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1.1.2Physical Chemistry

Mass Number and Isotopes — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 2 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6What Is an Isotope?

Atoms of the same element can have different masses.

Definition
  • Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Mass number
  • Mass number Different neutron numbers give different mass numbers, as in carbon-12 and carbon-14.
Notation
  • Notation Isotopes are written with their mass number, for example ³⁵Cl and ³⁷Cl, or chlorine-35 and chlorine-37.
Same atomic number
  • Same atomic number All isotopes of an element have the same atomic number because that number defines the element.

Part 2 of 6Chemical and Physical Properties

Chemical and Physical Properties
Chemical
  • Chemical Isotopes have the same electron configuration, so they react in the same way and form the same compounds.
Physical
  • Physical Differences in mass cause differences in density, melting point, boiling point and rate of diffusion.
Diffusion
  • Diffusion Heavier isotopes diffuse more slowly than lighter ones at the same temperature.
Example
  • Example Hydrogen has isotopes with 0, 1 and 2 neutrons, all of which form water with the same chemical behaviour.

Part 3 of 6Relative Atomic Mass from Abundance

Ar is the weighted mean of the isotopic masses.

1
List each isotope’s mass number and its percentage abundance.
2
Multiply each mass number by its percentage abundance.
3
Add all of these products together to give a single total for the sample.
4
Divide the total by 100 (or by the sum of the abundances if they are given as relative amounts).
5
Quote the answer to a sensible number of significant figures, and remember relative atomic mass has no units.

Part 4 of 6Reading Data

Percentages
  • Percentages If the abundances are given as percentages, they must add up to 100 as a quick check.
Ratios
  • Ratios If a mass spectrum shows relative peak heights, use the heights in place of percentages and divide by their total.
Check
  • Check The answer must lie between the lightest and heaviest isotope mass numbers, and nearer the more abundant isotope.
Not whole numbers
  • Not whole numbers Most Ar values are not whole numbers because they are averages over several isotopes.

Part 5 of 6Worked Example

Question

  • Boron has two isotopes: ¹⁰B (19.9 %) and ¹¹B (80.1 %).
  • Calculate the relative atomic mass of boron to 3 significant figures.

  1. Multiply10 × 19.9 = 199.0 and 11 × 80.1 = 881.1.
  2. Add the products199.0 + 881.1 = 1080.1.
  3. Divide by 100: 1080.1 ÷ 100 = 10.801.
  4. Round to 3 significant figures.

Answer Ar of boron = 10.8

Part 6 of 6Exam Checklist & Tips

1.1.2 – Key Points for the Exam

  • Isotopes Atoms of the same element with the same atomic number but different mass numbers.
  • Relative atomic mass The weighted mean mass of an atom of an element compared with one-twelfth of the mass of a carbon-12 atom.
  • Percentage abundance The proportion of atoms of an element that are a particular isotope.
  • Nucleon A particle in the nucleus, that is a proton or a neutron.

Common Mistakes

  • Saying isotopes have different numbers of protons; they differ only in neutrons.
  • Claiming isotopes react differently, when their identical electron arrangements give the same chemical behaviour.
  • Forgetting to divide by 100 after multiplying mass numbers by percentages.
  • Giving relative atomic mass units such as grams or g mol⁻¹.

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