Isotopes
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge IGCSE Chemistry (0620)
What are Isotopes?
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons - so they share the same atomic number but have different mass numbers.
- Same protons → same element, same atomic number
- Different neutrons → different mass number
- Same electrons (in a neutral atom) → identical electronic configuration
Examples
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 (12₆C) | 6 | 6 | 6 |
| Carbon-13 (13₆C) | 6 | 7 | 6 |
| Carbon-14 (14₆C) | 6 | 8 | 6 |
| Chlorine-35 (35₁₇Cl) | 17 | 18 | 17 |
| Chlorine-37 (37₁₇Cl) | 17 | 20 | 17 |
All three nuclei have 6 protons - the same element - but different numbers of neutrons, giving different mass numbers
Isotopes Have Identical Chemical Properties
- Chemical properties depend on the number and arrangement of electrons, not neutrons
- All isotopes of an element have the same number of electrons, so they have identical electronic configurations
- Therefore, isotopes of the same element react in exactly the same way chemically
The main difference between isotopes is their mass - heavier isotopes have a slightly greater mass and density, and may diffuse slightly more slowly, but this does not change how they react.
Calculating Relative Atomic Mass from Isotopic Abundance
Most elements exist naturally as a mixture of isotopes. The relative atomic mass (Ar) is the weighted average mass of all the isotopes, measured relative to 12C.
Worked Example: Relative Atomic Mass of Chlorine
- Question: chlorine exists as two isotopes, 35Cl (75% abundance) and 37Cl (25% abundance). Calculate the relative atomic mass of chlorine. [2]
- Answer: Ar = [(35 × 75) + (37 × 25)] ÷ 100 [1]; Ar = (2625 + 925) ÷ 100 = 35.5 [1]
This is why chlorine's relative atomic mass (35.5) is not a whole number - it is the weighted average of two isotopes present in different proportions.
Common Mistakes
- ❌ Thinking isotopes are different elements
- ✓ isotopes are different forms of the same element - they share the same atomic number (proton number), only their neutron count differs
- ❌ Thinking isotopes have different chemical properties
- ✓ isotopes have identical chemical properties because they have the same number of electrons; only some physical properties (like mass) differ slightly
- ❌ Simply averaging the mass numbers without weighting by abundance
- ✓ each isotope's mass must be multiplied by its percentage abundance before adding and dividing by 100 - an element that is 90% one isotope should be much closer to that isotope's mass, not a simple midpoint
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