---
title: "Half-Life and Activity"
description: "Activity is the number of decays each second, measured in becquerels, and it falls by half in every half-life. Reading a half-life off a decay curve turns an unpredictable single nucleus into a reliab"
canonical: https://lightmysky.com/learn/science/half-life-and-activity-mt_S5snTqUjyi
source: https://lightmysky.com/learn/science/half-life-and-activity-mt_S5snTqUjyi.md
retrieved: 2026-09-12
---

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# Half-Life and Activity

Activity is the number of decays each second, measured in becquerels, and it falls by half in every half-life. Reading a half-life off a decay curve turns an unpredictable single nucleus into a reliable count for a large sample.

Subject: Science · Area: Matter & Materials · Ages 15 to 16
Page: https://lightmysky.com/learn/science/half-life-and-activity-mt_S5snTqUjyi

## Ready when they can

- Reads a half-life from a graph of activity against time
- Calculates the fraction or the count rate left after a whole number of half-lives
- Explains that a single nucleus decays at random while a large sample follows a steady pattern

## Lesson: Halving again and again

Activity counts the decays each second, measured in becquerels, where 1 Bq means one decay per second. As unstable nuclei pop, fewer remain, so the activity falls over time. The half-life is the time for the activity to halve, and it is a fixed fingerprint of each isotope.

Read a half-life straight off an activity graph by finding where the curve drops to half its start value and reading across to the time axis. After each whole half-life the fraction left halves again. One half, one quarter, one eighth, then one sixteenth after four.

**Example.** Priya watches a counter fall from 800 to 100 Bq. She lists the drops: 800 to 400, then 200, then 100. That is three halvings, so three half-lives have passed. If one half-life is 5 minutes, the whole fall took 15 minutes.

**Tip.** Single nuclei are surprises, since no one can predict when one will pop. A sample holds trillions of them, and the crowd average smooths into a steady curve. Half-life describes the crowd, never the individual.

**Recap.** Activity halves each half-life, from a crowd no single atom can predict.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Radioactive Decay: Alpha, Beta and Gamma](https://lightmysky.com/learn/science/radioactive-decay-alpha-beta-and-gamma-mt_-AmjVwkR9t)
- [Radiometric Dating](https://lightmysky.com/learn/science/radiometric-dating-mt_bHbpLW1HUg)

## Opens up

- [Activity, Half-Life and Radioactive Dating](https://lightmysky.com/learn/science/activity-half-life-and-radioactive-dating-mt_LvZw4h5wlu)
- [Deep Time: Radiometric Dating and the Geological Column](https://lightmysky.com/learn/science/deep-time-radiometric-dating-and-the-geological-column-mt_w5DL8yRghy)
- [Irradiation, Contamination and Dose](https://lightmysky.com/learn/science/irradiation-contamination-and-dose-mt_WyxR7ZVEhi)