Direct Answer
In nuclear notation, the upper-left number is the nucleon number (mass number), while the lower-left number is the proton number (atomic number). For SL and HL IB Physics, write a nuclide as , where is the total number of protons and neutrons and is the number of protons.
How the Notation Works
| Symbol or position | Meaning |
|---|---|
| Chemical symbol identifying the element | |
| , upper left | Nucleon number: total number of protons and neutrons |
| , lower left | Proton number: number of protons |
| (A-Z) | Number of neutrons |
| Upper right, if shown | Overall ionic charge |
For example, carbon-14 is written as . Its proton number is 6, so it contains 6 protons. Its nucleon number is 14, so the number of neutrons is
The proton number determines the element: every carbon nucleus has 6 protons. Carbon-12 and carbon-14 are isotopes because they have the same proton number but different numbers of neutrons.
A common misconception is that the nucleon number gives the number of neutrons. It does not: it gives the total number of nucleons, meaning protons plus neutrons. Also, the ion's charge does not change the nucleus; it shows that electrons have been gained or lost.
Nuclear equations use the same bookkeeping. In alpha decay, uranium-238 becomes thorium-234 and an alpha particle. The upper values satisfy , while the lower values satisfy . This conservation check helps identify a missing particle or an incorrect daughter nucleus before you finalize an exam answer.
Exam Technique
This is assessed in E.1 Structure of the atom for both SL and HL. In a calculate question, show ; in a state or identify question, give only the requested value. In nuclear equations, check that the total nucleon number and total proton number are conserved on both sides. Do not confuse proton number with ionic charge.
