Isotopes and Nuclide Notation

Nuclide notation writes a nucleus as the element symbol with its nucleon number on the top left and proton number on the bottom left. Every decay equation in 0625 uses it, so one notation error cascades through a whole question. Examiners test it because it is quick to mark and exposes whether a student truly understands proton and nucleon numbers.

What is nuclide notation and how do you read it?

A nuclide is a specific type of nucleus. We write it with the element symbol, the nucleon number on the top left and the proton number on the bottom left. For carbon-14 the notation is $^{14}_{6}\text{C}$: nucleon number $A = 14$, proton number $Z = 6$.

Symbol	Name	Meaning
$Z$	Proton number	Number of protons in the nucleus
$A$	Nucleon number	Protons + neutrons
$A - Z$	Neutron count	Number of neutrons

In words: nucleon number equals proton number plus neutron number. So for $^{14}_{6}\text{C}$ the nucleus holds 6 protons and $14 - 6 = 8$ neutrons. A neutral carbon-14 atom also has 6 electrons, equal to the proton number.

What makes two atoms isotopes of each other?

Isotopes are atoms of the same element with the same proton number but different nucleon numbers. Same protons, different neutrons. Carbon-12 ($^{12}_{6}\text{C}$) and carbon-14 ($^{14}_{6}\text{C}$) are isotopes: both have 6 protons; they have 6 and 8 neutrons respectively. Because the proton number fixes the electron count, isotopes have identical chemical behaviour. Their nuclear behaviour differs: carbon-14 is radioactive while carbon-12 is stable, which is what makes radiocarbon dating work.

The definition is a one-mark gift, but Cambridge only accepts the precise version. “Same element, different mass” scores nothing. Write: same number of protons (same proton number), different number of neutrons (different nucleon number).

Nuclide notation also covers particles. The alpha particle is $^{4}_{2}\text{He}$ (or $^{4}_{2}\alpha$). The beta particle is $^{0}_{-1}\text{e}$ (or $^{0}_{-1}\beta$): nucleon number 0, charge $-1$. A neutron is $^{1}_{0}\text{n}$. You will balance these in the radioactivity subtopic.

Worked Exam Question

Uranium has the isotopes $^{235}_{92}\text{U}$ and $^{238}_{92}\text{U}$. (a) State the number of protons, neutrons and electrons in a neutral atom of $^{238}_{92}\text{U}$. (3 marks) (b) Explain why these two atoms are isotopes of the same element. (2 marks)

Solution. (a) Protons = proton number = 92. Neutrons = nucleon number − proton number $= 238 - 92 = 146$. Electrons = protons (neutral atom) = 92. (b) Both nuclei contain 92 protons, so both are uranium. They contain different numbers of neutrons (143 and 146), so their nucleon numbers differ, which is the definition of isotopes.

Mark scheme:

• B1: 92 protons.
• B1: 146 neutrons (must show or imply $238 - 92$).
• B1: 92 electrons, linked to the atom being neutral.
• B1: same proton number / same number of protons.
• B1: different number of neutrons (or different nucleon number).

Common Mistakes

• Swapping $A$ and $Z$. Fix: the bigger number is almost always the nucleon number $A$, written on top. $Z$ matches the periodic-table position.
• Neutrons stated as $A$, not $A - Z$. Fix: always subtract. Write the subtraction in your answer to bank the method.
• “Isotopes have different protons.” Fix: different protons means a different element. Isotopes differ only in neutrons.
• Forgetting electrons = protons for neutral atoms. Fix: state “neutral, so electrons = protons”, because examiners credit the reasoning.
• Sloppy beta notation. Fix: the beta particle is $^{0}_{-1}\text{e}$. The $-1$ sits where the proton number goes, which is how beta decay raises $Z$ by 1.

Exam Technique Tip

Before answering any nuclide question, write a three-line header in the margin: $Z$ = (bottom number), $A$ = (top number), $N = A - Z$ with the actual values. This 10-second habit removes nearly all notation slips and gives the examiner visible working for method credit when a later step goes wrong.

How This Is Examined

Isotopes is Core-tier content, so all candidates answer it at the same depth. Papers 1 and 2 use it for one MCQ per session: pick the isotope pair, or count neutrons. Papers 3 and 4 ask the definition (2 marks) and particle counts (2-3 marks), often as the opening of a longer radioactivity question: easy marks that set up the decay equations that follow. It has no practical-paper role. Medical and dating applications of isotopes appear as context, including questions on cobalt-60 use in radiotherapy at hospitals, a context Malaysian papers’ regional mocks reuse often.