Isotopes and Relative Atomic Mass

This lesson covers isotopes and how to calculate relative atomic mass — as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 1: Key Concepts in Chemistry. You need to understand what isotopes are, why they have the same chemical properties, and be able to calculate relative atomic mass from isotopic abundance data.

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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. This means they have the same atomic number but different mass numbers.

Key Points About Isotopes

• Isotopes have the same atomic number (same number of protons).
• Isotopes have different mass numbers (different numbers of neutrons).
• Isotopes have the same electron configuration (because the number of electrons equals the number of protons in a neutral atom).
• Isotopes have the same chemical properties because chemical reactions depend on electron configuration, which is the same for all isotopes of an element.
• Isotopes have different physical properties (e.g. different mass, different density, different rate of diffusion).

Exam Tip: When explaining why isotopes have the same chemical properties, you MUST say it is because they have the same number of electrons (or the same electron configuration). Chemical properties depend on electrons, not on neutrons. This is a very commonly examined point.

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Examples of Isotopes

Isotopes of Carbon

Isotope	Protons	Neutrons	Mass Number	Electron Configuration
Carbon-12 (¹²C)	6	6	12	2, 4
Carbon-13 (¹³C)	6	7	13	2, 4
Carbon-14 (¹⁴C)	6	8	14	2, 4

All three isotopes have 6 protons (because they are all carbon) and 6 electrons (same chemical properties). They differ only in the number of neutrons.

Isotopes of Chlorine

Isotope	Protons	Neutrons	Mass Number
Chlorine-35 (³⁵Cl)	17	18	35
Chlorine-37 (³⁷Cl)	17	20	37

Chlorine has two stable isotopes. In a natural sample of chlorine, approximately 75% of atoms are ³⁵Cl and 25% are ³⁷Cl.

Isotopes of Hydrogen

Hydrogen is unique in that its isotopes have individual names:

Isotope	Name	Protons	Neutrons	Mass Number
¹H	Hydrogen (protium)	1	0	1
²H	Deuterium	1	1	2
³H	Tritium	1	2	3

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Relative Atomic Mass (Aᵣ)

Most elements exist as a mixture of isotopes in nature. The relative atomic mass (Aᵣ) of an element is the weighted average of the mass numbers of all its naturally occurring isotopes, taking into account their relative abundance (how common each isotope is).

Definition

Relative atomic mass (Aᵣ) is the weighted mean mass of an atom of an element compared to one-twelfth of the mass of a carbon-12 atom.

This is why relative atomic mass values on the periodic table are often not whole numbers — they are averages. For example, the Aᵣ of chlorine is 35.5, not 35 or 37.

Exam Tip: In the Edexcel exam, if you are asked to define relative atomic mass, say it is the "weighted mean mass of an atom of an element compared to 1/12th of the mass of a carbon-12 atom." The word weighted is essential — it means the more abundant isotopes contribute more to the average.

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Calculating Relative Atomic Mass

To calculate the Aᵣ from isotopic abundance data, use this method:

Formula

Aᵣ = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2) + ... ÷ total abundance

If abundances are given as percentages, the total abundance is 100.

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Worked Example 1: Chlorine

Chlorine has two isotopes:

• ³⁵Cl: abundance = 75%
• ³⁷Cl: abundance = 25%

Calculation:

Aᵣ = (35 × 75) + (37 × 25) ÷ 100

Aᵣ = (2625 + 925) ÷ 100

Aᵣ = 3550 ÷ 100

Aᵣ = 35.5

This is why the periodic table shows chlorine's relative atomic mass as 35.5.

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Worked Example 2: Copper

Copper has two isotopes:

• ⁶³Cu: abundance = 69%
• ⁶⁵Cu: abundance = 31%

Calculation:

Aᵣ = (63 × 69) + (65 × 31) ÷ 100

Aᵣ = (4347 + 2015) ÷ 100

Aᵣ = 6362 ÷ 100

Aᵣ = 63.62

The periodic table shows copper's Aᵣ as approximately 63.5.

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Worked Example 3: Magnesium (Three Isotopes)

Magnesium has three isotopes:

• ²⁴Mg: abundance = 79%
• ²⁵Mg: abundance = 10%
• ²⁶Mg: abundance = 11%

Calculation:

Aᵣ = (24 × 79) + (25 × 10) + (26 × 11) ÷ 100

Aᵣ = (1896 + 250 + 286) ÷ 100

Aᵣ = 2432 ÷ 100

Aᵣ = 24.32

The periodic table shows magnesium's Aᵣ as approximately 24.3.

Exam Tip: Always show your working clearly in calculations. Even if you get the final answer wrong, you can still earn marks for your method. Remember to divide by the total abundance (usually 100 if given as percentages).

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Interpreting Relative Atomic Mass

If the relative atomic mass of an element is close to one isotope's mass number, that isotope must be the most abundant:

• Chlorine has Aᵣ = 35.5, which is closer to 35 than 37 → ³⁵Cl is more abundant.
• Copper has Aᵣ = 63.5, which is closer to 63 than 65 → ⁶³Cu is more abundant.
• If Aᵣ = 63.5 exactly, this means the two isotopes are not equally abundant — the lighter one (63) is more common.

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Mass Spectrometry (Brief Overview)

Mass spectrometry is the technique used to determine the isotopic composition of an element — that is, which isotopes exist and their relative abundance. A mass spectrometer:

1. Vaporises and ionises the sample.
2. Separates the ions based on their mass-to-charge ratio.
3. Detects the ions and produces a mass spectrum showing the relative abundance of each isotope.