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.
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.
The number of protons decides the element. This lesson asks what happens when only the neutrons change.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: two kinds of carbon
ExploreTry it on the board and compare what happens with your prediction.
Explore: make the carbon isotopes
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.
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Keep 6 protons and make carbon-13 and carbon-14 as well.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: isotopes
In your book: draw carbon-12 and carbon-14 side by side and label what is the same and what is different. Then write a sentence using the word "isotope".
Carbon-12 and carbon-14 atoms: same or different?
Number of protons
Number of electrons (neutral atoms)
Number of neutrons
Mass number
How they react with oxygenReactions depend on the electrons, and both have 6.
Whether the nucleus is stableCarbon-12 is stable; carbon-14 is radioactive.
Chlorine has two common isotopes, chlorine-35 and chlorine-37 (17 protons). How many neutrons are in chlorine-37?
Elaborate TypicalUse the same idea in a new context.
Elaborate: where carbon-14 comes from
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High in the air, a neutron from space hits a nitrogen-14 nucleus. The nucleus takes in the neutron and a proton is knocked out. Make the carbon-14 that results (a neutral atom).
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: chlorine's mass of 35.5
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Make chlorine-37.
Use the board. This ticks when you get there.
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)
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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.
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.
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.
Running the lesson
Engage: tally the predictions. 'Different element' means the student has not yet tied identity to protons.
Explore: ask what stayed the same at the top of the board as the neutrons changed, and when the stability line changed.
Explain: the definition, the sort and the chlorine number are the node's success criteria.
In your own words: the labelled pair of carbon atoms is the evidence.
The hard part (on purpose)
Separating 'same element' from 'same atom'. Ask which number on the board stayed the same and which changed, before giving the word isotope.
When students get stuck
A student adds protons to make carbon-14. Ask: What happened to the name at the top of the board? Fix: It changed to nitrogen, then oxygen. Protons decide the element; change the neutrons for a heavier carbon.
A student thinks carbon-14 reacts differently because it is heavier. Ask: Which particles take part in reactions? Fix: The electrons. Both isotopes have 6, arranged the same way, so they react the same way.
A student thinks 'radioactive' means the whole atom is dangerous to touch or reacts strongly. Ask: Which part of the atom is unstable? Fix: Only the nucleus. It decays now and then, giving out radiation; chemically it is ordinary carbon.
Stretch and support
Stretch: Hydrogen has three isotopes: hydrogen-1, -2 (deuterium) and -3 (tritium). Which is radioactive? Use the board to check. (Tritium, hydrogen-3.)
Support: Give the frame: 'Carbon-12 and carbon-14 both have … protons, so they are both … . Carbon-14 has … more neutrons, so its mass number is … .'
Where it 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.
What each step is for
Engage: two kinds of carbon: Asking what differs between carbon-12 and carbon-14 tests whether students separate the protons (the element) from the neutrons (the mass); the distractors change the protons, the electrons or the element.
Explore: make the carbon isotopes: Changing only the neutrons while the name stays 'Carbon' makes 'same element, different mass' something students see; the board's stable or radioactive line adds that isotopes can differ in stability.
Explain: isotopes: Sorting what is the same and what differs between two isotopes, and why the electrons stay the same, sets up both 'same chemistry' and 'different stability' for decay and half-life.
Elaborate: where carbon-14 comes from: 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.
Elaborate: chlorine's mass of 35.5: The relative atomic mass on the periodic table is an average over isotopes, which explains why it is not a whole number.
Now you can
Define isotopes.
Say what is the same and what is different about two isotopes of an element.
Name isotopes (carbon-14) and recognise that some are unstable.