Isotopes — same protons, different neutrons
Isotopes are atoms of the same element with the same proton (atomic) number but a different number of neutrons, and therefore a different mass (nucleon) number. Sharing a proton number means they occupy the same Periodic Table position — "isotope" is Greek for "same place". Carbon-12, carbon-13 and carbon-14 all have 6 protons (6, 7 and 8 neutrons); chlorine-35 and chlorine-37 both have 17 protons (18 and 20 neutrons).
Nuclide symbols — top is A, bottom is Z
A nuclide symbol carries the mass (nucleon) number (protons + neutrons) on top and the proton (atomic) number (protons only) below, so has 6 protons and neutrons. An ion adds its charge at the top right, e.g. : protons and neutrons read exactly as for the atom (17 and 18), but electrons no longer equal protons — one electron has been gained, giving 18.
(Extended) Same configuration, same chemistry
Chemical behaviour is set by the electron arrangement, above all by the number of outer-shell electrons. Every isotope of an element has the same proton number, so a neutral atom of each has the same number of electrons and therefore the same electron configuration — the extra neutrons add no electrons. Isotopes therefore react identically, yet physical properties that depend on mass, such as density and rate of diffusion, can still differ.
Drawn from real examiner reports.
Nucleon number, not "nucleus number"
The top number of a nuclide symbol is the mass number, also called the nucleon number — the total of protons and neutrons. "Nucleus number" is not accepted and loses the definition mark even when every figure in the working is correct. Use "nucleon number" or "mass number", and reserve "nucleus" for the structure itself.
Flagged Jun 2022 P12 Q15
Half a definition scores half marks
A vague answer — "atoms that are different", "atoms with different masses" — says neither what is the same nor what differs, and scores zero. The mark scheme wants both halves: the same number of protons (so the same element) AND a different number of neutrons (so a different mass number). Giving one half alone typically earns only one of the two marks.
(Extended) "Same electrons" misses the outer shell
Repeating the isotope definition — "same protons, different neutrons" — does not explain shared chemistry, because nucleons do not control reactions. The chain is: same proton number, so the same number of electrons, so the same electron configuration and outer shell; reactions are governed by outer-shell electrons. Stopping before the configuration step loses the mark.
Flagged Jun 2022 P42 Q8bi · Jun 2023 P42 Q8b · Nov 2023 P42 Q2aiii
The top number is not the proton count
Mass number, proton number and neutron number are three different quantities. Reading as having 35 protons, or adding the two numbers instead of subtracting them, are the recurring slips. Protons are always the bottom number, and neutrons are always top minus bottom: .
Flagged Nov 2023 P11 Q40
Same mass number ≠ isotopes
Isotopes must share a proton number, not a mass number. and both have mass number 40 but 18 and 20 protons, so they are different elements — argon and calcium — and not isotopes of each other. Always check the bottom number before calling two species isotopes.
Ion charge: add or subtract electrons?
A negative ion has gained electrons, so electrons = protons plus the size of the charge; a positive ion has lost them, so electrons = protons minus the charge. Applying the wrong sign turns into 14 electrons instead of 18. Protons and neutrons are never changed by forming an ion.
Isotopes are not identical in every way
Isotopes of an element have identical chemical properties, but their physical properties can differ because the neutron count changes the mass — density and rate of diffusion are the standard examples. "Isotopes are exactly the same" and "isotopes differ in every property" both overshoot; name chemical or physical explicitly.
Three fixed steps for any symbol
Protons = the bottom number , always. Neutrons = top minus bottom, , always. Electrons = , plus the charge for a negative ion or minus it for a positive one. Check on : 13 protons, neutrons, electrons.
The charge touches electrons only
Gaining or losing electrons does not change the nucleus, so the proton and neutron counts of an ion are read straight off the symbol exactly as for the neutral atom. Only the electron count is adjusted. State protons and neutrons first, then deal with the charge last.
Match define, state and explain
"Define isotopes" wants the full two-part definition. "State" a particle count wants the value with the subtraction shown. "Explain" (Extended) why isotopes share chemistry wants the electrons-to-configuration-to-outer-shell chain, not the definition repeated.
Cambridge 0654 spec reference: Section C2 "Atoms, elements and compounds", sub-topic C2.3. Core covers the definition of isotopes and interpreting/using nuclide symbols for atoms and ions. Extended (Supplement) adds the reasoning for why isotopes of the same element have identical chemical properties.
| Term | Mark-scheme-precise meaning |
|---|---|
| Isotopes | Atoms of the same element that have the same number of protons but a different number of neutrons |
| Mass (nucleon) number, | The total number of protons and neutrons in the nucleus of an atom -- the TOP number in a nuclide symbol |
| Proton (atomic) number, | The number of protons in the nucleus of an atom -- the BOTTOM number in a nuclide symbol |
Nuclide symbol (for an atom or ion):
Reading rules:
(Extended) Isotopes of the same element have the same chemical properties because they have the same number of electrons, and therefore the same electron configuration (same outer-shell arrangement) -- and chemical reactions are governed by outer-shell electrons. Isotopes can still differ in physical properties that depend on mass (e.g. density, rate of diffusion) because they have different numbers of neutrons.
For an ion, how do you find the number of electrons from the proton number and the charge?
Define the term isotopes. (2 marks)
