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1.2.3Additional lessons

Isotopes — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 5What Isotopes Are

1.2.3 Isotopes

Atoms of the same element are not always identical.

Definition
  • Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Mass difference
  • Different neutron numbers mean different mass numbers, for example carbon-12 and carbon-14.
Chemical behaviour
  • They have the same electron configuration, so chemical properties are essentially identical.
Physical behaviour
  • Physical properties that depend on mass, such as density and rate of diffusion, differ slightly.

Part 2 of 5Relative Atomic Mass

Most elements are a mixture of isotopes, so the relative atomic mass is a weighted mean.

1
Find the percentage abundance of each isotope, which is usually read from a mass spectrum.
2
Multiply each isotope's mass number by its abundance and add the results.
3
Divide the total by 100 if percentages were used, to give the weighted mean relative atomic mass.
4
The result is generally not a whole number because it averages over several isotopes.

Part 3 of 5Mass Spectrum

Mass Spectrum
Axes
  • The horizontal axis shows the mass-to-charge ratio, m/z, and the vertical axis shows relative abundance.
Peaks
  • Each peak represents one isotope, and the height of the peak shows how abundant it is.
Ionisation
  • In the spectrometer the atoms are turned into positive ions, then accelerated, deflected and detected.
Reading
  • For singly charged ions, m/z is the mass number of the isotope.

Part 4 of 5Worked Example

Question

  • Chlorine has two isotopes: ³⁵Cl (75.8%) and ³⁷Cl (24.2%).
  • Calculate its relative atomic mass to 3 significant figures.

  1. Contribution of ³⁵Cl = 35 × 75.8 = 2653.
  2. Contribution of ³⁷Cl = 37 × 24.2 = 895.4.
  3. Total = 3548.4, divided by 100 = 35.484.

Answer 35.5

Part 5 of 5Exam Checklist & Tips

1.2.3 – Key Points for the Exam

  • Isotopes Atoms of one element with the same atomic number but different mass numbers.
  • Relative atomic mass The weighted mean mass of an atom of an element relative to one twelfth of the mass of a carbon-12 atom.
  • Abundance The proportion of atoms of an element that are a particular isotope.
  • m/z Mass-to-charge ratio of an ion shown on a mass spectrum.

Common Mistakes

  • Saying that isotopes have a different number of protons.
  • Giving a simple average of mass numbers rather than a weighted mean.
  • Stating that isotopes of an element react differently because of different electron arrangements.
  • Forgetting to divide by 100 (or by total abundance) when calculating a weighted mean.

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