What are isotopes in the context of nuclear chemistry?

In nuclear chemistry, isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. Isotopes play a crucial role in nuclear reactions and are important in various scientific applications, including medical imaging and radiometric dating.

How do isotopes differ from each other in terms of physical and chemical properties?

Isotopes of an element have nearly identical chemical properties because they have the same electron configuration. However, they can have different physical properties due to their differing masses. For example, heavier isotopes may have higher melting and boiling points. In nuclear chemistry, isotopes can exhibit different nuclear stability, which affects their radioactive behavior.

Why are isotopes important in the IB MYP Science curriculum?

In the IB MYP Science curriculum, isotopes are important because they help students understand fundamental concepts in nuclear chemistry, such as atomic structure and nuclear reactions. Learning about isotopes also provides insight into real-world applications, including their use in medical diagnostics, treatment, and environmental studies, which are key components of the curriculum's focus on practical and applied science.

What is the role of isotopes in radiometric dating?

Isotopes are crucial in radiometric dating, a technique used to determine the age of materials. This method relies on the decay of radioactive isotopes, such as Carbon-14, into stable isotopes over time. By measuring the ratio of parent to daughter isotopes in a sample, scientists can calculate the time elapsed since the material was formed, providing valuable information in fields like geology and archaeology.

Can you give examples of isotopes and their applications?

Yes, there are several examples of isotopes with diverse applications. Carbon-14 is used in radiocarbon dating to determine the age of archaeological artifacts. Uranium-235 is utilized in nuclear reactors and weapons due to its ability to sustain a chain reaction. In medicine, isotopes like Iodine-131 are used for diagnosing and treating thyroid conditions. These examples highlight the significance of isotopes in both scientific research and practical applications.

Why is "Treating isotopes as different elements." a common mistake in Isotopes?

Why it happens: Different mass numbers feel like different substances. How to avoid it: Isotopes have the SAME atomic number — so they are the SAME element. Only the neutron count (and therefore the mass) differs.

Why is "Averaging just the mass numbers without weighting by abundance." a common mistake in Isotopes?

Why it happens: Looks like an average. How to avoid it: Multiply each mass by its abundance percent, sum, divide by 100. A 50:50 mix gives the simple average; otherwise it's weighted.

Nuclear chemistryIB MYP Sciences (Years 1-5)

Isotopes

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Isotopes — IB MYP Sciences: atoms of the same element with different masses

Carbon-12 and carbon-14 are both carbon — same number of protons, different number of neutrons. This MYP Sciences note covers what makes two atoms isotopes of the same element, why some isotopes are radioactive, and how relative atomic mass is averaged.

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Define isotopes and identify isotopic pairs from their notation.
MYP Sciences A — Find the number of protons, neutrons and electrons of a given isotope.
MYP Sciences A — Calculate the relative atomic mass from isotopic abundances.
MYP Sciences D — Identify a radioactive isotope and a use for it.

What are isotopes?

Same element (same Z), different mass (different number of neutrons).

Some elements have ONLY ONE natural form. Others have several isotopes — atoms that share the same atomic number (Z) but have different numbers of neutrons.

Examples:

Carbon: ¹²C (6p + 6n), ¹³C (6p + 7n), ¹⁴C (6p + 8n).
Hydrogen: ¹H (just 1 proton — protium), ²H (1p + 1n — deuterium), ³H (1p + 2n — tritium).
Chlorine: ³⁵Cl (17p + 18n) and ³⁷Cl (17p + 20n).
Uranium: ²³⁵U (used in nuclear reactors), ²³⁸U (the most abundant).

The chemistry is essentially IDENTICAL (same electron arrangement, same reactions) — but the mass differs, and some isotopes are radioactively unstable while others are stable.

All three are carbon (6 protons). Different neutron counts give different mass numbers. ¹²C and ¹³C are stable; ¹⁴C is radioactive.

Isotopes: same Z, different A.
Same chemistry (same electrons); different mass.
Some isotopes stable, others radioactive.

Relative atomic mass — a weighted average

Most elements are a MIX of isotopes; Aᵣ is the average.

If chlorine were 100% ³⁵Cl, its relative atomic mass (Aᵣ) would be 35.0. If 100% ³⁷Cl, it would be 37.0. In reality, natural chlorine is about 75% ³⁵Cl and 25% ³⁷Cl. The weighted average is:

$A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = \frac{2625 + 925}{100} = 35.5$

This is why the Periodic Table value for chlorine is 35.5 — it's a real mixture being averaged.

Similar averaging gives:

Copper (Aᵣ = 63.5) ← 69% ⁶³Cu + 31% ⁶⁵Cu.
Bromine (Aᵣ = 80) ← about 50:50 ⁷⁹Br and ⁸¹Br.
Carbon (Aᵣ ≈ 12.01) ← almost all ¹²C with a tiny percentage of ¹³C.

Worked example. Boron has two isotopes: ¹⁰B (20%) and ¹¹B (80%). Find its Aᵣ.

Aᵣ = (20 × 10 + 80 × 11) ÷ 100 = (200 + 880) ÷ 100 = 10.8.

Aᵣ = weighted average across natural isotopes.
Formula: Σ(mass × abundance) ÷ 100.
Why chlorine's Aᵣ is 35.5: it's really 3/4 ³⁵Cl and 1/4 ³⁷Cl.

Why we care about isotopes

Some isotopes are tools — medical tracers, dating, fingerprinting.

Even though isotopes of an element have identical chemistry, the differences in mass and stability give them useful applications:

Carbon-14 dating: living things absorb atmospheric ¹⁴C. When they die, ¹⁴C decays with a half-life of 5 730 years. Measuring how much remains dates the sample (up to ~50 000 years old).
Uranium-235 fuel: only this isotope (not the more common U-238) sustains a fission chain reaction. Reactor fuel is 'enriched' to raise the U-235 fraction.
Cobalt-60: a gamma source used in cancer treatment and to sterilise medical equipment.
Iodine-131: short-half-life beta/gamma emitter used as a medical tracer for thyroid problems.
Stable isotope tracers (¹³C, ¹⁵N, ²H): used in biochemistry and ecology to trace nutrient flows without radioactivity.

MYP criterion D: discuss whether the benefits of isotope use in medicine outweigh the radiation risks. Modern protocols (short half-life, targeted delivery, shielding) keep doses extremely low — usually justified by the diagnostic or treatment value.

C-14 dating: ages organic samples up to ~50 000 years.
U-235: sustains fission chain reaction (reactor fuel).
Medical isotopes: tracers (I-131), therapy (Co-60), imaging (Tc-99m).
Stable isotopes used in ecology and biochemistry tracing.

How it’s examined

Criterion A tests p/n/e calculations for isotopes, and Aᵣ from abundances. Criterion D often asks for an evaluation of a medical or industrial use of an isotope.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

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Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Isotopes, ready to print or save as PDF.

Step-by-step worked examples — Isotopes

Step-by-step solutions to past-paper-style questions on isotopes, written exactly the way a tutor would explain them at the board.

1p, n, e of a specific isotope
Getting started• isotopes
Question
An atom is ¹⁷O. Find the number of protons, neutrons and electrons.
Step-by-step solution
1. Step 1
  Oxygen → Z = 8 → 8 protons.
2. Step 2
  Neutrons = A − Z = 17 − 8 = 9.
3. Step 3
  Neutral atom → 8 electrons.
Answer
8 protons, 9 neutrons, 8 electrons.
2Relative atomic mass from abundance
Building confidence• Aᵣ
Question
Magnesium has three isotopes: ²⁴Mg (79%), ²⁵Mg (10%), ²⁶Mg (11%). Find its relative atomic mass.
Step-by-step solution
1. Step 1
  Aᵣ = Σ(mass × abundance%) ÷ 100.
2. Step 2
  (24 × 79 + 25 × 10 + 26 × 11) ÷ 100 = (1896 + 250 + 286) ÷ 100 = 2432 ÷ 100 = 24.32.
Answer
Aᵣ ≈ 24.3.
3Carbon-14 dating
Stretch• dating
Question
A wooden sample has 25% as much ¹⁴C as a fresh sample. How old is it (¹⁴C half-life = 5 730 years)?
Step-by-step solution
1. Step 1
  Activity halves each half-life: 100% → 50% → 25%. So 2 half-lives have passed.
2. Step 2
  Age = 2 × 5 730 = 11 460 years.
Answer
≈ 11 460 years old.

Key Formulae — Isotopes

The formulae you need to memorise for isotopes on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Relative atomic mass
$Aᵣ = Σ(mass × abundance%) / 100$
$mass$
mass number of each isotope
$abundance%$
% of that isotope in the natural mixture
When to use
Whenever an element has more than one natural isotope.

Key Definitions and Keywords — Isotopes

Definitions to memorise and the exact keywords mark schemes credit for isotopes answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Isotope
Examiner keyword
Atoms of the same element (same Z) with different mass numbers (different numbers of neutrons).
Relative atomic mass (Aᵣ)
Examiner keyword
The weighted average mass of all the naturally-occurring isotopes of an element.
Radioactive isotope (radioisotope)
Examiner keyword
An isotope with an unstable nucleus that decays, emitting radiation.
Carbon-14 dating
Using the decay of ¹⁴C (half-life 5 730 years) to estimate the age of organic samples up to ~50 000 years old.

Common Mistakes and Misconceptions — Isotopes

The traps other students keep falling into on isotopes questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Treating isotopes as different elements.
Why it happens
Different mass numbers feel like different substances.
How to avoid it
Isotopes have the SAME atomic number — so they are the SAME element. Only the neutron count (and therefore the mass) differs.
✕Averaging just the mass numbers without weighting by abundance.
Why it happens
Looks like an average.
How to avoid it
Multiply each mass by its abundance percent, sum, divide by 100. A 50:50 mix gives the simple average; otherwise it's weighted.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Isotopes — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Nuclear chemistryIB MYP Sciences (Years 1-5)

Isotopes

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

PreviousNext

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Short Study Notes — Isotopes

Start with these resources to cover the key concepts, then work through the practice questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Isotopes, ready to print or save offline.

Isotopes — IB MYP Sciences: atoms of the same element with different masses

What you’ll learn

Mapped to the IB MYP Sciences subject guide (2026 onwards).

MYP Sciences A — Define isotopes and identify isotopic pairs from their notation.
MYP Sciences A — Find the number of protons, neutrons and electrons of a given isotope.
MYP Sciences A — Calculate the relative atomic mass from isotopic abundances.
MYP Sciences D — Identify a radioactive isotope and a use for it.

What are isotopes?

Same element (same Z), different mass (different number of neutrons).

Some elements have ONLY ONE natural form. Others have several isotopes — atoms that share the same atomic number (Z) but have different numbers of neutrons.

Examples:

Carbon: ¹²C (6p + 6n), ¹³C (6p + 7n), ¹⁴C (6p + 8n).
Hydrogen: ¹H (just 1 proton — protium), ²H (1p + 1n — deuterium), ³H (1p + 2n — tritium).
Chlorine: ³⁵Cl (17p + 18n) and ³⁷Cl (17p + 20n).
Uranium: ²³⁵U (used in nuclear reactors), ²³⁸U (the most abundant).

The chemistry is essentially IDENTICAL (same electron arrangement, same reactions) — but the mass differs, and some isotopes are radioactively unstable while others are stable.

All three are carbon (6 protons). Different neutron counts give different mass numbers. ¹²C and ¹³C are stable; ¹⁴C is radioactive.

Isotopes: same Z, different A.
Same chemistry (same electrons); different mass.
Some isotopes stable, others radioactive.

Relative atomic mass — a weighted average

Most elements are a MIX of isotopes; Aᵣ is the average.

$A_r = \frac{(75 \times 35) + (25 \times 37)}{100} = \frac{2625 + 925}{100} = 35.5$

This is why the Periodic Table value for chlorine is 35.5 — it's a real mixture being averaged.

Similar averaging gives:

Copper (Aᵣ = 63.5) ← 69% ⁶³Cu + 31% ⁶⁵Cu.
Bromine (Aᵣ = 80) ← about 50:50 ⁷⁹Br and ⁸¹Br.
Carbon (Aᵣ ≈ 12.01) ← almost all ¹²C with a tiny percentage of ¹³C.

Worked example. Boron has two isotopes: ¹⁰B (20%) and ¹¹B (80%). Find its Aᵣ.

Aᵣ = (20 × 10 + 80 × 11) ÷ 100 = (200 + 880) ÷ 100 = 10.8.

Aᵣ = weighted average across natural isotopes.
Formula: Σ(mass × abundance) ÷ 100.
Why chlorine's Aᵣ is 35.5: it's really 3/4 ³⁵Cl and 1/4 ³⁷Cl.

Why we care about isotopes

Some isotopes are tools — medical tracers, dating, fingerprinting.

Even though isotopes of an element have identical chemistry, the differences in mass and stability give them useful applications:

Carbon-14 dating: living things absorb atmospheric ¹⁴C. When they die, ¹⁴C decays with a half-life of 5 730 years. Measuring how much remains dates the sample (up to ~50 000 years old).
Uranium-235 fuel: only this isotope (not the more common U-238) sustains a fission chain reaction. Reactor fuel is 'enriched' to raise the U-235 fraction.
Cobalt-60: a gamma source used in cancer treatment and to sterilise medical equipment.
Iodine-131: short-half-life beta/gamma emitter used as a medical tracer for thyroid problems.
Stable isotope tracers (¹³C, ¹⁵N, ²H): used in biochemistry and ecology to trace nutrient flows without radioactivity.

C-14 dating: ages organic samples up to ~50 000 years.
U-235: sustains fission chain reaction (reactor fuel).
Medical isotopes: tracers (I-131), therapy (Co-60), imaging (Tc-99m).
Stable isotopes used in ecology and biochemistry tracing.

How it’s examined

Criterion A tests p/n/e calculations for isotopes, and Aᵣ from abundances. Criterion D often asks for an evaluation of a medical or industrial use of an isotope.

Sources: IB MYP Sciences guide (IBO, official subject guide). Last reviewed 2026-05-25.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Isotopes, ready to print or save as PDF.

Step-by-step worked examples — Isotopes

Step-by-step solutions to past-paper-style questions on isotopes, written exactly the way a tutor would explain them at the board.

1p, n, e of a specific isotope
Getting started• isotopes
Question
An atom is ¹⁷O. Find the number of protons, neutrons and electrons.
Step-by-step solution
1. Step 1
  Oxygen → Z = 8 → 8 protons.
2. Step 2
  Neutrons = A − Z = 17 − 8 = 9.
3. Step 3
  Neutral atom → 8 electrons.
Answer
8 protons, 9 neutrons, 8 electrons.
2Relative atomic mass from abundance
Building confidence• Aᵣ
Question
Magnesium has three isotopes: ²⁴Mg (79%), ²⁵Mg (10%), ²⁶Mg (11%). Find its relative atomic mass.
Step-by-step solution
1. Step 1
  Aᵣ = Σ(mass × abundance%) ÷ 100.
2. Step 2
  (24 × 79 + 25 × 10 + 26 × 11) ÷ 100 = (1896 + 250 + 286) ÷ 100 = 2432 ÷ 100 = 24.32.
Answer
Aᵣ ≈ 24.3.
3Carbon-14 dating
Stretch• dating
Question
A wooden sample has 25% as much ¹⁴C as a fresh sample. How old is it (¹⁴C half-life = 5 730 years)?
Step-by-step solution
1. Step 1
  Activity halves each half-life: 100% → 50% → 25%. So 2 half-lives have passed.
2. Step 2
  Age = 2 × 5 730 = 11 460 years.
Answer
≈ 11 460 years old.

Key Formulae — Isotopes

The formulae you need to memorise for isotopes on the IB MYP Sciences paper, with every variable defined in plain English and a note on when to use it.

Relative atomic mass
$Aᵣ = Σ(mass × abundance%) / 100$
$mass$
mass number of each isotope
$abundance%$
% of that isotope in the natural mixture
When to use
Whenever an element has more than one natural isotope.

Key Definitions and Keywords — Isotopes

Definitions to memorise and the exact keywords mark schemes credit for isotopes answers — sharpened from recent examiner reports for the 2026 IB MYP Sciences sitting.

Isotope
Examiner keyword
Atoms of the same element (same Z) with different mass numbers (different numbers of neutrons).
Relative atomic mass (Aᵣ)
Examiner keyword
The weighted average mass of all the naturally-occurring isotopes of an element.
Radioactive isotope (radioisotope)
Examiner keyword
An isotope with an unstable nucleus that decays, emitting radiation.
Carbon-14 dating
Using the decay of ¹⁴C (half-life 5 730 years) to estimate the age of organic samples up to ~50 000 years old.

Common Mistakes and Misconceptions — Isotopes

The traps other students keep falling into on isotopes questions — taken from recent IB MYP Sciences examiner reports and mark schemes — and how to avoid them.

✕Treating isotopes as different elements.
Why it happens
Different mass numbers feel like different substances.
How to avoid it
Isotopes have the SAME atomic number — so they are the SAME element. Only the neutron count (and therefore the mass) differs.
✕Averaging just the mass numbers without weighting by abundance.
Why it happens
Looks like an average.
How to avoid it
Multiply each mass by its abundance percent, sum, divide by 100. A 50:50 mix gives the simple average; otherwise it's weighted.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Isotopes — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Isotopes

Short Study Notes in the form of Slides

Short Study Notes — Isotopes

Detailed Notes

What you’ll learn

How it’s examined

1p, n, e of a specific isotope

2Relative atomic mass from abundance

3Carbon-14 dating

Relative atomic mass

Isotope

Relative atomic mass (Aᵣ)

Radioactive isotope (radioisotope)

Carbon-14 dating

✕Treating isotopes as different elements.

✕Averaging just the mass numbers without weighting by abundance.

Practice questions

Past paper style quiz

Assess your understanding

Isotopes — frequently asked questions

Isotopes

Short Study Notes in the form of Slides

Short Study Notes — Isotopes

Detailed Notes

What you’ll learn

How it’s examined

1p, n, e of a specific isotope

2Relative atomic mass from abundance

3Carbon-14 dating

Relative atomic mass

Isotope

Relative atomic mass (Aᵣ)

Radioactive isotope (radioisotope)

Carbon-14 dating

✕Treating isotopes as different elements.

✕Averaging just the mass numbers without weighting by abundance.

Practice questions

Past paper style quiz

Assess your understanding

Isotopes — frequently asked questions