Isotopes and atomic symbols
Read atomic symbols and distinguish a neutron change from an electron change.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Use mass number = protons + neutrons.
- Find electron count from proton number and charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Mass number counts which particles?
What you will learn
- Identify isotopes using proton and neutron numbers.
- Interpret mass number, proton number and charge in atomic or ionic symbols.
Same element, different neutron count
Isotopes are atoms of one element with the same proton number but different neutron numbers. Carbon-12 and carbon-14 both have six protons; their neutron counts are six and eight. A changed neutron count changes mass number, while a changed proton count would identify a different element.
Changing electrons instead makes an ion. Isotopes and ions describe different comparisons, so an isotope can also be present as an ion. To classify a pair, compare protons first, then neutrons and electrons separately.
Read each part of the symbol
In the notation ³⁵₁₇Cl⁻, the upper-left 35 is the mass number and the lower-left 17 is the proton number. There are 17 protons and 35−17=18 neutrons. The upper-right minus sign gives the charge, so there are 18 electrons. A missing charge usually denotes a neutral atom in this context.
The upper-left mass number belongs to that isotope and is a whole nucleon count. Do not substitute a decimal average from a Periodic Table when finding the neutron count of a named isotope. Charge counts the imbalance between protons and electrons; it is not an extra neutron.
Sources for this lesson
Pause and explain
Which pair represents isotopes?
Worked example
Compare ²⁴₁₂Mg and ²⁶₁₂Mg. Show that they are isotopes.
Show the worked solution
- Both have proton number 12, identifying magnesium.
- The first has 24−12=12 neutrons; the second has 26−12=14.
- Same protons but different neutrons meets the isotope definition.
Answer Magnesium isotopes with 12 and 14 neutrons.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find protons, neutrons and electrons in neutral ¹⁴₆C.
Give me a hint
Read Z first, then subtract it from A.
Compare my reasoning
- Proton number 6 gives six protons.
- Neutrons =14−6=8.
- Neutral charge means six electrons.
6 protons, 8 neutrons, 6 electrons.
Look for these in your work
- The isotope's whole mass number is used.
- Neutral charge gives equal proton and electron counts.
Find the particle counts in ⁴⁰₂₀Ca²⁺.
Give me a hint
Charge affects electrons; it does not affect A or Z.
Compare my reasoning
- There are 20 protons.
- Neutrons =40−20=20.
- The 2+ charge means two electrons have been lost, leaving 18.
20 protons, 20 neutrons, 18 electrons.
Look for these in your work
- The nucleus counts stay independent of charge.
- The positive ion has fewer electrons than protons.
Particles X and Y each have 11 protons. X has 12 neutrons and 11 electrons; Y has 13 neutrons and 10 electrons. Compare isotope identity and charge.
Give me a hint
Treat neutrons and electrons as separate comparisons.
Compare my reasoning
- Both are sodium because both have 11 protons.
- Different neutron counts give mass numbers 23 and 24, so they are different isotopes.
- X is neutral; Y has one fewer electron than proton and is a +1 ion.
Different sodium isotopes: X is neutral sodium-23; Y is a sodium-24 ion with charge +1.
Look for these in your work
- Isotope and charge comparisons are both made.
- Neither isotope nor ion formation is treated as a new element.
Common mistakes
- Defining isotopes as atoms with different proton numbers.
- Using a decimal relative atomic mass to calculate a single isotope's neutron count.
What can you explain now?
Find the electron count in ¹⁹₉F⁻.
Compare with the explanation
10
The nine protons identify fluorine; charge −1 means ten electrons. The isotope has 19−9=10 neutrons.
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Make it stick
Read an atomic symbol aloud as proton count, neutron count and electron count. Explain which number distinguishes isotopes and which sign identifies an ion.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.