Neutrons = A − Z = 238 − 92 = 146

(d)Electrons = protons = 92 (neutral atom has equal protons and electrons)

Isotopes

Define the term isotope. Give an example using nuclide notation. Core

[2]

Reveal Answer

Isotopes are atoms of the same element that have the same proton number (Z) but different nucleon numbers (A) — they have different numbers of neutrons.

Example¹²₆C and ¹⁴₆C — both have 6 protons but different neutron numbers (6 and 8).

Key phrase: "same proton number, different nucleon number." Without both parts, you lose a mark.

Supplement

Nuclear Fission and Fusion

Process	What happens	Mass change	Energy
Fission	A heavy nucleus splits into two smaller nuclei + neutrons	Small mass lost → converted to energy	Energy released
Fusion	Two light nuclei join together to form a heavier nucleus	Small mass lost → converted to energy	Large energy released

Nuclear fusion powers the Sun — hydrogen nuclei fuse to form helium. Research into controlled fusion for electricity generation is ongoing. Both facts appear regularly on Paper 4.

MCQ1

Which process occurs in the Sun to release energy?

📡 5.2.1 Detection of Radioactivity & Background Radiation

Core

Background radiation is ionising radiation that exists in the environment at all times, even without a radioactive source present.

Sources of Background Radiation

Source	Details
Radon gas	Naturally occurring radioactive gas in the air — largest source in many countries
Rocks and buildings	Granite and other rocks contain naturally radioactive minerals
Food and drink	Small amounts of radioactive materials (e.g. carbon-14, potassium-40)
Cosmic rays	High-energy radiation from space — higher at altitude (aircrew receive more)
Medical/industrial sources	X-rays, nuclear power stations (small contribution)

Always subtract background count before calculating activity of a source. Cambridge specifically states this requirement.

A detector records 80 counts per minute near a radioactive source. The background count rate is 20 counts per minute. Calculate the corrected count rate. Supplement

[1]

Reveal Answer

Corrected count rate = 80 − 20 = 60 counts per minute

Always subtract background radiation when calculating the true count rate from a source. This is tested on both Paper 4 and Paper 6.

☢️ 5.2.2 The Three Types of Nuclear Emission

Core

Property	Alpha (α)	Beta (β⁻)	Gamma (γ)
Nature	Helium nucleus: ⁴₂He (2 protons, 2 neutrons)	Fast electron from nucleus (neutron → proton + electron)	Electromagnetic wave (photon) — no mass, no charge
Charge	+2	−1	0
Mass	4 u (heaviest)	Negligible	0
Ionising ability	Strongest (dense — causes most ionisation)	Moderate	Weakest
Penetrating ability	Weakest (stopped by few cm of air or paper)	Moderate (stopped by few mm of aluminium)	Strongest (reduced by many cm of lead or metres of concrete)
Speed	Slowest (~0.05c)	Fast (~0.9c)	Speed of light (c)

Remember: Ionising ability and penetrating ability are INVERSELY related. Alpha is most ionising but least penetrating. Gamma is least ionising but most penetrating. This inverse relationship is a guaranteed mark.

Penetrating Power of Alpha, Beta and Gamma Radiation

Behaviour in Electric and Magnetic Fields

Supplement

Radiation	In Electric Field	In Magnetic Field
Alpha (α, charge +2)	Deflected toward negative plate	Deflected (large radius — heavy)
Beta (β⁻, charge −1)	Deflected toward positive plate — opposite to alpha	Deflected (small radius — light, fast); opposite direction to alpha
Gamma (γ, charge 0)	Not deflected	Not deflected

Explain why alpha radiation has the greatest ionising ability but the least penetrating power. Supplement High-Frequency

[3]

Reveal Answer

IonisingAlpha particles carry a charge of +2 and have relatively large mass and low speed. They interact strongly with atoms in their path, removing electrons and ionising many atoms per unit length — hence strongest ionising ability.

PenetratingBecause alpha particles lose energy rapidly through all this ionisation, they quickly run out of energy and stop. This makes them least penetrating.

Strong ionising → rapid energy loss → short range → least penetrating.

The key link is: more ionisation per cm → faster energy loss → shorter range. Cambridge expects this causal chain.

MCQ2

Which radiation is stopped by a few centimetres of air or a thin sheet of paper?

💥 5.2.3 Radioactive Decay Equations

Core

Radioactive decay is spontaneous (not triggered by anything external) and random (cannot predict which nucleus decays next or when).

During alpha or beta decay, the nucleus changes to a different element (proton number changes).

Supplement

Decay Equations — Rules

Decay type	Change to A	Change to Z	Particle emitted
Alpha decay	A decreases by 4	Z decreases by 2	⁴₂He (alpha particle)
Beta decay	A unchanged	Z increases by 1	⁰₋₁e (beta particle / electron)
Gamma emission	A unchanged	Z unchanged	γ (photon — no mass or charge)

Conservation Rules for Decay Equations

Top numbers (A) balance | Bottom numbers (Z) balance

Total nucleon number before = Total nucleon number after | Total proton number before = Total proton number after

Write a balanced nuclear equation for the alpha decay of radium-226 (²²⁶₈₈Ra). Identify the daughter nucleus. Supplement High-Frequency Paper 4

[3]

Reveal Answer

Equation²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He

Check A226 = 222 + 4 ✓

Check Z88 = 86 + 2 ✓

Daughter nucleus: Radon-222 (²²²₈₆Rn)

Alpha decay: A drops by 4, Z drops by 2. Always check both top and bottom numbers balance. Alpha particle is always ⁴₂He.

Do NOT write alpha as ⁴₂α. It must be written as ⁴₂He to show it is a helium nucleus. Cambridge is strict about this.

Write a balanced nuclear equation for the beta decay of carbon-14 (¹⁴₆C). Supplement

[3]

Reveal Answer

Equation¹⁴₆C → ¹⁴₇N + ⁰₋₁e

Check A14 = 14 + 0 ✓

Check Z6 = 7 + (−1) ✓

Carbon-14 → Nitrogen-14 + beta particle

Nuclear changeIn β decay: neutron → proton + electron (the electron is emitted as the beta particle)

Beta decay: A stays the same, Z increases by 1. The beta particle is ⁰₋₁e — note the bottom number is −1 (not 0).

⏳ 5.2.4 Half-Life

Core

Definition of Half-Life (MUST KNOW Word-Perfect)

t₁/₂ = time for half the nuclei of that isotope in any sample to decay

Equivalently: time for the count rate / activity to halve

Radioactive Decay Curve — Activity vs Time

Each half-life: activity halves. After n half-lives: activity = A₀ × (½)ⁿ. The curve is exponential — never reaches zero.

A radioactive source has an initial activity of 6400 counts/min. Its half-life is 3 hours. Calculate the activity after 12 hours. Core FREE MARKS

[3]

Reveal Answer

Step 1Number of half-lives = 12 hours / 3 hours = 4 half-lives

Step 2After each half-life, activity halves: 6400 → 3200 → 1600 → 800 → 400

Activity after 12 hours = 400 counts/min

Always divide total time by the half-life to find the number of half-lives. Then halve the activity that many times. Never use a formula — the step-by-step halving method is clearest and earns all marks.

Q10

The count rate from a source falls from 800 counts/s to 100 counts/s in 24 hours. Calculate the half-life of the source. Supplement High-Frequency Paper 4

[3]

Reveal Answer

Step 1800 → 400 → 200 → 100. That is 3 halvings = 3 half-lives.

Step 2Total time = 24 hours for 3 half-lives.

Step 3Half-life = 24 / 3

Half-life = 8 hours

Count how many times you halve to get from initial to final value — that's the number of half-lives. Then divide total time by that number.

Applications of Radioisotopes

Supplement

Application	Isotope type	Why this half-life / radiation?
Smoke alarms	Alpha, short half-life (Am-241: 432 years)	Alpha ionises air between plates; long enough half-life to last years without replacing
Cancer treatment	Gamma, appropriate half-life	Gamma penetrates tissue to reach tumour; decays to safe level after treatment
Medical tracers	Gamma, short half-life (hours–days)	Detected outside body; short half-life minimises patient's radiation dose
Food irradiation	Gamma	Penetrates food to kill bacteria without making food radioactive
Thickness gauges	Beta (paper/metal sheets)	Penetrates material to correct depth; reading varies with thickness
Sterilisation (equipment)	Gamma	Penetrates packaging to kill all microorganisms

MCQ3

A medical radioactive tracer should have which combination of properties?

🦺 5.2.5 Safety Precautions

Core

Ionising radiation damages living cells — it can cause cell death, mutations (leading to cancer), and at high doses, radiation sickness.

Safe Handling of Radioactive Sources

Precaution	Reason
Use long tongs / remote handling	Increases distance from source → reduces dose (intensity ∝ 1/d²)
Minimise time near source	Less exposure time → less total dose received
Use lead containers for storage	Lead absorbs radiation — reduces exposure when not in use
Lead/concrete shielding	Absorbs gamma radiation particularly
Do not point source at people	Reduces direct irradiation
Wear dosimeters (film badges)	Monitor total dose received

Supplement

Q11

State THREE precautions for handling radioactive materials and explain each in terms of reducing radiation exposure. Supplement FREE MARKS

[6]

Reveal Answer

1. DistanceUse long tongs to hold source. Increasing distance greatly reduces intensity of radiation received (inverse square law — intensity ∝ 1/d²).

2. TimeMinimise time spent near the source. Less time → less total energy absorbed by body tissue → smaller dose.

3. ShieldingUse lead containers or screens. Lead absorbs radiation (especially gamma) before it reaches the body.

6 marks: 1 for each precaution (name/action) + 1 for each explanation. All three explanations must reference reducing EXPOSURE or DOSE.

🌌 Topic 6: Space Physics

🪐 6.1 The Earth and the Solar System

Core

Key Orbital Facts

Object	Orbital period (approx.)	Notes
Earth rotates on axis	24 hours (1 day)	Causes apparent daily motion of Sun; day and night cycle
Earth orbits Sun	365 days (1 year)	Causes seasons (tilted axis); tilted axis ≠ changing distance
Moon orbits Earth	≈ 1 month (27.3 days)	Causes Moon's phases (cycle of appearance)

The Solar System — Structure

Component	Details
The Sun	1 star — contains most of the mass of the Solar System → planets orbit the Sun due to its gravity
8 Planets (inner to outer)	Mercury, Venus, Earth, Mars (rocky, small) → Jupiter, Saturn, Uranus, Neptune (gaseous, large)
Moons	Natural satellites orbiting planets
Dwarf planets	E.g. Pluto — in the Kuiper Belt
Asteroids	Rocky minor planets, mostly in asteroid belt between Mars and Jupiter
Comets	Ice and dust — highly elliptical orbits; travel faster when closer to the Sun

Planet order: My Very Educated Mother Just Served Us Noodles — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. First 4 rocky/small; last 4 gaseous/large.

Q12

Explain why the four inner planets (Mercury, Venus, Earth, Mars) are small and rocky while the four outer planets are large and gaseous. Reference the accretion model. Core

[3]

Reveal Answer

AccretionThe Solar System formed from a rotating cloud (nebula) of gas and dust. Gravity caused material to clump together (accretion) and form a disc.

Inner planetsClose to the hot young Sun — light gases were blown away by solar wind. Only heavy rocky materials remained to form small, rocky planets.

Outer planetsFar from the Sun — cold enough for light gases (hydrogen, helium) to condense and remain. These planets accumulated vast amounts of gas, becoming large and gaseous.

Supplement

Average Orbital Speed

v = 2πr / T

v = orbital speed (m/s) | r = average orbital radius (m) | T = orbital period (s)

Q13

Earth's average orbital radius is 1.5 × 10¹¹ m and orbital period is 3.15 × 10⁷ s. Calculate Earth's average orbital speed. Supplement

[3]

Reveal Answer

Formulav = 2πr / T

Substitutev = 2π × (1.5 × 10¹¹) / (3.15 × 10⁷)

Calculatev = (9.42 × 10¹¹) / (3.15 × 10⁷)

v ≈ 2.99 × 10⁴ m/s ≈ 30 000 m/s = 30 km/s

Always use radian measure (2π, not 360°) in this formula. The period must be in seconds.

MCQ4

Why does a comet travel faster when it is closer to the Sun?

⭐ 6.2.1 & 6.2.2 The Sun as a Star — Stellar Life Cycle

Core

The Sun is a star of medium size, consisting mostly of hydrogen and helium. It radiates energy mainly in the infrared, visible and ultraviolet regions of the EM spectrum.

Supplement

Stellar Life Cycle — Flowchart

Life Cycle of Stars — from Nebula to Final State

Q14

Describe how a protostar becomes a stable main sequence star. Explain what maintains this stability. Supplement High-Frequency

[4]

Reveal Answer

FormationAn interstellar cloud of gas and dust collapses under gravity. As it collapses, gravitational potential energy converts to kinetic energy — the protostar heats up.

Nuclear fusion beginsWhen the core reaches sufficient temperature and pressure, hydrogen nuclei fuse to form helium — releasing large amounts of energy.

StabilityThe star becomes stable when the inward force of gravitational attraction is balanced by the outward pressure from the high-temperature fusion reactions in the core.

Stable star = gravitational force inward balanced by radiation pressure outward from fusion.

This balance is the key idea. Cambridge expects both forces named and the concept of balance. "Equilibrium between gravity and fusion pressure" is the examiner's expected phrasing.

🌌 6.2.3 The Universe — Redshift & Big Bang

Core

Key Facts about the Universe

Fact	Detail
The Milky Way	Our galaxy — one of many billions of galaxies in the Universe; diameter ≈ 100 000 light-years
Light-year	Distance light travels in one year in vacuum ≈ 9.5 × 10¹⁵ m (Supplement)
Our Sun's position	The Sun is a star in the Milky Way; other Milky Way stars are much further from Earth than the Sun

Redshift — Evidence for Expanding Universe

Q15

Describe what is meant by redshift and explain how it provides evidence that the Universe is expanding. Core High-Frequency Paper 4

[4]

Reveal Answer

DefineRedshift is an increase in the observed wavelength of electromagnetic radiation (light) emitted from distant galaxies — the light is shifted toward the red (longer wavelength) end of the spectrum.

CauseRedshift occurs because galaxies are moving away from us — the Doppler effect causes the wavelength of light to stretch as the source recedes.

EvidenceLight from distant galaxies is redshifted — this means those galaxies are moving away from us. Since all distant galaxies show redshift in all directions, the entire Universe must be expanding.

Big BangThe expansion of the Universe supports the Big Bang Theory — the idea that the Universe began from a single point about 13.8 billion years ago.

Cambridge always asks for: (1) definition of redshift, (2) what causes it (galaxies moving away), (3) what it tells us (Universe expanding), (4) link to Big Bang. All four points earn four marks.

Supplement

The Hubble Constant & Age of the Universe

Hubble Constant

H₀ = v / d

H₀ = Hubble constant | v = recessional speed of galaxy (m/s) | d = distance to galaxy (m)

Current estimate: H₀ = 2.2 × 10⁻¹⁸ s⁻¹ | Age of Universe ≈ 1/H₀

Q16

A galaxy is moving away from Earth at 2.2 × 10⁶ m/s. H₀ = 2.2 × 10⁻¹⁸ s⁻¹. Calculate: (a) the distance to the galaxy, (b) an estimate for the age of the Universe. Supplement A* Level

[4]

Reveal Answer

(a) Formulad = v / H₀ = (2.2 × 10⁶) / (2.2 × 10⁻¹⁸)

(a) d = 1.0 × 10²⁴ m

(b) FormulaAge ≈ 1/H₀ = 1 / (2.2 × 10⁻¹⁸)

(b) Age ≈ 4.5 × 10¹⁷ s ≈ 14 billion years

Age = 1/H₀ represents the time since the Big Bang if expansion has been at a constant rate. Cambridge expects this formula and a numerical answer.

Supplement

Cosmic Microwave Background Radiation (CMBR)

Q17

Describe what the cosmic microwave background radiation (CMBR) is and explain how it supports the Big Bang Theory. Supplement

[3]

Reveal Answer

What it isCMBR is microwave radiation of a specific frequency observed uniformly at all points in space around us in all directions.

OriginIt was produced shortly after the Big Bang — the Universe was originally very hot and dense, producing high-energy radiation. As the Universe expanded and cooled, this radiation was stretched to microwave wavelengths.

EvidenceThe existence and uniformity of CMBR supports the Big Bang Theory — it is the "afterglow" of the initial explosion.

MCQ5

Which observation provides evidence that the Universe is expanding?

📄 Paper 4 — Structured Extended Questions

Question 1 — Radioactive Decay and Half-Life[12 marks]

Data: Iodine-131 (¹³¹₅₃I) is a beta-emitting isotope used in medical diagnosis. Its half-life is 8 days. A patient receives a dose giving an initial activity of 3200 counts/minute.

(a) [3] Write a balanced nuclear decay equation for the beta decay of ¹³¹₅₃I.

Answer (a)

¹³¹₅₃I → ¹³¹₅₄Xe + ⁰₋₁e

Check A131 = 131 + 0 ✓

Check Z53 = 54 + (−1) ✓

3 marks: 1 for correct daughter nucleus (Xe-131), 1 for beta particle notation, 1 for balanced A and Z.

(b) [3] Calculate the activity after 24 days.

Answer (b)

Half-livesn = 24 / 8 = 3 half-lives

Halving3200 → 1600 → 800 → 400

Activity = 400 counts/min

(c) [2] Explain why iodine-131 is suitable for medical use rather than an alpha-emitting isotope.

Answer (c)

Beta particles penetrate body tissue and can be detected outside the body (gamma detectors can track it). Alpha particles would be absorbed immediately within the body and cause intense local damage — they cannot be detected externally. Also, beta decay has moderate ionising ability — less tissue damage than alpha.

2 marks: 1 for "beta can be detected outside" / penetrates tissue, 1 for "alpha absorbed immediately / causes local damage / cannot be detected."

(d) [2] State why the half-life of 8 days makes iodine-131 suitable for this medical application.

Answer (d)

8 days is long enough to allow time for imaging/diagnosis procedures to be completed, but short enough that the patient's radiation dose falls to a safe level relatively quickly (after a few half-lives, activity is negligible).

(e) [2] The activity of the source must fall below 100 counts/min before the patient is discharged. How many complete days must pass?

Answer (e)

Halving3200 → 1600 (8d) → 800 (16d) → 400 (24d) → 200 (32d) → 100 (40d)

After 5 half-lives = 5 × 8 = 40 days, activity = 100 counts/min. Must pass below 100, so at least 40 days needed. (Exactly 100 at 40 days — so strictly must wait beyond 40 days.)

Question 2 — Stars and the Universe[10 marks]

Context: A galaxy is detected at a distance of 4.0 × 10²³ m from Earth. Its light is redshifted, and the galaxy is moving away at a speed v. H₀ = 2.2 × 10⁻¹⁸ s⁻¹.

(a) [2] Calculate the recessional speed of the galaxy.

Answer (a)

Formulav = H₀ × d = 2.2 × 10⁻¹⁸ × 4.0 × 10²³

v = 8.8 × 10⁵ m/s

(b) [2] Explain what the redshift of the galaxy's light tells us about the Universe.

Answer (b)

The redshift shows that the galaxy is moving away from us. Since all distant galaxies in all directions show redshift, the Universe as a whole is expanding. This supports the Big Bang Theory.

(c) [2] Estimate the age of the Universe.

Answer (c)

FormulaAge ≈ 1/H₀ = 1/(2.2 × 10⁻¹⁸)

Age ≈ 4.5 × 10¹⁷ s ≈ 14 billion years

(d) [2] Describe the complete life cycle of a star more massive than our Sun, starting from a nebula.

Answer (d)

Nebula → protostar (collapses under gravity, heats up) → main sequence star (H fusion, stable) → red supergiant (H exhausted, expands) → supernova (violent explosion, heavier elements formed) → neutron star or black hole.

2 marks: 1 for correct sequence of stages (at least 4), 1 for supernova + final state (neutron star/black hole).

(e) [2] Explain the role of nuclear fusion in maintaining a star's stability during its main sequence lifetime.

Answer (e)

During the main sequence, hydrogen is fused into helium in the core, releasing enormous amounts of energy. This creates an outward radiation pressure that exactly balances the inward force of gravity. The star remains stable as long as this balance is maintained. When hydrogen runs out, the balance is broken and the star evolves.

⚡ Quick-Fire MCQ — Topics 5 & 6 Full Review

Papers 1 & 2

16 rapid MCQs. Aim for 13+ correct.

Relative charge of a neutron is...

²³⁸₉₂U has how many neutrons?

Which radiation has the greatest ionising power?

In alpha decay, mass number A changes by...

After 3 half-lives, what fraction of atoms remain?

Rutherford's gold foil experiment showed that...

Gamma radiation is NOT deflected in electric or magnetic fields because...

The Sun is powered by nuclear fusion of...

Redshift in galaxy light is evidence that...

Which is the final stage of a massive star?

Activity = 1600. After 2 half-lives, activity =?

Largest source of background radiation in most countries is...

In beta decay, proton number Z...

CMBR is evidence for the...

Isotopes of an element have the same...

Earth orbits the Sun once in approximately...

📋 Topics 5 & 6 — Complete Formula & Key Facts Summary

FREE MARKS — Revise Daily

Concept	Key Fact / Formula	Core/Supp
Particle charges	Proton +1 \| Neutron 0 \| Electron −1	Core
Nuclide notation	A/Z X: A = nucleons, Z = protons, neutrons = A−Z	Core
Isotopes	Same Z, different A (different neutrons)	Core
Alpha particle	⁴₂He: A−4, Z−2	Core / Supplement
Beta particle	⁰₋₁e: A unchanged, Z+1	Core / Supplement
Gamma	EM wave: no mass, no charge; A and Z unchanged	Core
Penetration	α: paper \| β: 3mm Al \| γ: many cm lead	Core
Half-life definition	Time for half the nuclei / activity to halve	Core
After n half-lives	Activity = A₀ × (½)ⁿ	Core
Background radiation	Subtract before calculating corrected count rate	Supplement
Orbital speed	v = 2πr/T	Supplement
Hubble's Law	H₀ = v/d \| H₀ = 2.2 × 10⁻¹⁸ s⁻¹	Supplement
Age of Universe	≈ 1/H₀ ≈ 4.5 × 10¹⁷ s ≈ 14 billion years	Supplement
1 light-year	9.5 × 10¹⁵ m	Supplement
Milky Way diameter	≈ 100 000 light-years	Core
Star stability	Gravity inward = fusion pressure outward	Supplement
Redshift	Increased wavelength from receding sources → Universe expanding	Core

Top 12 Exam Tips — Topics 5 & 6

#	Tip
1	Neutrons = A − Z. Always calculate this rather than guessing.
2	Alpha decay: A−4, Z−2. Beta decay: A unchanged, Z+1. Memorise both.
3	Write alpha as ⁴₂He, not ⁴₂α — Cambridge requires the helium notation.
4	Always check both A and Z balance in nuclear equations.
5	Half-life: subtract background FIRST, then halve to find number of half-lives.
6	Count halvings to find number of half-lives — never use a formula at IGCSE.
7	Rutherford: 3 conclusions — mostly empty space, most mass in nucleus, nucleus positive.
8	Ionising ability ↑ = penetrating power ↓ (inverse relationship). Alpha/gamma are extremes.
9	Stellar life cycle: less massive → white dwarf; more massive → neutron star/black hole.
10	Redshift = increased wavelength = galaxies moving away = Universe expanding = Big Bang.
11	Age of Universe = 1/H₀. CMBR = afterglow of Big Bang. Both are Supplement A* marks.
12	Planet order: Mercury, Venus, Earth, Mars (rocky), Jupiter, Saturn, Uranus, Neptune (gaseous).