The big idea: Isotopes are atoms of the same element that have different numbers of neutrons.
Because they are the same element, they have the same number of protons — the same atomic number, Z. What differs is the number of neutrons, so they have a different mass number, A.
In short: same protons, different neutrons.
Define the terms: - Atomic number, Z — the number of protons (this is what makes an element that element). - Mass number, A — the total number of protons + neutrons. - Isotopes — atoms with the same Z but different A (because they have different numbers of neutrons).
Chlorine-35: 17 protons, 18 neutrons. Electron arrangement 2,8,7.
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Two isotopes of chlorine: Both atoms below are chlorine because both have 17 protons. Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons — that is the only difference.
Chlorine-37: 17 protons, 20 neutrons. Same electron arrangement 2,8,7.
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An atom's chemistry is decided by its electrons — specifically the electron arrangement (the number of outer electrons). Neutrons have no charge and sit in the nucleus, so adding or removing them does not change how the atom bonds or reacts.
Isotopes of an element have the same number of protons, therefore the same number of electrons and the same electron arrangement. This is why isotopes show identical chemical properties — they react in exactly the same way.
Stays the SAME (chemical)
- Number of protons (same Z)
- Number of electrons and electron arrangement
- Chemical reactions and bonding — identical
CHANGES (physical)
- Number of neutrons → different mass number A
- Mass of the atom (heavier or lighter)
- Physical properties that depend on mass — density, rate of diffusion, boiling/melting point
| Chlorine-35 | Chlorine-37 | |
|---|---|---|
| Atomic number, Z (protons) | 17 | 17 |
| Mass number, A | 35 | 37 |
| Neutrons (A − Z) | 18 | 20 |
| Electrons | 17 | 17 |
| Electron arrangement | 2,8,7 | 2,8,7 |
| Chemical behaviour | identical | identical |
| Mass / density | lighter | heavier |
Why this matters: Heavier isotopes make a heavier, slightly denser substance that diffuses more slowly. But the chemistry is unchanged — for example, water made with heavy hydrogen (deuterium, ²H) reacts just like ordinary water, even though it is denser.
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Some isotopes have an unstable nucleus (too many or too few neutrons). These radioisotopes decay and give out radiation. Because chemistry is unchanged, a radioisotope is taken up by the body or a reaction in exactly the same way as the stable isotope — which makes it useful as a 'tag'.
Everyday uses: - Carbon-14 — radioactive dating of once-living material. - Cobalt-60 — gamma source used in cancer radiotherapy and to sterilise equipment. - Iodine-131 — diagnosis and treatment of the thyroid (the body handles it like ordinary iodine).
Reading isotope notation: An isotope can be written as a name + mass number (e.g. carbon-14) or in nuclear notation with the mass number A on top and the atomic number Z below the symbol.
Neutrons = A − Z. For carbon-14: neutrons = 14 − 6 = 8.
How this is tested: Isotopes show up as a quick Paper 1A MCQ — usually 'which property is shared by two isotopes?' (answer: a chemical property, because the electron arrangement is the same) or 'which physical property differs?' (answer: mass / density).
On Paper 2 it becomes a short explain: why isotopes react identically but differ physically.
The marking idea: Tie 'same chemistry' to the electrons / electron arrangement, and 'different physical properties' to the extra neutrons → greater mass. Naming the link earns the mark — don't just state the difference.
IB-style question — isotopes of magnesium
Magnesium-24 and magnesium-26 are isotopes of the same element. Explain why they have identical chemical properties but different physical properties. [3]
How to score the marks
- Mark 1 — same electrons. Both isotopes have the same number of protons (12), so they have the same number of electrons and the same electron arrangement (2,8,2).
- Mark 2 — link to chemistry. Chemical properties depend on the (outer) electrons, so identical electron arrangements give identical chemical reactions.
- Mark 3 — the physical difference. Magnesium-26 has two more neutrons than magnesium-24, giving it a greater mass (and so a higher density / slower diffusion). Neutrons do not affect bonding, so only physical properties change.
Final answer
Same protons → same electrons / electron arrangement → identical chemistry; Mg-26 has more neutrons → greater mass → different physical properties (e.g. density).