Isotopes are atoms of the same element with the same proton number but different numbers of neutrons. They are the same element because the proton number names the element, and they have different masses because the neutron count differs. Isotope questions ask you to compare particle counts, explain why chemical behaviour matches, or calculate an average atomic mass.
This lesson extends reading proton and nucleon numbers and sits within atoms, elements and isotopes.
How do I compare two isotopes?
Write both symbols and count the particles, using the same steps as before. Then state what is the same and what differs.
| Feature | Same or different? | Reason |
|---|---|---|
| Protons | Same | Defines the element |
| Electrons (neutral atom) | Same | Equal to protons |
| Neutrons | Different | The only particle that varies |
| Nucleon number | Different | Protons + neutrons |
| Chemical reactions | Same | Same electron arrangement |
Worked example
Chlorine has two main isotopes, 35/17 Cl and 37/17 Cl. Compare them, then calculate the relative atomic mass.
The percentages below are rounded teaching values, not exact measurements: 75% is 35/17 Cl and 25% is 37/17 Cl.
Step 1, particles in 35/17 Cl: 17 protons, 17 electrons, 35 − 17 = 18 neutrons.
Step 2, particles in 37/17 Cl: 17 protons, 17 electrons, 37 − 17 = 20 neutrons.
Step 3, compare: both have 17 protons and 17 electrons, so both are chlorine with identical chemical behaviour. They differ by two neutrons.
Step 4, weighted average: multiply each nucleon number by its share, then add.
(35 × 0.75) + (37 × 0.25) = 26.25 + 9.25 = 35.5
Answer: relative atomic mass = 35.5. No single atom weighs 35.5, because it is an average over the mixture of isotopes.
Check by reasoning: 75% of atoms are the lighter one, so the average should sit closer to 35 than to 37. It does.
The mistake to watch for
A common slip is to say isotopes have different numbers of protons, or that they are different elements.
Mistaken answer: “35/17 Cl and 37/17 Cl are different elements because their masses are different.”
The student linked identity to mass instead of to proton number.
The correction is to ask “what defines the element?” The answer is always the proton number. If two atoms share a proton number, they are the same element, whatever their neutron counts.
Another slip is to give the average as a simple mean, (35 + 37) ÷ 2 = 36. That ignores how much of each isotope is present.
Check yourself
1. Atoms X, Y and Z are written 16/8 X, 18/8 Y and 18/9 Z. Which two are isotopes of the same element?
Show answer
X and Y both have proton number 8 but different nucleon numbers (16 and 18), so they are isotopes of the same element. Z has 9 protons, so it is a different element.
X and Y
2. Give the number of neutrons in 12/6 C and 14/6 C, and explain why both react in the same way.
Show answer
12/6 C has 12 − 6 = 6 neutrons. 14/6 C has 14 − 6 = 8 neutrons. Both have 6 protons and 6 electrons, so the electron arrangement is the same and the chemical behaviour is the same.
3. An element has two isotopes, nucleon numbers 10 and 11. Use the invented teaching figures of 20% and 80% to find the relative atomic mass.
Show answer
(10 × 0.20) + (11 × 0.80) = 2.0 + 8.8 = 10.8.
10.8. It sits closer to 11 because 11 is the more common isotope.
Where this leads next
Isotopes have the same chemistry because their electron arrangements match, which is the subject of using an electron arrangement to explain a pattern. Finish the module with the mixed practice set. The mole and equation-ratio tutor later uses relative atomic masses like the one calculated here.
If the weighted average or the reasoning sentence is where your marks slip, our teachers can work on exactly that in online one-to-one Chemistry tuition.