Radioactivity and changes in the nucleus | ICSE Class 10 Physics Notes

This note covers nuclear structure, atomic number and mass number, isotopes, spontaneous radioactivity, alpha, beta and gamma radiations, nuclear decay equations, radioisotope uses, background radiation, X-rays, harmful effects, safety precautions and radioactive waste.

What do atomic number and mass number tell us about a nucleus?

An atom is a particle of an element with a central nucleus and electrons outside it. An element is identified by the number of protons in its nucleus. A proton has positive electric charge; an electron has negative electric charge.

A neutron is an electrically neutral nuclear particle. Protons and neutrons are called nucleons. A neutron has very nearly the same mass as a proton. The nucleus contains most of the mass of an atom and occupies a much smaller volume than the atom.

How are the nuclear counts written?

The atomic number, Z, is the number of protons in a nucleus. The mass number, A, is the total number of protons and neutrons. The neutron number, N, is the number of neutrons. These are counts, so they have no physical unit.

A = Z + N

N = A − Z

In nuclear notation, the mass number is written at the upper left and the atomic number at the lower left of an element's chemical symbol. A nuclide is a nuclear species specified by its proton and neutron counts.

For example, ¹⁹⁷₇₉Au represents gold with mass number 197 and atomic number 79; Au is the chemical symbol for gold. It has 79 protons and 118 neutrons. A neutral gold atom also has 79 electrons outside its nucleus.

Worked example 1. A gold nucleus has A = 197 and Z = 79. Find its neutron number. Answer: N = A − Z = 197 − 79 = 118. Thus, the nucleus contains 118 neutrons; the result is a particle count.

Mass number is not the mass of the atom in kilograms. It counts nucleons. Also, the number of electrons belongs to the description of the whole atom, not to the composition of its nucleus. These distinctions prevent mistakes when writing nuclear equations.

What is radioactivity, and how is it related to isotopes?

Definition: Radioactivity is the spontaneous disintegration or transformation of an unstable atomic nucleus, accompanied by the emission of radiation. An unstable nucleus is one that can undergo radioactive decay.

Spontaneous means that the decay occurs without being triggered by an external collision or an external supply of energy. It is a nuclear process. Ordinary heating, cooling, pressure changes and chemical combination do not appreciably change the radioactive decay of a given isotope.

The nucleus before decay is the parent nucleus; the nucleus formed is the daughter nucleus. A daughter need not be stable. Further radioactive changes may occur if it is unstable. Do not assume that every decay ends the sequence of nuclear changes.

What makes atoms isotopes?

Isotopes are atoms of the same element with the same atomic number but different mass numbers. Their nuclei contain the same number of protons but different numbers of neutrons. Their chemical properties are similar because their electronic arrangements are the same.

Hydrogen has the isotopes protium, deuterium and tritium. H is the chemical symbol for hydrogen. Their nuclear symbols are ¹₁H, ²₁H and ³₁H respectively. Each nucleus has one proton; their neutron numbers are zero, one and two respectively.

Three schematic atoms are labelled Protium, Deuterium and Tritium. Each has one blue electron on a circular path. Their central groups contain one red particle and, respectively, zero, one and two green particles, representing their different neutron counts.

See Fig. 8.12 in your NCERT textbook

A radioisotope is an isotope with an unstable nucleus that undergoes radioactive decay. Isotope describes nuclear composition; radioisotope additionally describes radioactive behaviour. Merely being an isotope does not mean that an atom is radioactive.

The emitted radiations considered here are alpha, α, beta-minus, β⁻, and gamma, γ. The Greek letters label the radiation types. Beta-minus means electron emission; the minus sign specifies its negative charge.

How do alpha, beta and gamma radiations differ?

An alpha particle is a helium nucleus containing two protons and two neutrons. He is the chemical symbol for helium. The nuclear symbol ⁴₂He therefore also represents an alpha particle. It is not a neutral helium atom because it has no surrounding electrons.

A beta-minus particle is a fast electron emitted during a nuclear transformation. A gamma ray is high-energy electromagnetic radiation emitted by a nucleus. Electromagnetic radiation carries energy through electric and magnetic disturbances; a photon is a packet of this radiation's energy.

Let e denote the magnitude of the charge on one proton or electron. An alpha particle has charge +2e; a beta-minus particle has charge −e. A gamma photon has no electric charge. Its rest mass, meaning mass associated with a particle at rest, is zero. A vacuum is space without matter.

What do ionising and penetrating powers mean?

Ionisation is the formation of charged atoms or molecules, for example by removing electrons from them. Such charged particles are called ions. Ionising power describes how effectively radiation produces ions while passing through matter.

Penetrating power describes the ability of radiation to pass through matter. Strong ionisation does not mean deep penetration. Alpha particles produce many ions over a short path and lose their energy rapidly. Gamma rays can penetrate much farther before their energy is absorbed.

These are qualitative comparisons. Actual penetration depends on the radiation's energy and the material it encounters. A radiation type with low penetrating power is not automatically harmless, especially if the radioactive material enters the body.

How can fields and absorbers distinguish the three radiations?

An electric field is a region where an electric charge experiences a force. Between oppositely charged plates, alpha particles bend towards the negative plate because they are positively charged. Beta-minus particles bend towards the positive plate because they are negatively charged.

Gamma rays continue without electric deflection because they are uncharged. Alpha and beta-minus paths therefore curve in opposite directions, while the gamma path remains straight. This comparison identifies the sign of charge carried by the radiation.

A magnetic field is a region of magnetic influence in which a moving charge can experience a force. When their motion is perpendicular to the field, alpha and beta-minus particles deflect in opposite directions for the same initial direction of travel. Gamma rays are not deflected. The amount of bending depends on particle properties and speed.

Draw a narrow beam entering horizontally between two plates. Label the upper plate positive and the lower plate negative. Show β⁻ curving upwards, α curving downwards and γ continuing horizontally. Label every path and add arrowheads showing travel.

What happens when matter lies in the path?

An absorber is material that reduces the radiation passing through it. A sheet of paper can stop alpha particles. Beta particles penetrate paper but can be stopped by a suitable thickness of aluminium. Gamma radiation requires much thicker shielding, such as lead or concrete, to reduce the radiation transmitted.

Draw three labelled paths towards successive barriers labelled paper, aluminium and thick lead or concrete. End the alpha path at paper and the beta path at aluminium. Show the gamma path weakened through the thick shield, rather than implying that every photon is stopped.

The diagram expresses a relative comparison, not a universal specification of shield thickness. Shielding means placing suitable absorbing material between a source and people or equipment. The material and thickness must be chosen for the radiation concerned.

Field deflection and absorption answer different questions. Deflection gives evidence about charge; absorption compares passage through matter. Lack of deflection does not mean that gamma rays carry no energy or cannot damage living tissue.

What changes occur in a nucleus during alpha decay?

In alpha decay, a parent nucleus emits a particle containing two protons and two neutrons. Its daughter therefore has two fewer protons and four fewer nucleons. The atomic number decreases by two, while the mass number decreases by four.

Derivation: How does alpha emission change the nuclear counts?

Use $A$, $Z$ and $N$ for the parent's mass, atomic and neutron numbers. The corresponding daughter numbers are $A_1$, $Z_1$ and $N_1$. The subscript 1 identifies the daughter; it does not mean multiplication.

1. An alpha particle is a helium nucleus with two protons and two neutrons. It carries away four nucleons, so $A=A_1+4$, giving $A_1=A-4$.
2. The loss of two protons gives $Z=Z_1+2$. Rearranging gives $Z_1=Z-2$, so the daughter is a different element from the parent.
3. Neutron number is mass number minus atomic number. Therefore, $N_1=A_1-Z_1=(A-4)-(Z-2)=A-Z-2=N-2$.

Result: $A_1=A-4$, $Z_1=Z-2$ and $N_1=N-2$. Alpha emission removes two protons and two neutrons. These numbers are particle counts and have no physical units.

How is the uranium example balanced?

Uranium-238, written ²³⁸₉₂U, can decay to thorium-234, written ²³⁴₉₀Th, by alpha emission. U and Th are the chemical symbols for uranium and thorium. The arrow in a nuclear equation means “changes into”, and the plus sign separates the products.

²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He

1. Read the parent's mass number, 238, and atomic number, 92.
2. Subtract the emitted alpha particle's mass number, 4, to obtain 234.
3. Subtract its atomic number, 2, to obtain the daughter atomic number, 90.
4. Check both totals: 238 = 234 + 4, and 92 = 90 + 2.

The parent neutron number is 238 − 92 = 146. The daughter neutron number is 234 − 90 = 144. This confirms the loss of two neutrons along with the two protons.

Note: Balancing mass numbers checks the number of nucleons, not equality of the particles' measured masses. Nuclear energy release does not contradict the balance of the integer mass numbers.

Why does beta-minus decay increase atomic number without changing mass number?

During beta-minus decay, a neutron inside the nucleus changes into a proton, and an electron is emitted. An electron antineutrino, an electrically neutral elementary particle, is also emitted. It is represented here by ν̅ₑ, where the bar identifies an antineutrino and the small e specifies electron type.

The neutron is represented by ¹₀n and the proton by ¹₁p. In nuclear equations, the emitted electron is written ⁰₋₁e. Its upper zero means that it carries no nucleon number; it does not mean that an electron has zero mass.

¹₀n → ¹₁p + ⁰₋₁e + ν̅ₑ

Derivation: How does beta-minus emission change the nuclear counts?

Use the parent and daughter symbols already defined. In beta-minus decay, one neutron changes into a proton, with an electron and an electron antineutrino emitted.

1. One neutron is replaced by one proton. Since each is a nucleon, the total nucleon count is unchanged: $A_1=A$.
2. The new proton increases the proton count by one: $Z_1=Z+1$. The emitted electron is not a proton and does not reduce this count.
3. Using neutron number as mass number minus atomic number gives $N_1=A_1-Z_1=A-(Z+1)=A-Z-1=N-1$.

Result: $A_1=A$, $Z_1=Z+1$ and $N_1=N-1$. Beta-minus emission changes the proton and neutron counts while leaving the mass number unchanged.

How does carbon-14 illustrate the change?

Carbon-14, ¹⁴₆C, undergoes beta-minus decay to nitrogen-14, ¹⁴₇N. C and N are the chemical symbols for carbon and nitrogen. Here N within a nuclide symbol means nitrogen; the separately defined variable N in counting formulas means neutron number.

¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅ₑ

The carbon nucleus contains six protons and eight neutrons. The nitrogen nucleus contains seven protons and seven neutrons. Each has fourteen nucleons. The emitted electron's negative charge balances the increase in the daughter's positive nuclear charge.

The mass-number check is 14 = 14 + 0; the charge-number check is 6 = 7 + (−1). The antineutrino contributes zero to both checks. It is included to show the complete physical decay process.

Note: The beta electron is created in the nuclear transformation. It is not an electron knocked out of an atomic shell, a region occupied by electrons outside the nucleus, and it was not stored inside the nucleus before emission.

The rule that atomic number increases applies specifically to beta-minus decay. Beta-plus decay emits a positron, a particle with the electron's mass but positive charge. Keeping the minus sign in β⁻ prevents the electron-emission rule from being applied to every kind of beta decay.

What changes during gamma emission, and how can decay equations be checked?

An excited nucleus has more energy than a lower-energy state of the same nucleus. It can emit the excess energy as a gamma photon. During this change, the numbers of protons and neutrons remain unchanged.

Gamma emission therefore changes the nuclear energy state without changing the element or its mass number. A nucleus may be left excited after alpha or beta decay and subsequently emit gamma radiation. The gamma step is distinct from the particle-emission step.

In the symbolic statement “excited nucleus → lower-energy nucleus + γ”, the two nuclei have the same proton and neutron counts. Gamma radiation removes energy but no nucleons. Do not subtract four from the mass number merely because energy has left the nucleus.

Which checks should accompany an equation?

1. Identify the parent nuclide and write its mass number and atomic number in the correct positions.
2. Identify the emitted radiation, including the sign when the emission is beta-minus.
3. Balance the mass numbers, then balance the electric charge numbers on the two sides.
4. Calculate the daughter neutron number by subtracting its atomic number from its mass number.

Conservation of electric charge means that the total electric charge remains the same through the process. For beta-minus decay, apply this to the daughter and emitted electron together. The daughter's nuclear charge alone is not unchanged.

An equation can balance arithmetically without establishing that the proposed decay occurs in nature. Use a known decay example or a stated transformation. Arithmetic checks whether its particle counts are consistent; they do not determine nuclear stability on their own.

How are radioisotopes useful?

A radioisotope emits radiation that can be detected or used to transfer energy to matter. This makes radioisotopes useful in medicine and scientific investigation. Their usefulness depends on selecting an appropriate isotope and controlling exposure to its radiation.

Radiotherapy means treating disease using radiation. Radiation can damage or destroy cells, so carefully controlled treatment can target cancer cells. Cancer is a disease in which normal control of cell division breaks down. The same ability to damage cells also explains why surrounding healthy tissue needs protection.

The thyroid is a gland in the neck, and goitre is enlargement of that gland. The stated iodine application concerns medical use under specialist control. It should not be read as a suggestion to handle or administer radioactive material.

Why must an application name the isotope?

Isotopes of one element have different nuclei. Naming cobalt-60 identifies the particular radioisotope used, rather than treating every cobalt atom as an interchangeable radioactive source. Likewise, carbon-14 identifies a radioactive form of carbon rather than carbon in general.

Radioactive dating uses radioactive change to estimate age. In carbon-14 dating, the relevant material is ancient carbon-containing material of biological origin. The method is not a general test for the age of any object merely because that object is old.

A useful explanation links the isotope to its purpose and then recognises the need for control. Radiation's beneficial and harmful effects have the same physical basis: it transfers energy to matter. The context and management of that transfer determine how it is used.

What are background radiation, X-rays and radioactive fallout?

Background radiation is the ionising radiation present in our surroundings even when no particular experimental radioactive source has been introduced. It has natural sources and can also include contributions from human activities. Background is therefore not evidence by itself of a nearby nuclear accident.

Natural contributions include cosmic radiation, radiation reaching Earth from space, and radiation from naturally occurring radioactive materials in rocks, soil and building materials. Naturally occurring radioisotopes in living bodies also contribute to exposure.

Radon is a naturally occurring radioactive gas that can escape from the ground and accumulate in enclosed spaces. Background levels vary with location and surroundings. It is incorrect to assume that the background reading must be identical everywhere.

How are X-rays different from radioactive emissions?

X-rays are high-energy electromagnetic radiation. A common way of producing them is to direct high-energy electrons onto a metal target. Their production in an X-ray tube is different from the spontaneous decay of a radioactive nucleus.

X-rays are used as a diagnostic tool in medicine and in treating certain forms of cancer. Diagnosis means identifying a disease or injury. Because X-rays can damage living tissue, unnecessary exposure and overexposure must be avoided.

Both X-rays and gamma rays are uncharged electromagnetic radiation. Wavelength means the distance between successive wave crests. Their wavelength ranges overlap, so wavelength alone is not an absolute dividing line. In this comparison, X-rays arise from processes involving electrons, while gamma rays arise from nuclear changes.

What does fallout mean?

Radioactive fallout is radioactive material carried through the atmosphere that settles on the ground or other surfaces. It can follow nuclear explosions or accidental releases of radioactive material from nuclear facilities. It is matter containing radioactive nuclei, not simply a beam of radiation.

Fallout can contaminate soil, water and food. Contamination means unwanted radioactive material is present on or inside something. Once deposited, that material can continue emitting radiation as its nuclei decay. Preventing its spread is therefore different from merely shielding an external radiation source.

Why is radiation harmful, and how are exposure and waste controlled?

Ionising radiation can alter molecules in living cells and damage DNA, deoxyribonucleic acid, the molecule carrying genetic information. Such damage can kill cells or contribute to cancer. Radiation's ability to damage tissue explains both its medical applications and its hazards.

The biological effect depends on factors including radiation type, energy absorbed, exposure time and the tissue exposed. A radiation dose describes the amount of radiation received, with different dose quantities used for energy absorption and biological effect. A numerical safety limit cannot be inferred simply from the radiation's name.

Irradiation means exposure to radiation. A person may be irradiated by an external source without radioactive material being deposited on them. Contamination involves the material itself being present, and can continue causing exposure until that material is removed or decays.

What precautions reduce exposure?

• Time: Keep time near a radioactive source as short as practicable, so less radiation is received.
• Distance: Work farther from the source when possible. Remote-handling tools help avoid unnecessary close contact.
• Shielding: Use barriers selected for the radiation. Penetrating gamma radiation needs suitable thick shielding.
• Containment: Keep radioactive material in appropriate sealed, labelled containers to reduce its spread.
• Monitoring: A dosimeter is a device used to monitor radiation dose. It records exposure; it does not act as a shield.
• Clean handling: Prevent radioactive material entering the body through inhalation, swallowing or wounds. Follow controlled handling procedures.

Alpha radiation has low external penetration, but alpha-emitting material inside the body can deposit energy directly in living tissue. Gamma radiation can be a significant external hazard because of its greater penetration. Neither observation justifies calling the other radiation types harmless.

Why does radioactive waste need special management?

Radioactive waste is discarded material containing radioactive substances for which there is no further intended use. It can remain a radiation source after the activity that produced it has ended. Ordinary burning or chemical treatment does not remove radioactivity merely by changing the material's chemical form.

Safe management requires identifying and separating waste, suitable containment and shielding, secure storage, monitoring and an authorised disposal route appropriate to the waste. The aim is to prevent exposure and the escape of radioactive material into air, water or soil.

Waste differs in the radiation it emits and how long it remains radioactive. A single storage period or disposal method is therefore not suitable for every kind. Nuclear facilities need strict safety measures during operation and continuing control over radioactive waste afterwards.

Glossary

• Nucleus — The small, positively charged central region containing most of an atom's mass.
• Nucleon — A proton or neutron, counted as one constituent particle of an atomic nucleus.
• Atomic number — The number of protons in a nucleus, identifying the chemical element.
• Mass number — The total number of protons and neutrons in a particular atomic nucleus.
• Isotopes — Atoms of the same element with equal proton numbers but different neutron numbers.
• Radioisotope — An isotope whose unstable nuclei undergo radioactive decay and emit radiation.
• Radioactivity — Spontaneous nuclear disintegration or transformation accompanied by the emission of radiation.
• Alpha particle — A helium nucleus containing two protons and two neutrons, carrying positive charge.
• Beta-minus particle — An electron emitted during a nuclear transformation in which a neutron becomes a proton.
• Gamma radiation — High-energy electromagnetic radiation emitted when a nucleus changes to a lower-energy state.
• Ionisation — Formation of electrically charged atoms or molecules, for example through electron removal.
• Background radiation — Ionising radiation present in the surroundings without introducing a particular experimental radioactive source.
• Radioactive contamination — Unwanted radioactive material present on a surface or inside an object or organism.
• Radioactive fallout — Radioactive material transported through the atmosphere and deposited on the ground or other surfaces.

Common errors and misconceptions

• Misconception: Mass number gives an atom's mass in kilograms. Correct: It counts the protons and neutrons in the nucleus and has no physical unit.
• Misconception: All isotopes are radioactive. Correct: Isotopes have the same proton number but different neutron numbers; the term radioisotope identifies an isotope with an unstable nucleus.
• Misconception: Alpha radiation is made of neutral helium atoms. Correct: Alpha particles are helium nuclei without surrounding electrons and carry positive charge.
• Misconception: Beta-minus emission removes an electron already stored in the nucleus. Correct: The emitted electron is created when a neutron changes into a proton.
• Misconception: Beta-minus decay lowers atomic number because something leaves. Correct: A neutron becomes a proton, so atomic number increases by one while mass number remains unchanged.
• Misconception: Gamma emission leaves the nucleus unchanged in every respect. Correct: Its proton and neutron counts remain unchanged, but its energy decreases.
• Misconception: Low penetration makes alpha-emitting material harmless. Correct: If it enters the body, it can cause strong ionisation in nearby living tissue.
• Misconception: A dosimeter prevents radiation reaching a worker. Correct: It monitors exposure; controlling time, distance, shielding and contamination provides protection.

Exam-style questions with model answers

Q1. Define radioactivity and explain the word spontaneous in this definition. [2 marks]

1. Radioactivity is the disintegration or transformation of an unstable atomic nucleus, accompanied by the emission of radiation.
2. Spontaneous means that the decay occurs without needing an external collision or an external supply of energy to initiate it.

Q2. Uranium-238 has atomic number 92 and emits one alpha particle, ⁴₂He. Thorium has atomic number 90; U and Th denote uranium and thorium. Write the decay equation, determine the daughter's mass and atomic numbers, and calculate its neutron number. [4 marks]

1. The nuclear equation is ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He: uranium-238 changes into thorium-234 with alpha emission.
2. The daughter mass number is 238 − 4 = 234 because the emitted alpha particle carries four nucleons.
3. The daughter atomic number is 92 − 2 = 90 because the alpha particle carries two protons.
4. The daughter neutron number is 234 − 90 = 144. Subtract the daughter proton number from its own mass number.

Q3. Carbon-14, ¹⁴₆C, undergoes beta-minus decay to nitrogen, N, of atomic number 7. The emitted electron is ⁰₋₁e; an electron antineutrino, ν̅ₑ, carries zero charge and zero nucleon number. Explain the nuclear conversion, write the equation, and check mass number and charge number separately. [4 marks]

1. A neutron in the carbon nucleus changes into a proton, emitting an electron and an electron antineutrino. The proton count rises by one.
2. The decay equation is ¹⁴₆C → ¹⁴₇N + ⁰₋₁e + ν̅ₑ. The nitrogen daughter has the same mass number as the parent.
3. The mass-number check is 14 = 14 + 0 + 0. No nucleon is lost in the neutron-to-proton conversion.
4. The charge-number check is 6 = 7 + (−1) + 0. The emitted negative electron balances the daughter's increased positive charge.

Q4. A nucleus emits a gamma photon. Explain what happens to its proton number, mass number and nuclear energy. [3 marks]

1. The proton number remains unchanged because gamma emission does not remove or create a proton. The nucleus therefore remains that of the same element.
2. The mass number remains unchanged because no nucleons are emitted. Both the proton count and the neutron count stay the same.
3. The nuclear energy decreases as the excited nucleus changes to a lower-energy state. The emitted gamma photon carries away energy.

Q5. Compare alpha, beta-minus and gamma radiations under five headings: nature, electric charge, electric-field deflection, relative ionising power and relative penetrating power. [5 marks]

1. Nature: Alpha radiation consists of helium nuclei; beta-minus radiation consists of fast electrons; gamma radiation consists of high-energy electromagnetic photons emitted by nuclei.
2. Charge: Alpha particles carry positive charge equal to twice the proton charge; beta-minus particles carry one electron's negative charge; gamma photons are uncharged.
3. Electric-field deflection: Alpha particles bend towards the negative plate and beta-minus particles towards the positive plate. Gamma rays pass without electric deflection.
4. Ionising power: In the usual qualitative comparison, alpha radiation is most strongly ionising, beta radiation is intermediate and gamma radiation is least strongly ionising.
5. Penetration: The usual order is reversed: gamma has the greatest penetration, beta is intermediate and alpha has the least. Actual penetration also depends on energy and material.

Q6. State five precautions for work involving radioactive materials. Explain how limiting time, increasing distance, using shielding, preventing contamination and monitoring dose each help. [5 marks]

1. Limit time: Keep the time spent near the source as short as practicable. Shorter exposure reduces the radiation received during the work.
2. Increase distance: Avoid unnecessary close contact and use remote-handling equipment where appropriate. Greater separation helps reduce exposure from the source during handling.
3. Use shielding: Place suitable absorbing barriers between people and the source. Choose the shielding material and thickness for the radiation being emitted.
4. Prevent contamination: Keep radioactive material securely contained and follow clean handling procedures. This helps prevent its spread and entry into the body.
5. Monitor dose: Use appropriate dosimeters to record exposure and support control of the work. A dosimeter provides information and does not itself shield the wearer.

Q7. Give one application each of cobalt-60, iodine-131 and carbon-14. [3 marks]

1. Cobalt-60 is used in radiation treatment for cancer. Radiation is directed at cancerous tissue under controlled medical conditions.
2. Iodine-131 is used in treating thyroid conditions, including goitre and thyroid cancer. Goitre means enlargement of the thyroid gland.
3. Carbon-14 is used to estimate the age of ancient organic remains and artefacts. This application uses its radioactive change in suitable carbon-containing material.

Q8. Distinguish background radiation, radioactive fallout and irradiation. Give one natural source of background radiation and explain why fallout can cause continuing exposure. [3 marks]

1. Background radiation is ionising radiation already present in the surroundings without an introduced experimental source. Cosmic radiation arriving from space is one natural contribution.
2. Radioactive fallout is radioactive material deposited from the atmosphere. It can cause continuing exposure because its unstable nuclei keep emitting radiation after deposition.
3. Irradiation is exposure to radiation. It does not by itself mean radioactive material has been deposited on the person or object; that would be contamination.

Key takeaways

• Atomic number counts protons; mass number counts protons and neutrons. Subtract atomic number from mass number to find neutron number.
• Radioactivity is a spontaneous nuclear change. Isotopes share proton number, while radioisotopes additionally have unstable nuclei.
• Alpha particles are helium nuclei, beta-minus particles are electrons, and gamma rays are high-energy electromagnetic radiation.
• Alpha decay lowers mass number by four and atomic number by two because two protons and two neutrons leave.
• Beta-minus decay raises atomic number by one without changing mass number; gamma emission changes energy without changing either count.
• Balance both mass numbers and charge numbers in a nuclear equation, including the negative charge of an emitted beta electron.
• Radioisotopes have useful applications in medicine and dating, but their ability to transfer energy also creates hazards.
• Control radiation exposure through time, distance, shielding, containment and monitoring; radioactive waste requires secure, appropriate long-term management.

Test yourself

What distinguishes atomic number from mass number?

Atomic number counts protons alone; mass number counts protons and neutrons together. Neither count has a physical unit.

A gold nucleus has mass number 197 and atomic number 79. How many neutrons does it contain?

It contains 197 − 79 = 118 neutrons, found by subtracting the proton count from the total nucleon count.

Why is an alpha particle positively charged?

It contains two positively charged protons and two neutral neutrons, with no electrons to balance the proton charges.

Why does beta-minus decay leave mass number unchanged?

One neutron changes into one proton. The total number of nucleons stays the same even though their composition changes.

Does gamma emission change the nucleus's element?

No. Gamma emission leaves proton number unchanged, so the element is unchanged, although the nuclear energy decreases.

Why can alpha-emitting material be dangerous inside the body?

Alpha particles can produce strong ionisation directly in nearby living tissue when the emitting material is inside the body.

Why is radioactive contamination different from irradiation?

Contamination means radioactive material is present on or inside something. Irradiation means exposure to radiation, without necessarily acquiring that material.

Why must radioactive waste remain controlled after a facility finishes using it?

Its radioactive nuclei can continue decaying and emitting radiation. Secure containment and suitable management help prevent exposure and environmental contamination.