IB Physics SLTopic 5 — The Atomic & Nuclear WorldPaper 1 & 2AZX · neutrons = A − Z~7 min read
Nuclear Notation
Now we know the atom has a tiny nucleus, let’s name what’s inside it — and give every nucleus a neat two-number label. With just a symbol and two numbers, physicists can tell you exactly how many protons and neutrons a nucleus holds. Learn to read that label and a whole chunk of nuclear physics falls into place.
📘 What you need to know
Atoms are built from three particles: protons and neutrons in the nucleus, with electrons orbiting around it
Relative charges: proton +1, neutron 0, electron −1 (one elementary charge e = 1.60 × 10−19 C)
Protons and neutrons each have a mass of about 1 u; the electron is almost 2000 times lighter, so nearly all the atom’s mass is in the nucleus
A nucleus is written AZX: A is the nucleon (mass) number, Z is the proton (atomic) number, and X is the element’s symbol
Number of neutrons = A − Z; in a neutral atom the number of electrons = Z
Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A)
Inside the Atom
Every atom has a dense central nucleus containing protons and neutrons (together called nucleons), surrounded by a cloud of orbiting electrons. Each particle has its own charge and mass, usually quoted in two handy sets of units: charge in coulombs (C) or in elementary chargese, and mass in kilograms (kg) or in atomic mass units (u), where 1 u is roughly the mass of one nucleon.
Protons (red) and neutrons (green) sit together in the nucleus; electrons (blue) orbit far outside. Because the electron is nearly 2000 times lighter than a nucleon, almost all the atom’s mass lives in that tiny nucleus.
Labelling a Nucleus: AZX
Rather than write out “carbon with six protons and six neutrons” every time, we use a compact symbol. The element symbol X carries two numbers on its left: the nucleon number on top and the proton number on the bottom.
The top number A counts all the nucleons (protons + neutrons); the bottom number Z counts just the protons. Subtract to find the neutrons, and for a neutral atom the electrons match the protons.
A (top) = nucleon number
= protons + neutrons
Z (bottom) = protons
A − Z
→
number of neutrons
Isotopes
Keep the number of protons the same but change the neutrons and you still have the same element — just a heavier or lighter version of it. These are called isotopes: same Z, different A. Carbon-12 and carbon-14, for instance, both have 6 protons (so both are carbon) but 6 and 8 neutrons respectively. Because chemistry is set by the electrons — and hence by Z — isotopes behave the same chemically but differ in mass.
Quick recap: atoms are protons + neutrons (nucleus) plus orbiting electrons. In AZX, A is the nucleon number and Z the proton number, so neutrons = A − Z and (for a neutral atom) electrons = Z. Isotopes share Z but differ in A.
🧭 Reading a nuclide AZX
Bottom number Z → the number of protons (this fixes which element it is)
Top number A → the number of nucleons (protons + neutrons)
Neutrons = A − Z — subtract the bottom from the top
Neutral atom? Then electrons = Z (protons and electrons balance)
Same Z, different A? They’re isotopes of the same element
WE 1
A neutral atom of aluminium is written 2713Al. State the number of protons, neutrons and electrons it contains.
Read off Z and A
Z = 13 (bottom), A = 27 (top)
Protons = Z13 protonsNeutrons = A − Z27 − 13 = 1414 neutronsNeutral atom → electrons = Z13 electrons
WE 2
Carbon-12 is written 126C and carbon-14 is written 146C. (a) State the number of neutrons in each. (b) State what makes them isotopes of the same element.
Part (a) — neutrons = A − Zcarbon-12: 12 − 6 = 6 neutronscarbon-14: 14 − 6 = 8 neutronsPart (b) — why isotopes
Both have Z = 6, so both are carbon (same element)…
…but different A means a different number of neutrons
same protons, different neutrons → isotopes
💡 Top tips
A on top, Z on the bottom. Top = nucleon (mass) number, bottom = proton (atomic) number — never swap them
Neutrons = A − Z, not A. The top number counts protons and neutrons together
In a neutral atom, electrons = protons = Z. If a question mentions an ion, that balance changes
Almost all the mass is in the nucleus — the electron is ~1/1836 of a nucleon, so it barely adds to the mass
⚠ Common mistakes
Swapping A and Z — reading the atomic number as the mass number or vice versa
Using A as the neutron count instead of subtracting: neutrons = A − Z
Thinking isotopes have different protons — they have the same Z; it’s the neutrons that differ
Counting the electron mass as significant — it’s negligible next to a nucleon
Up next: we shine light on atoms. The next page looks at emission and absorption spectra — the sharp coloured (or dark) lines atoms produce, and how those lines are the fingerprints that reveal an atom’s electron energy levels.
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