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# Same element, different atoms?

Years 9 and 10 Chemistry · Inside the atom

<!-- Speaker note: 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). About 25 minutes (Engage and Explore 7 min · Explain 11 min · Evaluate 3 min · Typical adds 4 min, Wider about 4 min). -->

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## Today

**We are learning to** isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. They react in the same way but have different masses, and some are unstable: they are radioactive.

- 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.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. It builds on What's inside an atom? (node 1), so set that first. Not a hinge node, but its idea carries the next two: decay (only unstable isotopes decay) and half-life (carbon-14 dating). The prediction separates "more neutrons" from the usual slips: more protons, more electrons, or a different element. Board notes: for elements 1 to 20 the board says whether the nucleus you built is stable or radioactive (from the list of stable isotopes; very long-lived calcium-48 counts as stable). It does not say how an unstable nucleus decays: that is the next lesson's board. Nuclei far from the stable ones are listed as radioactive even though some would not hold together at all. Teacher: In your own words: the labelled pair of carbon atoms is the evidence. -->

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## Why it matters

> 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.

<!-- Speaker note: The number of protons decides the element. This lesson asks what happens when only the neutrons change. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**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

<!-- Speaker 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. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: tally the predictions. 'Different element' means the student has not yet tied identity to protons. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fisotopes%2Fstep-2&code=widget%3A+atom%0Astart%3A+6%2C+6%2C+6%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/isotopes/?full=step-2#step-2)

<!-- Speaker note: Use **+** and **−** for the neutrons and watch the name at the top, the mass number and the line that says whether the nucleus is stable. Try a few more neutrons, or fewer. -->

<!-- Speaker note: Teacher: ask what stayed the same at the top of the board as the neutrons changed, and when the stability line changed. -->

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## What happened?

* 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.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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## 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

* Answer: Atoms of the same element with different numbers of neutrons

<!-- Speaker note: Same protons (so the same element), different neutrons (so a different mass number). -->

<!-- Speaker note: Teacher: the definition, the sort and the chlorine number are the node's success criteria. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Same or different

**Carbon-12 and carbon-14 atoms: same or different?**

Groups: Same · Different

Number of protons · Number of electrons (neutral atoms) · Number of neutrons · Mass number · How they react with oxygen · Whether the nucleus is stable

* 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

<!-- Speaker note:  -->

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## Chlorine

**Chlorine has two common isotopes, chlorine-35 and chlorine-37 (17 protons). How many neutrons are in chlorine-37?**

* Answer: 20

<!-- Speaker note: 37 − 17 = 20 neutrons. Chlorine-35 has 18. -->

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## 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

* Answer: Reactions depend on the electrons, and both have 6 arranged the same way

<!-- Speaker note: Isotopes of an element have the same number of electrons, so they react in the same way. Only the nucleus differs. -->

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## 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

* Answer: The nucleus will decay, giving out radiation and often becoming a different element

<!-- Speaker note: 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. -->

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## Elaborate: where carbon-14 comes from

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fisotopes%2Fstep-4&code=widget%3A+atom%0Astart%3A+7%2C+7%2C+7%0Atarget%3A+carbon-14%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/isotopes/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Making carbon-14 from nitrogen-14 shows that a nucleus can change into another element, ready for decay, and gives carbon-14 a real source. -->

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## 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

* 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

<!-- Speaker note: 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). -->

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## Elaborate: chlorine's mass of 35.5

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fchemistry-9-10%2Fisotopes%2Fstep-5&code=widget%3A+atom%0Astart%3A+17%2C+18%2C+17%0Atarget%3A+chlorine-37%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/chemistry-9-10/isotopes/?full=step-5#step-5)

<!-- Speaker note: What this step is for: The relative atomic mass on the periodic table is an average over isotopes, which explains why it is not a whole number. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## 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: 35.5

<!-- Speaker note: (3 × 35 + 1 × 37) ÷ 4 = 142 ÷ 4 = 35.5. That is why the periodic table gives chlorine a relative atomic mass of 35.5. -->

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## 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.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Uranium-235 and uranium-238 both have 92 protons. How many neutrons does each have, and are they the same element?**

* 143 and 146 neutrons. Both are uranium (92 protons): they are isotopes.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: alpha, beta and gamma (node 3), then half-life (node 4), which uses carbon-14 for dating. In class: a "same and different" table for isotopes of hydrogen, carbon and chlorine; relative atomic mass from isotope abundances (the Wider step) if the class has met averages. -->