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1.2.3Additional lessons

Isotopes — IB Chemistry HL Revision Notes & Topical Questions

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📚 IB Chemistry HL 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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Section 1 of 5What Isotopes Are

Atoms of the same element can have different numbers of neutrons.

Definition
  • Definition Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Notation
  • Notation Isotopes are distinguished by mass number, such as carbon-12 and carbon-14, or ¹²C and ¹⁴C.
Same chemistry
  • Same chemistry The electron arrangement is the same, so isotopes undergo the same chemical reactions.
Different physics
  • Different physics Mass and properties depending on mass, such as density and rate of diffusion, are different.
IsotopeProtonsNeutronsElectrons (atom)
¹H101
²H111
³⁵Cl171817
³⁷Cl172017

Section 2 of 5Relative Atomic Mass

Relative Atomic Mass
Meaning
  • Meaning The relative atomic mass Ar is the weighted mean mass of the atoms of an element relative to 1/12 of a ¹²C atom.
Calculation
  • Calculation Multiply each isotopic mass by its percentage abundance, add the results and divide by 100.
Why not whole numbers
  • Why not whole numbers Ar is an average over several isotopes, so it is usually not a whole number.
Use
  • Use The Ar values in the data booklet are used for calculating molar masses.

Section 3 of 5Radioactive Isotopes

Some isotopes have unstable nuclei and are radioactive.

1
Carbon-14 has two more neutrons than carbon-12 and is radioactive, decaying over thousands of years.
2
Its decay is used in radiocarbon dating of organic remains.
3
Cobalt-60 emits gamma radiation that is used to sterilise medical equipment.
4
Iodine-131 is used in medicine because the thyroid gland absorbs iodine.
5
Each application relies on the fact that the chemistry of the radioactive isotope matches that of the stable isotope.

Section 4 of 5Worked Example

Question

  • Chlorine is 75.8% ³⁵Cl and 24.2% ³⁷Cl.
  • Calculate its relative atomic mass.

  1. Ar = (75.8 × 35 + 24.2 × 37) ÷ 100.
  2. = (2653 + 895.4) ÷ 100 = 35.48.

Answer Ar = 35.5 (3 s.f.)

Section 5 of 5Exam Checklist & Tips

1.2.3 – Key Points for the Exam

  • Isotopes Atoms of an 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 1/12 of the mass of carbon-12.
  • Abundance The percentage of atoms of an element that are a given isotope.
  • Radioactive Describes a nucleus that is unstable and emits radiation.

Common Mistakes

  • Saying isotopes have different numbers of electrons or protons.
  • Stating that isotopes have different chemical properties.
  • Forgetting to divide by 100 when using percentage abundances.
  • Quoting Ar as a whole number when the data show a mix of isotopes.

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