--- title: "From uranium to lead" subtitle: "Years 9 and 10 Chemistry · Inside the atom · Lesson 5" --- # From uranium to lead **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** A grain of uranium doesn't decay once. It sets off a chain of fourteen steps, passing through radon, a gas that seeps into some basements, and ends as lead. How can you follow it? :::teacher-note **In one sentence:** Students trace a decay sequence, track atomic and mass numbers at each step, and explain how the sequence ends at a stable nucleus (AC9S9U06, node 5, an integration node). **Students should be able to say:** I can follow a decay chain from an unstable isotope to a stable one. I can work out the atomic and mass numbers after each alpha or beta decay. I can say why a chain stops. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **A uranium-238 nucleus decays by emitting an alpha particle. What is the new nucleus like?** ☐ The same uranium, only lighter ☐ Thorium-234, which is also unstable and decays again ☐ Stable lead straight away ☐ A pile of loose protons My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Thorium-234 (atomic number 90, mass number 234), which is unstable too. The chain carries on through a series of alpha and beta decays until a stable nucleus, lead-206, is reached. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: tracking numbers :::keep-together ### Atomic number **Uranium-238 has atomic number 92. After one alpha decay, what is the new atomic number?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 90 An alpha particle carries away 2 protons, so 92 − 2 = 90 (thorium). ::: :::keep-together ### Mass number **And the new mass number, after the alpha decay from mass number 238?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 234 An alpha particle has 4 particles (2 protons and 2 neutrons), so 238 − 4 = 234. ::: :::keep-together ### Beta step **Thorium-234 (atomic number 90) then decays by beta emission. What is the new atomic number?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 91 A neutron becomes a proton, so the atomic number goes up by 1: 91 (protactinium). The mass number stays at 234. ::: :::keep-together ### Total fall **Uranium-238 ends as lead-206. By how much does the mass number fall in total?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 32 238 − 206 = 32. ::: :::keep-together ### Alpha count **Only alpha decays change the mass number, by 4 each time. How many alpha decays are in the chain?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 8 32 ÷ 4 = 8 alpha decays. They lower the atomic number by 16, from 92 to 76; beta decays raise it back to 82. ::: :::keep-together ### Why it stops **Why does the chain stop at lead-206?** ☐ It has become too small ☐ It has run out of energy ☐ Its nucleus has a stable balance of protons and neutrons, so it doesn't decay ::: :::teacher-note **Answer:** Its nucleus has a stable balance of protons and neutrons, so it doesn't decay Uranium-238's nucleus is too big, with too many neutrons, to last. Alpha decays make it smaller; beta decays turn spare neutrons into protons. When it reaches a mix of protons and neutrons that lasts, nothing more happens. ::: :::typical ## Elaborate: radon in the home :::keep-together **Radon-222 is a gas made in the uranium chain, and it seeps up from rock into some buildings. Why is it of more concern than the solid isotopes before it?** ☐ It is hotter ☐ It has a longer half-life than uranium ☐ As a gas it can be breathed in, and it decays to more radioactive solids inside the lungs ::: :::teacher-note **Answer:** As a gas it can be breathed in, and it decays to more radioactive solids inside the lungs The earlier solids stay in the rock. A gas gets out and can be breathed in, so its later decay products are released next to lung tissue. Good ventilation helps. ::: ::: :::wider ## Elaborate: missing mass :::keep-together **The total mass of the products of a decay is slightly less than the mass of the original nucleus. What does physics say happened to the 'missing' mass?** ☐ It became the energy carried away by the radiation (E = mc²) ☐ It was destroyed ☐ It left as a gas ::: :::teacher-note **Answer:** It became the energy carried away by the radiation (E = mc²) A tiny amount of mass becomes a large amount of energy, which is why a small amount of radioactive material releases so much energy. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - A **decay chain** is a series of decays that ends only at a **stable** nucleus. - **Alpha** decay: atomic number −2, mass number −4. **Beta** decay: atomic number +1, mass number unchanged. - Uranium-238 ends as lead-206 after **8 alpha** and **6 beta** decays. - Alpha decays make the nucleus smaller and beta decays fix its balance of neutrons to protons, until it is stable. ## Exit ticket :::keep-together **Radium-226 (atomic number 88) decays by alpha emission. What are the new atomic and mass numbers?** {{lines-3}} ::: :::teacher-note **Answer:** Atomic number 86, mass number 222: radon-222. :::