The Nuclear Model of the Atom and Isotopes
The atom is mostly empty space, with a tiny, dense, positively charged nucleus at its centre. This page covers that model, the notation that describes it, and the experiment that proved it.
1. Structure of the atom
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | nucleus | +1 | 1 |
| Neutron | nucleus | 0 | 1 |
| Electron | orbiting shells | −1 | ~1/1840 (negligible) |
The nucleus contains the protons and neutrons (together called nucleons), holds virtually all the mass, and is positively charged.
The electrons orbit in shells, occupy almost all the volume, and have negligible mass.
A neutral atom has equal numbers of protons and electrons, so the charges cancel.
The atom is mostly empty space. The nucleus is roughly 1/10 000 of the atom’s diameter — a fact the gold foil experiment established directly.
2. Nuclide notation
ᴬ𝑍X A = nucleon number (mass number) — protons + neutrons, written on top Z = proton number (atomic number) — number of protons, on the bottom
Number of neutrons = A − Z.
Example: ²³₁₁Na has 11 protons, 11 electrons (neutral) and 23 − 11 = 12 neutrons.
Nucleon number and mass number are the SAME thing — two names for A. Treating them as different was a recorded error.
Nucleon number is not the neutron number. A counts protons and neutrons together; to get neutrons you must subtract. This confusion was recorded repeatedly and is the most common error in the topic.
Don’t confuse the proton number with the nucleon number. The smaller number (bottom) is the proton number and it identifies the element; the larger (top) is the total nucleons.
Reading a value carefully matters: for caesium, ¹³⁷₅₅Cs has A = 137 and Z = 55. A recorded error read the pair as 135 and 56 — check both digits against the periodic table.
3. Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
So they have the same Z and a different A.
Example: carbon-12 (¹²₆C) and carbon-14 (¹⁴₆C) both have 6 protons; carbon-12 has 6 neutrons and carbon-14 has 8.
It is the NEUTRON number that differs, not the electron number. A recorded error had the number of electrons changing — but changing the electrons would make an ion, not an isotope.
Different numbers of protons means a DIFFERENT ELEMENT, not an isotope. Also recorded. The proton number is what defines the element, so it must stay the same.
Properties:
- Isotopes have identical chemical properties, because chemistry depends on the electrons
- They have different physical properties (mass, density), and some are radioactive while others are stable
4. The Rutherford gold foil (alpha scattering) experiment
Alpha particles were fired at a very thin sheet of gold foil, and their paths detected.
The three observations, and what each proves:
| Observation | Conclusion |
|---|---|
| Most passed straight through, undeflected | The atom is mostly empty space |
| A few were deflected through large angles | The nucleus is positively charged (it repels the positive alpha particles) |
| A very few bounced almost straight back | The nucleus is very small and very dense, containing most of the mass |
Match each observation to its conclusion. These are usually three separate marks, and giving observations without conclusions (or the wrong pairing) loses them.
The deflection shows the nucleus is POSITIVE, because alpha particles are positive and were repelled. Confusion about how the experiment shows the core is charged, and about why particles deflected, were both recorded — the answer is electrostatic repulsion between like charges.
Why it mattered: it replaced the earlier “plum pudding” model (charge spread evenly throughout the atom), which could not explain large-angle deflections. If charge were spread out, nothing would deflect sharply.
A thin foil is used so that most alpha particles pass through only one layer of atoms, and the experiment is done in a vacuum so the alphas aren’t absorbed by air.
5. Ions
An ion is an atom that has lost or gained electrons, giving it an overall charge.
- Lost electrons → positive ion
- Gained electrons → negative ion
Ions differ in ELECTRONS; isotopes differ in NEUTRONS. Keeping these two apart is the cleanest way to avoid the errors above.
6. Mistakes that cost marks
Saying a neutron has a charge. It is neutral.
Confusing nucleon number with neutron number.
Confusing proton number with nucleon number.
Misreading the numbers in nuclide notation.
Saying isotopes have different numbers of electrons.
Saying isotopes have different numbers of protons.
Giving gold foil observations without conclusions.
Saying deflection proves the nucleus is negative.
Forgetting the atom is mostly empty space.
Confusing ions with isotopes.
Frequently asked questions
What is in the nucleus? Protons and neutrons — together called nucleons.
What are the charges of the three particles? Proton +1, neutron 0, electron −1.
What is the nucleon number? The total number of protons and neutrons — also called the mass number.
What is the proton number? The number of protons, which defines the element.
How do I find the number of neutrons? A − Z — nucleon number minus proton number.
What are isotopes? Atoms of the same element with the same protons but different neutrons.
Do isotopes have different chemical properties? No — chemistry depends on electrons, which are unchanged.
What did the gold foil experiment show? The atom is mostly empty space, with a small, dense, positively charged nucleus.
Why did some alpha particles bounce back? They met the tiny, dense, positive nucleus and were repelled.
What is the difference between an ion and an isotope? An ion has different electrons; an isotope has different neutrons.
Quick revision checklist
- I know the charge and mass of protons, neutrons and electrons
- I know the nucleus holds the mass and the positive charge
- I know the atom is mostly empty space
- I can read nuclide notation and identify A and Z
- I can calculate the number of neutrons
- I know nucleon number = mass number
- I can define isotopes correctly
- I know isotopes differ in neutrons, not electrons or protons
- I know why isotopes share chemical properties
- I can give all three gold foil observations with their conclusions
- I can explain why alpha particles deflect
- I know why the foil is thin and the experiment is in a vacuum
- I can distinguish an ion from an isotope
These notes cover the nuclear model of the atom and isotopes in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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