Radioactivity and Nuclear Energy notes: ICSE Class 10 Physics (Science Paper 1)

1. The nucleus; Z, A and isotopes

Nucleus: protons (+1, ~1 u) and neutrons (0, ~1 u). Atomic number $Z$ = number of protons; mass number $A$ = protons + neutrons; neutrons $N = A - Z$ . Notation $\,{}^{A}_{Z}\text{X}$ .

Isotopes: same $Z$ , different $A$ (different numbers of neutrons); same chemical properties. Examples: protium, deuterium, tritium ( $\,{}^{1}_{1}\text{H}$ , $\,{}^{2}_{1}\text{H}$ , $\,{}^{3}_{1}\text{H}$ ); carbon-12 and carbon-14.

Table of protons and neutrons in three nuclides
Nuclide	Protons	Neutrons
Na-23	11	12
Co-60	27	33
C-14	6	8

Formulas

Number of neutrons
$N = A - Z$

Key terms

Atomic number: The number of protons in the nucleus of an atom.
Mass number: The total number of protons and neutrons in the nucleus.
Isotopes: Atoms of the same element with the same atomic number but different mass numbers.

2. Radioactivity and the three radiations

Radioactivity: spontaneous disintegration of unstable nuclei, emitting α, β or γ radiation; not affected by temperature, pressure or chemical state.

Property	α	β	γ
Nature	Helium nucleus	Fast electron (from nucleus)	Electromagnetic wave
Charge	+2e	−e	0
Mass	4 u	≈ 1/1840 u	0
Speed	≈ 0.1c	up to 0.99c	c
Ionising power	Highest	Medium	Lowest
Penetrating power	Lowest (paper)	Medium (few mm Al)	Highest (cm of lead)
In electric field	Towards − plate	Towards + plate (more)	Undeflected

Table of absorbers for each radiation
Radiation	Stopped by
α	Paper
β	Aluminium (few mm)
γ	Lead (several cm)

Key terms

Radioactivity: The spontaneous disintegration of the nucleus of an unstable atom with the emission of alpha, beta or gamma radiation.
Ionising power: The ability of radiation to knock electrons out of atoms of a gas.

3. Changes in the nucleus

α decay: $A \to A - 4$ , $Z \to Z - 2$ : $\,{}^{226}_{88}\text{Ra} \rightarrow {}^{222}_{86}\text{Rn} + {}^{4}_{2}\text{He}$ .
β decay: a neutron becomes a proton + electron; $A$ unchanged, $Z \to Z + 1$ : $\,{}^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}e$ .
γ emission: no change in $A$ or $Z$ (the nucleus loses excess energy).
In every equation, totals of $A$ and of $Z$ balance on both sides.

Table of changes in A and Z
Emission	ΔA	ΔZ
α	−4	−2
β	0	+1
γ	0	0

Formulas

Alpha decay
${}^{A}_{Z}\text{X} \rightarrow {}^{A-4}_{Z-2}\text{Y} + {}^{4}_{2}\text{He}$
Beta decay
${}^{A}_{Z}\text{X} \rightarrow {}^{A}_{Z+1}\text{Y} + {}^{0}_{-1}e$

Key terms

Alpha decay: Emission of an alpha particle, which lowers the mass number by 4 and the atomic number by 2.
Beta decay: Emission of an electron from the nucleus, which raises the atomic number by 1 and leaves the mass number unchanged.

4. Uses, hazards and safety

Uses: Co-60 (radiotherapy), I-131 (thyroid), Na-24 (blood circulation studies), P-32 (tracer in agriculture), C-14 (dating), γ sources (sterilising instruments, checking welds, pipe leaks), β sources (thickness gauges).

Harmful effects: burns, radiation sickness, cancer (leukaemia), genetic mutations.

Background radiation: cosmic rays, radioactive rocks and soil, radon gas, K-40 in food and the body, medical X-rays, fallout.

Safety: long tongs, lead containers and aprons, distance, short exposure time, film badges (dosimeters), no eating near sources. X-rays are also ionising; radioactive fallout is radioactive dust spread by nuclear explosions or accidents.

Table of three radioisotopes and their uses
Isotope	Use
Co-60	Cancer therapy
I-131	Thyroid
C-14	Dating

Key terms

Background radiation: Low-level radiation always present in the environment from natural and man-made sources.
Radioisotope: A radioactive isotope of an element.

5. Fission, fusion and nuclear waste

Fission: a heavy nucleus splits into two lighter nuclei when struck by a slow neutron, releasing energy and 2–3 neutrons; chain reaction (controlled in reactors, uncontrolled in atom bombs). $\,{}^{235}_{92}\text{U} + {}^{1}_{0}n \rightarrow {}^{141}_{56}\text{Ba} + {}^{92}_{36}\text{Kr} + 3\,{}^{1}_{0}n + \text{energy}$ .

Fusion: light nuclei join to form a heavier nucleus at extremely high temperatures, releasing energy (Sun, stars, hydrogen bomb). $\,{}^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + {}^{1}_{0}n + \text{energy}$ .

Energy comes from a loss of mass ( $E = mc^{2}$ ). Nuclear waste: cool in ponds, seal in glass/steel and concrete, store deep underground; it remains radioactive for thousands of years.

Table contrasting fission and fusion
Fission	Fusion
Heavy splits	Light join
Reactors	Sun

Formulas

Mass–energy equivalence
$E = mc^{2}$

Key terms

Nuclear fission: The splitting of a heavy nucleus into two lighter nuclei with the release of energy.
Nuclear fusion: The joining of light nuclei to form a heavier nucleus with the release of energy.
Chain reaction: A self-sustaining series of fissions in which neutrons from one fission cause further fissions.

Exam tips

Common mistakes

Saying β particles come from the electron shells.
Getting the change in Z wrong for β decay.
Confusing fission (splitting) with fusion (joining).
Forgetting the neutrons when balancing fission equations.

How marks are usually awarded

Decay equations (1–2 marks): correct A and Z, symbol of the radiation.
Comparisons (1 mark per property).
Uses and safety (1 mark each): named isotope or precaution with a reason.

1. The nucleus; Z, A and isotopes

Formulas

Key terms

2. Radioactivity and the three radiations

Key terms

3. Changes in the nucleus

Formulas

Key terms

4. Uses, hazards and safety

Key terms

5. Fission, fusion and nuclear waste

Formulas

Key terms

Exam tips

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