AQA A-Level Chemistry 3.1.1.2 – What Are Mass Numbers and Isotopes?

Mastering mass numbers and isotopes gives students an edge in calculations, confident terminology use, and interpretation of real-world data.

Section 3.1.1.2 of the AQA A-Level Chemistry syllabus builds on the concept of fundamental particles by introducing two crucial ideas: mass number and isotopes.

While this topic is often introduced at GCSE, A-Level requires students to apply the knowledge more deeply — including in calculations and interpretation of data like mass spectrometry. In this post, we’ll explain what’s covered, why it matters, and how students can use it to boost their exam technique and scientific understanding.

What Is Mass Number?

The mass number of an atom is the total number of protons and neutrons in its nucleus.

It is represented by the symbol A, and sits at the top in nuclear notation:

E.g.
¹⁴₆C
Mass number (A) = 14
Atomic number (Z) = 6

Students should be confident in:

• Recognising mass number in isotope notation

• Using it to calculate the number of neutrons:
  Neutrons = Mass number − Atomic number

What Are Isotopes?

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

For example, carbon has two common isotopes:

• ¹²C → 6 protons, 6 neutrons

• ¹⁴C → 6 protons, 8 neutrons

Both are carbon, because they have 6 protons (atomic number = 6), but they differ in mass number due to extra neutrons.

Key Points Students Need to Know

• The atomic number (Z) identifies the element (number of protons).

• The mass number (A) tells us the total of protons and neutrons.

• Electrons are not included in the mass number.

• Isotopes have the same chemical properties (due to same electron configuration) but different physical properties (due to different mass).

Why Is This Topic Important?

Understanding mass number and isotopes is foundational for many areas of chemistry. It directly links to:

• Relative atomic mass (Ar)

• Mass spectrometry (next topic in the syllabus)

• Nuclear equations and reactions

• The periodic table – which uses weighted average of isotopes

• Radioactive decay (in nuclear chemistry and environmental chemistry)

Without clarity on these definitions and concepts, students will struggle with both calculations and conceptual reasoning in future topics.

Opportunities for Skills Development

Like other early A-Level topics, 3.1.1.2 helps students sharpen key scientific and exam skills:

1. Mathematical Skills

• Subtracting atomic number from mass number to calculate neutrons

• Interpreting and calculating weighted averages for relative atomic mass (Ar) from isotopic abundances

• Rearranging and applying formulas

• Using percentage or decimal abundance data

2. Symbol Interpretation

• Reading and writing standard isotope notation

• Linking numbers to real particle counts

• Recognising ions and charged particles

3. Scientific Reasoning

• Explaining why isotopes of the same element behave identically in reactions

• Applying particle numbers to make predictions about behaviour and trends

4. Exam Technique
Common exam questions include:

• “State the number of protons, neutrons, and electrons in…”

• “Explain why two isotopes have the same chemical properties.”

• “Calculate the relative atomic mass from given data.”

These are often short but high-reward questions that test precision and understanding, rather than extended writing.

How Relative Atomic Mass Links In

Once students understand isotopes, they can appreciate why the Ar on the periodic table is rarely a whole number.

For example:

Chlorine has two common isotopes:
³⁵Cl and ³⁷Cl
If 75% is ³⁵Cl and 25% is ³⁷Cl:

Ar = (75 × 35 + 25 × 37) ÷ 100 = 35.5

This kind of calculation often appears in Paper 1 and reinforces concepts introduced here.

Tips for Students Studying This Section

• Always subtract to calculate neutrons: mass number − atomic number

• Practise Ar calculations with sample data from isotopes

• Remember: isotopes = same protons, different neutrons

• Use visual models to see how different isotopes of the same element are structured

• Know your terminology — don’t confuse atomic number and mass number

Table: Quick Reference for Isotope Notation

SymbolAtomic Number (Z)Mass Number (A)Neutrons¹²C6126¹³C6137¹⁴C6148

Summary: A Key Link in Atomic Understanding

Section 3.1.1.2 may be short, but it unlocks a deeper understanding of the structure of matter. It’s the bridge between simple atomic theory and more advanced applications like relative mass, spectroscopy, and radioactivity.

Mastering mass numbers and isotopes gives students an edge in calculations, confident terminology use, and interpretation of real-world data. It’s an excellent chance to consolidate their early progress at A-Level and build toward success.

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