IGCSE Isotopes: Complete Guide for Cambridge IGCSE Chemistry

IGCSE isotopes are atoms of the same element with different numbers of neutrons. Mastering isotope notation, relative atomic mass calculations, and understanding how isotopes affect element properties is essential for achieving top grades in IGCSE Chemistry exams.

This comprehensive IGCSE isotopes guide covers everything you need to know, including what isotopes are, how to identify them, isotope notation, calculating relative atomic mass, worked examples, common exam questions, and expert tips from Tutopiya’s IGCSE chemistry tutors.

🎯 What you’ll learn: By the end of this guide, you’ll know how to define isotopes, identify isotopes of elements, calculate relative atomic mass, and apply these concepts to solve problems in IGCSE Chemistry exams.

Already studying with Tutopiya? Practice these skills with our dedicated IGCSE Chemistry practice deck featuring exam-style questions and instant feedback.

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Why IGCSE Isotopes Matter

IGCSE isotopes are important concepts that appear regularly in IGCSE Chemistry. Here’s why:

• High frequency topic: Isotope questions appear in many IGCSE chemistry papers
• Foundation concept: Essential for understanding relative atomic mass
• Exam weight: Typically worth 4-8 marks per paper
• Real-world applications: Used in radiometric dating, medical imaging, and nuclear applications

Key insight from examiners: Students often confuse isotopes with ions or struggle with relative atomic mass calculations. This guide will help you master these concepts.

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What are Isotopes?

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Key Characteristics

• Same atomic number: Same number of protons (same element)
• Different mass number: Different numbers of neutrons
• Same chemical properties: Same electron configuration
• Different physical properties: Slightly different masses

Example: Carbon Isotopes

Carbon-12 (¹²C):

• 6 protons, 6 neutrons
• Mass number = 12
• Most common isotope

Carbon-13 (¹³C):

• 6 protons, 7 neutrons
• Mass number = 13

Carbon-14 (¹⁴C):

• 6 protons, 8 neutrons
• Mass number = 14
• Radioactive (used in carbon dating)

All are carbon because they all have 6 protons.

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Isotope Notation

Isotopes are written as: ᴬX where A = mass number, X = element symbol

Examples:

• ¹²C - Carbon-12
• ²³⁵U - Uranium-235
• ¹H - Hydrogen-1 (protium)
• ²H - Hydrogen-2 (deuterium)
• ³H - Hydrogen-3 (tritium, radioactive)

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Relative Atomic Mass

Relative atomic mass (Ar) is the weighted average mass of all isotopes of an element compared to carbon-12.

Calculating Relative Atomic Mass

Formula: Relative Atomic Mass = Σ (isotope mass × % abundance) / 100

Or: Relative Atomic Mass = Σ (isotope mass × fractional abundance)

Worked Example

Chlorine has two isotopes:

• Chlorine-35: 75% abundance
• Chlorine-37: 25% abundance

Calculate relative atomic mass.

Solution: Relative Atomic Mass = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5

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Common Examiner Traps

• Confusing isotopes with ions - Isotopes differ in neutrons; ions differ in electrons
• Calculation errors - Remember to use abundance as percentage or fraction correctly
• Not averaging correctly - Must use weighted average, not simple average

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Differences Between Isotopes and Ions

Isotopes vs Ions

Feature	Isotopes	Ions
Difference	Different number of neutrons	Different number of electrons
Protons	Same	Same
Neutrons	Different	Same
Electrons	Same (in neutral atoms)	Different
Element	Same element	Same element
Chemical properties	Same	Different (due to charge)

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More Examples of Isotopes

Hydrogen Isotopes

Hydrogen-1 (Protium):

• 1 proton, 0 neutrons
• Most common (99.98%)
• Notation: ¹H

Hydrogen-2 (Deuterium):

• 1 proton, 1 neutron
• Notation: ²H or D

Hydrogen-3 (Tritium):

• 1 proton, 2 neutrons
• Radioactive
• Notation: ³H or T

Chlorine Isotopes

Chlorine-35:

• 17 protons, 18 neutrons
• 75% abundance
• Mass number = 35

Chlorine-37:

• 17 protons, 20 neutrons
• 25% abundance
• Mass number = 37

Relative atomic mass = 35.5 (weighted average)

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Radioactive Isotopes

Some isotopes are radioactive (unstable) and decay over time.

Uses of Radioactive Isotopes

Carbon-14 Dating:

• Used to date organic materials
• Carbon-14 decays at known rate

Medical Applications:

• Radioactive tracers in medicine
• Cancer treatment

Industrial Applications:

• Checking for leaks
• Sterilization

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Calculating Relative Atomic Mass - Detailed Examples

Example 1: Two Isotopes

Boron has two isotopes:

• Boron-10: 20% abundance
• Boron-11: 80% abundance

Calculate relative atomic mass.

Solution: Relative Atomic Mass = (10 × 20 + 11 × 80) / 100 = (200 + 880) / 100 = 1080 / 100 = 10.8

Example 2: Three Isotopes

Magnesium has three isotopes:

• Magnesium-24: 79% abundance
• Magnesium-25: 10% abundance
• Magnesium-26: 11% abundance

Calculate relative atomic mass.

Solution: Relative Atomic Mass = (24 × 79 + 25 × 10 + 26 × 11) / 100 = (1896 + 250 + 286) / 100 = 2432 / 100 = 24.32

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Step-by-Step Method for Isotope Problems

1. Identify the isotopes - What are the mass numbers?
2. Check abundances - Are they given as percentages or fractions?
3. Apply formula - Use weighted average formula
4. Calculate carefully - Show all working
5. Check answer - Should be between the isotope masses

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Worked Examples

Example 1: Identifying Isotopes

Explain why ¹²₆C and ¹⁴₆C are isotopes of carbon.

Solution: Both have:

• Same atomic number (6) → same number of protons → same element (carbon)
• Different mass numbers (12 vs 14) → different numbers of neutrons
• Carbon-12 has 6 neutrons; Carbon-14 has 8 neutrons
• Therefore, they are isotopes of carbon

Example 2: Relative Atomic Mass

Copper has two isotopes: ⁶³Cu (69%) and ⁶⁵Cu (31%). Calculate relative atomic mass.

Solution: Relative Atomic Mass = (63 × 69 + 65 × 31) / 100 = (4347 + 2015) / 100 = 6362 / 100 = 63.62

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Common Examiner Traps (and How to Dodge Them)

• Confusing isotopes with ions - Isotopes: different neutrons; Ions: different electrons
• Using simple average - Must use weighted average based on abundance
• Abundance units - Check if percentage or fraction, convert if needed
• Calculation errors - Double-check arithmetic
• Not understanding notation - Mass number is top left number

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IGCSE Isotopes Practice Questions

Question 1: Definition

What are isotopes?

Solution: Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, resulting in different mass numbers.

Question 2: Identification

Explain why ¹H, ²H, and ³H are all isotopes of hydrogen.

Solution: All have 1 proton (atomic number 1), so they are the same element (hydrogen). They have different mass numbers (1, 2, 3) because they have different numbers of neutrons (0, 1, 2). Therefore, they are isotopes.

Question 3: Relative Atomic Mass

An element X has two isotopes:

• X-20: 90% abundance
• X-22: 10% abundance

Calculate the relative atomic mass of X.

Solution: Relative Atomic Mass = (20 × 90 + 22 × 10) / 100 = (1800 + 220) / 100 = 2020 / 100 = 20.2

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Frequently Asked Questions About IGCSE Isotopes

What are isotopes?

Isotopes are atoms of the same element (same number of protons) that have different numbers of neutrons, giving them different mass numbers.

How do isotopes differ from each other?

Isotopes differ only in the number of neutrons. They have the same number of protons (same element) and the same number of electrons (in neutral atoms), but different mass numbers.

Do isotopes have the same chemical properties?

Yes, isotopes have the same chemical properties because they have the same electron configuration. However, they may have slightly different physical properties due to different masses.

How do you calculate relative atomic mass?

Use the weighted average formula: Relative Atomic Mass = Σ (isotope mass × abundance) / 100 (if abundances are percentages).

What’s the difference between isotopes and ions?

Isotopes have different numbers of neutrons; ions have different numbers of electrons (and thus different charges).

Why do we use relative atomic mass instead of mass number?

Most elements exist as mixtures of isotopes. Relative atomic mass gives the average mass based on natural abundance, which is more accurate than using a single isotope’s mass.

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Related IGCSE Chemistry Resources

Strengthen your IGCSE Chemistry preparation with these comprehensive guides:

• IGCSE Atomic Structure and the Periodic Table: Complete Guide - Master atomic structure and periodic trends
• IGCSE Ions and Ionic Bonds: Complete Guide - Master ionic bonding
• IGCSE Chemistry Revision Notes, Syllabus and Preparation Tips - Complete syllabus overview, topic breakdown, and revision strategies
• IGCSE Past Papers Guide - Access free IGCSE past papers and exam resources

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