2.3 Isotopes
- Syllabus
- 0620–2026–2027
- Topic
- 2.3
- Level
- —
Learning objectives
2.3.1Isotopes as different atoms of the• Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons2.3.2Symbols for atoms and ions• Interpret/use symbols for atoms and ions, e.g. 12C and 35Cl- notation with mass/nucleon and proton/atomic numbers2.3.3Isotopes of the same element have the• State: isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration2.3.4Relative atomic mass of an element• Calculate the relative atomic mass of an element from the relative masses and abundances of its isotopes
Identify isotopes of the same element
Isotopes are different atoms of the same element with the same number of protons but different numbers of neutrons.
| Feature | Isotopes of one element |
|---|---|
| proton number Z | same, so they are the same element |
| neutron number | different |
| mass number A | different because A = protons + neutrons |
| electrons in neutral atoms | same as the proton number |
To test a pair, compare proton numbers first: if they differ, the atoms are different elements. If proton numbers match, compare neutron or mass numbers; a difference confirms isotopes.
Same mass number does not make two atoms isotopes. Isotopes must first have the same proton number and therefore the same element identity.
Read and write atom and ion symbols
Write a particle as ᴬ_ZX with ionic charge at the upper right: X is the element symbol, A is mass/nucleon number and Z is proton/atomic number.
| Symbol information | Particle count |
|---|---|
| proton number Z | protons = Z |
| mass number A | neutrons = A − Z |
| no ionic charge | electrons = Z |
| charge n+ | electrons = Z − n |
| charge n− | electrons = Z + n |
To build a symbol from counts, use the proton number to identify X, add protons and neutrons for A, then compare protons with electrons to find the charge. More protons than electrons gives positive charge; more electrons gives negative charge.
For ³⁷₁₇Cl⁻: protons = 17, neutrons = 37 − 17 = 20 and electrons = 18. The charge changes electron count, not A or Z.
Do not put the ionic charge at the lower left or use it to change neutron number. A and Z describe the nucleus; charge describes the proton–electron imbalance.
Explain why isotopes react alike
Neutral atoms of isotopes of the same element have the same proton number, so they have the same number of electrons and the same electronic configuration.
| Nuclear comparison | Electron consequence | Chemical consequence |
|---|---|---|
| same proton number | same total electrons in neutral atoms | same electronic configuration |
| same electronic configuration | same number of outer-shell electrons | same chemical properties and reactions |
| different neutron number | different mass | does not change the electron arrangement |
A complete explanation links the same number of electrons to the same electronic configuration, especially the same outer-shell arrangement that controls bonding and reactions.
Different neutron numbers can change mass and some physical or nuclear properties, but they do not give isotopes different ordinary chemical properties.
Calculate relative atomic mass from isotopes
Relative atomic mass, Aᵣ, is the abundance-weighted mean of the relative masses of an element's naturally occurring isotopes.
Aᵣ = Σ(isotope relative mass × abundance) ÷ Σ(abundance). For percentage abundances, the denominator is 100; for a ratio, it is the sum of the ratio parts.
| Isotope mass | Percentage abundance | Weighted contribution |
|---|---|---|
| 63 | 70 | 63 × 70 = 4410 |
| 65 | 30 | 65 × 30 = 1950 |
| total | 100 | 6360 |
| Aᵣ | 6360 ÷ 100 = 63.6 |
The answer must lie between the lightest and heaviest isotope masses and be closer to the mass of the more abundant isotope. Round only as requested after completing the weighted sum.
Do not take a simple mean unless the isotopes are equally abundant, and do not use proton number in place of isotope mass.