--- title: "Same element, different atoms?" subtitle: "Years 9 and 10 Chemistry · Inside the atom · Lesson 2" --- # Same element, different atoms? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** Every living thing contains a tiny amount of carbon that is slowly turning into nitrogen. That carbon is how scientists date bones and charcoal tens of thousands of years old. :::teacher-note **In one sentence:** Students define isotopes as atoms of the same element with different numbers of neutrons, explain why isotopes react alike, and recognise that some isotopes are unstable (AC9S9U06, node 2). **Students should be able to say:** I can define isotopes. I can say what is the same and what is different about two isotopes of an element. I can name isotopes (carbon-14) and recognise that some are unstable. **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 **Carbon-12 and carbon-14 both have 6 protons. What is different about a carbon-14 atom?** ☐ It has 2 more neutrons ☐ It has 2 more protons ☐ It has 2 more electrons ☐ It is a different element My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 2 more neutrons. Both are carbon because both have 6 protons. The mass number counts protons and neutrons, so 14 − 6 = 8 neutrons, against 6 in carbon-12. Carbon-12 and carbon-14 are isotopes of carbon. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: isotopes :::keep-together ### What isotopes are **What are isotopes?** ☐ Atoms of the same element with different numbers of neutrons ☐ Atoms that have gained or lost electrons ☐ Atoms of different elements with the same mass number ::: :::teacher-note **Answer:** Atoms of the same element with different numbers of neutrons Same protons (so the same element), different neutrons (so a different mass number). ::: :::keep-together ### Same or different **Carbon-12 and carbon-14 atoms: same or different?** Groups: Same, Different - Number of protons: {{field-15}} - Number of electrons (neutral atoms): {{field-15}} - Number of neutrons: {{field-15}} - Mass number: {{field-15}} - How they react with oxygen: {{field-15}} - Whether the nucleus is stable: {{field-15}} ::: :::teacher-note **Answer:** Number of protons → Same; Number of electrons (neutral atoms) → Same; Number of neutrons → Different; Mass number → Different; How they react with oxygen → Same; Whether the nucleus is stable → Different ::: :::keep-together ### Chlorine **Chlorine has two common isotopes, chlorine-35 and chlorine-37 (17 protons). How many neutrons are in chlorine-37?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 20 37 − 17 = 20 neutrons. Chlorine-35 has 18. ::: :::keep-together ### Same reactions **Why does carbon-14 burn in oxygen just like carbon-12?** ☐ The neutrons do the reacting ☐ Reactions depend on the electrons, and both have 6 arranged the same way ☐ It doesn't: carbon-14 does not burn ::: :::teacher-note **Answer:** Reactions depend on the electrons, and both have 6 arranged the same way Isotopes of an element have the same number of electrons, so they react in the same way. Only the nucleus differs. ::: :::keep-together ### Unstable isotopes **Some isotopes are unstable. What does that mean?** ☐ The nucleus will decay, giving out radiation and often becoming a different element ☐ The electrons keep leaving the atom ☐ The atom will react violently with water ::: :::teacher-note **Answer:** The nucleus will decay, giving out radiation and often becoming a different element An unstable (radioactive) nucleus has a mix of protons and neutrons that does not last. It decays, giving out radiation. Most elements have both stable and unstable isotopes. ::: :::typical ## Elaborate: where carbon-14 comes from :::keep-together ### Living things **Plants take in carbon dioxide, including a little made with carbon-14. Animals eat the plants. Why does that matter for dating old bones?** ☐ Every living thing has the same small share of carbon-14; after death no new carbon-14 comes in, and what is there slowly decays ☐ Bones make carbon-14 after death ☐ Carbon-14 turns into carbon-12 as bones age ::: :::teacher-note **Answer:** Every living thing has the same small share of carbon-14; after death no new carbon-14 comes in, and what is there slowly decays While alive, an animal or plant keeps topping up its carbon-14. After death the share falls steadily. Measuring how much is left tells how long ago it died (you will see how in the half-life lesson). ::: ::: :::wider ## Elaborate: chlorine's mass of 35.5 :::keep-together ### The average **About 3 in every 4 chlorine atoms are chlorine-35, and 1 in 4 is chlorine-37. What is the average mass number? (to 1 decimal place)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 35.5 (3 × 35 + 1 × 37) ÷ 4 = 142 ÷ 4 = 35.5. That is why the periodic table gives chlorine a relative atomic mass of 35.5. ::: ::: ## Evaluate: look back Write one sentence: "At first I thought… Now I think… because…" **Revision summary** - **Isotopes** are atoms of the same element (same number of protons) with different numbers of neutrons, so different mass numbers. We name them by mass number: carbon-12, carbon-13, carbon-14. - Isotopes have the same number of electrons, so they react in the same way. - Some isotopes are **stable**. Others are **unstable** (radioactive): their nuclei decay, giving out radiation. - Carbon-14 is made in the air from nitrogen-14 and taken in by living things, which is why it can date things that were once alive. ## Exit ticket :::keep-together **Uranium-235 and uranium-238 both have 92 protons. How many neutrons does each have, and are they the same element?** {{lines-3}} ::: :::teacher-note **Answer:** 143 and 146 neutrons. Both are uranium (92 protons): they are isotopes. :::