--- title: "What comes out of a decaying nucleus?" subtitle: "Years 9 and 10 Chemistry · Inside the atom · Lesson 3" --- # What comes out of a decaying nucleus? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Radon is a gas that seeps out of some rocks and into houses. It is invisible and has no smell, yet the air in a closed cellar can carry atoms that are turning, one by one, into lead. :::teacher-note **In one sentence:** Students describe alpha, beta and gamma decay by what leaves the nucleus, explain how alpha and beta decay change the atomic and mass numbers, and compare how far each gets (AC9S9U06, node 3, a hinge node). **Students should be able to say:** I can describe what is given out in alpha, beta and gamma decay. I can explain how alpha and beta decay change the atomic number and mass number. I can compare how far alpha, beta and gamma get through materials. **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 **Radon-222 has 86 protons. Its nucleus gives out an alpha particle: 2 protons and 2 neutrons. What is left?** ☐ Radon-220 (86 protons) ☐ Polonium-218 (84 protons) ☐ Radon-222: nothing changes ☐ Radium-226 (88 protons) My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** Polonium-218. Losing 2 protons takes the atomic number from 86 to 84, which is polonium; losing 4 particles in all takes the mass number from 222 to 218. A different number of protons means a different element. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: three kinds of radiation :::keep-together ### Three kinds **Alpha, beta or gamma?** Groups: Alpha, Beta, Gamma - 2 protons and 2 neutrons (a helium nucleus): {{field-15}} - A fast electron made when a neutron turns into a proton: {{field-15}} - Energy only: no mass and no charge: {{field-15}} - Atomic number down 2, mass number down 4: {{field-15}} - Atomic number up 1, mass number the same: {{field-15}} - Atomic number and mass number both the same: {{field-15}} - Stopped by a sheet of paper or a few centimetres of air: {{field-15}} - Stopped by a few millimetres of aluminium: {{field-15}} - Reduced by thick lead or concrete, never stopped completely: {{field-15}} ::: :::teacher-note **Answer:** 2 protons and 2 neutrons (a helium nucleus) → Alpha; A fast electron made when a neutron turns into a proton → Beta; Energy only: no mass and no charge → Gamma; Atomic number down 2, mass number down 4 → Alpha; Atomic number up 1, mass number the same → Beta; Atomic number and mass number both the same → Gamma; Stopped by a sheet of paper or a few centimetres of air → Alpha; Stopped by a few millimetres of aluminium → Beta; Reduced by thick lead or concrete, never stopped completely → Gamma ::: :::keep-together ### Alpha **Uranium-238 (92 protons) decays by alpha decay. What is the mass number of the new nucleus?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 234 An alpha particle takes 4 particles (2 protons, 2 neutrons): 238 − 4 = 234. The atomic number drops to 90, which is thorium: thorium-234. ::: :::keep-together ### Beta **Carbon-14 (6 protons) decays by beta decay. What is the atomic number of the new nucleus?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 7 A neutron turns into a proton, so 6 + 1 = 7 protons: nitrogen. The mass number stays 14, so carbon-14 becomes nitrogen-14, which is stable. ::: :::keep-together ### Gamma **A nucleus gives out a gamma ray. What changes?** ☐ It becomes the next element ☐ It loses 4 from its mass number ☐ Only its energy: it is the same isotope, calmer ::: :::teacher-note **Answer:** Only its energy: it is the same isotope, calmer Gamma is energy, with no particles, so the protons and neutrons stay the same. Gamma rays usually come straight after an alpha or beta decay, carrying away the new nucleus's extra energy. ::: :::keep-together ### Stopping power **A detector picks up radiation from a source through paper, but nothing once 5 mm of aluminium is in the way. What is the source giving out?** ☐ Alpha ☐ Gamma ☐ Beta ::: :::teacher-note **Answer:** Beta Through paper but stopped by a few millimetres of aluminium: beta. ::: :::keep-together ### Why **Why is alpha stopped so much sooner than gamma?** ☐ Paper absorbs helium ☐ Alpha particles are big, slow and charged, so they hit and knock electrons off atoms almost at once; gamma has no mass or charge ☐ Alpha particles have less energy than gamma rays ::: :::teacher-note **Answer:** Alpha particles are big, slow and charged, so they hit and knock electrons off atoms almost at once; gamma has no mass or charge The bigger and more charged the radiation, the more it collides with atoms in its path and the sooner it gives up its energy. That also makes alpha the most damaging if its source gets inside the body. ::: :::typical ## Elaborate: radiation in medicine :::keep-together ### Why beta **Iodine-131 collects in the thyroid gland and gives out beta particles. Why does that suit treating thyroid cancer?** ☐ Beta turns cancer cells into xenon ☐ Beta goes straight through the body, so it does no harm ☐ Beta travels only a few millimetres in the body, so it damages cells near the iodine and few others ::: :::teacher-note **Answer:** Beta travels only a few millimetres in the body, so it damages cells near the iodine and few others Inside the body, beta travels only millimetres, so its energy is delivered to the thyroid itself. ::: :::keep-together ### Seeing with gamma **To make a picture of an organ, doctors inject a substance that gives out gamma rays and use a camera outside the body. Why gamma?** ☐ It gets out of the body to the camera, and it is less damaging on the way than alpha or beta ☐ It is stopped inside the organ ☐ It changes the organ into another element ::: :::teacher-note **Answer:** It gets out of the body to the camera, and it is less damaging on the way than alpha or beta Gamma passes out of the body, so a camera can detect where it came from. Isotopes for imaging also have short half-lives, so they don't stay active for long (half-life is next). ::: ::: :::wider ## Elaborate: in the home :::keep-together ### On the ceiling **Why is the americium in a smoke alarm safe to have on the ceiling?** ☐ Americium does not decay ☐ Its alpha particles are stopped by a few centimetres of air and the alarm's case ☐ Alpha particles can't harm living cells ::: :::teacher-note **Answer:** Its alpha particles are stopped by a few centimetres of air and the alarm's case Alpha cannot get out of the alarm. Inside, it knocks electrons off air molecules so a small current flows; smoke reduces the current, and the alarm sounds. ::: :::keep-together ### In the lungs **Radon gas gives out alpha particles too. Why is breathing it in a health risk, when the alarm is not?** ☐ Breathing makes radon decay faster ☐ Inside the lungs there is nothing between the alpha particles and living cells, and alpha does the most damage close up ☐ Radon's alpha particles are a different kind that go through walls ::: :::teacher-note **Answer:** Inside the lungs there is nothing between the alpha particles and living cells, and alpha does the most damage close up Outside the body, skin and air stop alpha. Inside, alpha delivers all its energy to the cells next to it. That is why homes over some rocks are tested for radon and ventilated. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - An unstable nucleus decays by giving out radiation. It is random for one atom, but predictable for many. - **Alpha**: 2 protons and 2 neutrons (a helium nucleus). Atomic number −2, mass number −4: a new element. Stopped by paper or a few centimetres of air. - **Beta**: a neutron turns into a proton and a fast electron, which leaves. Atomic number +1, mass number the same: a new element. Stopped by a few millimetres of aluminium. - **Gamma**: energy only, usually just after alpha or beta. No change to the atomic or mass number. Reduced by thick lead or concrete. - The more massive and charged the radiation, the sooner it is stopped, and the more damage it does close up. ## Exit ticket :::keep-together **Cobalt-60 (27 protons) decays by beta decay and gives out gamma rays. What is it now, and which of the radiation would get through 1 cm of aluminium?** {{lines-3}} ::: :::teacher-note **Answer:** Nickel-60 (28 protons, mass number still 60). Only the gamma rays get through the aluminium. :::