Ch 5 · 5.2.3 Radioactive Decay · O Level Physics 5054

Topic 5 of 8 in Nuclear Physics — O Level Physics 5054 — 59 lessons.

Introduction

When a radioactive nucleus decays, it can turn into a completely different element. Radium becomes radon; carbon-14 becomes nitrogen. Decay equations let us track exactly what happens to the protons and neutrons. (Syllabus section 5.2.3.)

Key ideas — What radioactive decay is

Some nuclei are unstable: they have too many neutrons, too many protons, or too much energy. Radioactive decay is a change in an unstable nucleus that makes it more stable. It can result in the emission of an α-particle, a β-particle and/or γ-radiation.

🧑‍🏫 Teacher's explanation

Imagine a large, crowded room of people where everyone is trying to find a comfortable spot to sit. Some people are standing in the middle of a walkway, feeling very 'unstable' because they are likely to be bumped. Eventually, they will move to a seat to become 'stable'. In a nucleus, if there are too many protons or neutrons, the nucleus feels 'unstable' and undergoes a change to reach a more stable state. This change is what we call radioactive decay.

📖 Definitions

🔍 Worked example — Easy

A nucleus of Uranium-238 (_92^238U) undergoes α-decay. Write the balanced decay equation in nuclide notation.

🔍 Worked example — Standard

A nucleus undergoes β-decay and transforms into an isotope of Phosphorus (_15^32P). Identify the parent nucleus.

🔍 Worked example — Exam-style

An unstable nucleus emits an α-particle and a β-particle. If the parent nucleus was _90^238Th, what is the mass number and atomic number of the resulting nucleus?

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Writing decay equations

In nuclide notation:

🧑‍🏫 Teacher's explanation

Imagine a busy, crowded party where a few guests feel extremely uncomfortable and decide to leave the room to find peace. In the world of atoms, some nuclei are 'unstable' because they have too much energy or an awkward mix of protons and neutrons. To become stable, they undergo radioactive decay, which is a spontaneous and random process—you can't predict exactly which nucleus will leave or when, only how many might leave over time.

📖 Definitions

🔍 Worked example — Easy

An isotope of an element decays by emitting an α-particle. The parent nucleus is ^238_92U. Write the balanced decay equation for the daughter nucleus.

🔍 Worked example — Standard

A nucleus undergoes β-decay. The parent nucleus is ^14_6C. Write the balanced decay equation.

🔍 Worked example — Exam-style

An unstable nucleus ^210_84Po emits an α-particle and a γ-photon. Write the complete decay equation.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Alpha decay

The nucleus loses 2 protons and 2 neutrons:

🧑‍🏫 Teacher's explanation

Imagine a crowded elevator that is far too heavy for the cable to support. The elevator is 'unstable.' To stabilize itself, it doesn't just lose weight gradually; it suddenly and unpredictably ejects a heavy chunk of weight to try and find balance. In a nucleus, this 'chunk' is an alpha particle.

📖 Definitions

🔍 Worked example — Easy

A nucleus with a nucleon number of 238 and a proton number of 92 undergoes alpha decay. Write the nuclide notation for the new daughter nucleus.

🔍 Worked example — Standard

Complete the following decay equation: ^210_84Po arrow\ ^206_82Pb + ?

🔍 Worked example — Exam-style

An isotope of Radium (^226_88Ra) decays via alpha emission. Determine the mass number and atomic number of the resulting nucleus and show that the equation is balanced.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Beta decay

A neutron in the nucleus changes into a proton and an electron, and the electron is emitted as the β-particle:

🧑‍🏫 Teacher's explanation

Imagine a crowded elevator where one person suddenly decides to jump out. You can't predict exactly when that person will jump, nor can you predict which person it will be. This is how radioactive decay works: it is spontaneous (happens without being forced) and random (unpredictable).

📖 Definitions

🔍 Worked example — Easy

A nucleus of Carbon-14 (^14_6C) undergoes beta decay. Complete the decay equation: ^14_6C arrow\ ^14_xN + ^0_-1β. Find the value of x.

🔍 Worked example — Standard

A radioactive isotope of Phosphorus (^32_15P) undergoes beta decay to become Sulfur (^32_16S). Write the complete balanced decay equation using nuclide notation.

🔍 Worked example — Exam-style

An unstable nucleus undergoes beta decay. The mass number A remains 24 and the proton number Z increases from 11 to 12. Write the decay equation.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) Write the decay equation for this process using nuclide notation. [2] (b) State the name of the new element formed. [1] [3 marks]

Key ideas — Gamma emission

Gamma rays are electromagnetic waves, with no mass and no charge. A nucleus that has extra energy (often just after an α or β decay) gives it out as a γ-ray:

🧑‍🏫 Teacher's explanation

Imagine a person who has just finished a high-intensity sprint. Their body is physically the same (same mass, same height), but they are 'excited' and have excess energy that they need to release to return to a stable, resting state. In the nucleus, after an alpha or beta decay, the nucleus often finds itself in an 'excited state'—it has too much energy packed into its nucleons.

Ch 5 · 5.2.3 Radioactive Decay · O Level Physics 5054

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