Radioactivity – JSS3 Basic Science Lesson Note (Third Term)

Radioactivity is the spontaneous disintegration of the nucleus of an unstable atom, with the emission of radiation and energy.

Behavioural objectives

By the end of the lesson, students should be able to:

1. define radioactivity and radioactive elements;
2. name the three types of radiation and state their properties;
3. state how alpha and beta emission change the mass number and atomic number of an isotope;
4. distinguish nuclear fission from nuclear fusion;
5. state the uses and dangers of radioactivity and the uses of X-rays.

What is radioactivity?

Radioactivity is the spontaneous disintegration of the nucleus of an atom of an element.

• Nuclear reaction: radioactivity involves changes in the nucleus, not in the electrons around it.
• Unstable atom: the force holding the nucleus together is not strong enough, so the atom breaks up on its own.
• Emission: the atom gives out radiation and releases energy as it becomes stable.
• Becquerel (1896): the first scientist to observe spontaneous emission of radiation, from uranium salt.
• Pierre and Marie Curie: later isolated two new radioactive elements, polonium and radium.

What are radioactive elements?

Radioactive elements are isotopes with unstable nuclei that break up spontaneously, emitting radiation and releasing large amounts of energy as they reach a stable composition.

• Isotopes: atoms of the same element with equal numbers of protons but different numbers of neutrons.
• Radioisotope: an isotope whose nucleus is unstable and radioactive.

Examples of radioactive isotopes

• Carbon-14: used in carbon dating.
• Uranium-238 and uranium-235: used as fuel in nuclear power stations.
• Cobalt-60: used in cancer treatment.
• Hydrogen-3: a radioactive isotope of hydrogen.
• Radium-226: a radioactive isotope of radium.
• Thorium-232 and potassium-40: other naturally occurring radioisotopes.

What are the types of radiation and their properties?

There are three types of radiation from radioactive isotopes: alpha (α), beta (β) and gamma (γ).

What is alpha emission (α)?

Alpha emission is the release of alpha particles, which are helium nuclei represented by ⁴₂He.

• Effect on mass number: it is reduced by 4.
• Effect on atomic number: it is reduced by 2.
• Example: uranium-238 (atomic number 92) emits an alpha particle and becomes thorium-234 (atomic number 90).

Properties of alpha emission

• Charge: positive.
• Composition: helium nuclei.
• Penetrating power: low, stopped by a sheet of paper.
• Ionising power: high.
• Electrostatic field: deflected toward the negative plate.

What is beta emission (β)?

Beta emission is the release of beta particles, which are fast-moving electrons from the nucleus.

• How it happens: a neutron changes into a proton, and an electron (the beta particle) is emitted.
• Effect on atomic number: it increases by 1.
• Effect on mass number: it does not change.
• Example: carbon-14 (atomic number 6) emits a beta particle and becomes nitrogen-14 (atomic number 7).

Properties of beta emission

• Charge: negative.
• Mass: relatively small.
• Penetrating power: more than alpha particles, stopped by a thin sheet of aluminium.
• Ionising power: lower than alpha particles.
• Electrostatic field: deflected toward the positive plate.

What is gamma emission (γ)?

Gamma emission is the release of gamma rays (γ), which are electromagnetic waves of very high energy.

• Occurrence: gamma rays are often emitted along with alpha and beta particles.
• Effect on mass and atomic number: neither changes.

Properties of gamma emission

• Nature: electromagnetic waves, like light but with a much shorter wavelength.
• Charge: none.
• Penetrating power: the highest of the three, stopped only by thick lead.
• Ionising power: the least of the three.
• Electrostatic field: not deflected.
• Fluorescence: causes sodium iodide and zinc sulphide to glow.

How do alpha, beta and gamma radiation compare?

What are the types of nuclear reaction?

There are two types of nuclear reaction: nuclear fission and nuclear fusion.

What is nuclear fission?

Nuclear fission is the splitting of a heavy radioactive nucleus into smaller (lighter) nuclei of other elements.

• Example: the splitting of uranium-235 in a nuclear power station.

What is nuclear fusion?

Nuclear fusion is the joining of two or more light nuclei to form a heavier nucleus.

• Example: the joining of hydrogen nuclei in the Sun.

What is the difference between nuclear fission and nuclear fusion?

What are X-rays?

X-rays are high-energy electromagnetic waves that pass through most solid materials.

• Medicine: soft X-rays are used to take photographs of parts of the human body, such as bones.
• Crystal study: they are used to study how particles are arranged in crystal lattices, especially proteins.

What are the uses of radioactivity?

Radioactivity is used in power generation, medicine, food preservation, tracing and dating.

• Electricity generation: energy from nuclear fission is used to produce electricity.
• Treatment of cancer: radiation, for example from cobalt-60, destroys cancer cells.
• Food preservation: radiation kills decay-causing bacteria and mould in food.
• Tracer: a small amount of a radioisotope is followed to trace its path, for example in the body or in pipes.
• Carbon dating: the amount of carbon-14 left in a specimen shows its age.

What are the dangers of radioactivity?

Radioactive rays are dangerous because they damage living cells.

• Cell damage: the rays destroy body cells and can cause burns and cancer.
• Hereditary effects: exposure can cause harmful changes passed on to children.
• Death: heavy overexposure can kill.
• Safety: workers use lead shielding, protective clothing and limited exposure time.

Evaluation

1. Define radioactivity.
2. State three properties of alpha particles.
3. What happens to the atomic number and mass number when a beta particle is emitted?
4. Differentiate between nuclear fission and nuclear fusion.
5. List three uses of radioactivity.

Assignment

Answer all four questions.

1. Gamma rays have/are: A. positively charged B. deflected by a force field C. negatively charged D. most penetrating power
2. The dangers of radioactive rays do not include: A. destruction of body cells B. undesirable hereditary effects C. tracer of body effects D. death due to overexposure
3. The radiation used to examine soft body tissue is ………… ray. A. γ B. β C. α D. X
4. Radioactivity is the spontaneous decay of the: A. nucleus B. atom C. electron D. proton