What is the nuclear model of the atom in the context of Cambridge IGCSE Coordinated Science?

The nuclear model of the atom, as studied in Cambridge IGCSE Coordinated Science, describes an atom as having a small, dense nucleus at its center, composed of protons and neutrons. This nucleus is surrounded by electrons that orbit in shells. This model was developed following the discovery that atoms are not indivisible, as previously thought, but have a complex internal structure.

How did Rutherford's gold foil experiment lead to the nuclear model of the atom?

Rutherford's gold foil experiment was crucial in developing the nuclear model of the atom. In this experiment, alpha particles were directed at a thin sheet of gold foil. Most particles passed through, but some were deflected at large angles. This led to the conclusion that the atom's positive charge and most of its mass are concentrated in a small nucleus, with electrons orbiting around it, rather than being spread out as in the earlier plum pudding model.

What are the main components of the nuclear atom according to the IGCSE syllabus?

According to the IGCSE syllabus, the nuclear atom consists of three main components: protons, neutrons, and electrons. Protons and neutrons form the nucleus at the center of the atom, while electrons orbit the nucleus in defined energy levels or shells. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge.

Why is the nuclear model of the atom important in understanding atomic physics?

The nuclear model of the atom is fundamental in atomic physics because it provides a framework for understanding the structure and behavior of atoms. It explains atomic properties such as atomic number, mass number, and isotopes. This model also underpins the principles of chemical bonding and reactions, as well as the behavior of elements in the periodic table, which are essential concepts in the Cambridge IGCSE Coordinated Science curriculum.

How does the nuclear model explain the stability of atoms?

The nuclear model explains the stability of atoms through the balance of forces within the nucleus and the arrangement of electrons. The strong nuclear force holds protons and neutrons together in the nucleus, overcoming the repulsive electromagnetic force between positively charged protons. Electrons orbit the nucleus in energy levels, and their arrangement minimizes repulsion between them, contributing to the atom's stability. This understanding is crucial for explaining phenomena such as radioactivity and nuclear reactions in atomic physics.

Why is "Confusing mass number with atomic number" a common mistake in The Nuclear Atom?

Why it happens: Both are numbers associated with an atom; students do not remember which is on top and which is on the bottom in nuclide notation. How to avoid it: In ^A_ZX: A (top) = mass number (total nucleons); Z (bottom) = atomic number (protons). Neutrons = A - Z.

Why is "Saying isotopes have different numbers of electrons (in neutral atoms)" a common mistake in The Nuclear Atom?

Why it happens: Students think 'different mass number → different everything'. How to avoid it: Neutral atoms always have the same number of electrons as protons. Isotopes differ only in neutron number. Electron number is unchanged between isotopes of the same element.

Why is "Describing the pre-Rutherford model correctly but attributing it to Rutherford" a common mistake in The Nuclear Atom?

Why it happens: Students mix up Thomson's plum-pudding model with Rutherford's nuclear model. How to avoid it: Rutherford's experiment disproved the plum-pudding model. The nuclear model (tiny dense nucleus) was Rutherford's conclusion, not Thomson's.

Atomic PhysicsCambridge IGCSE Coordinated Sciences 0654

The Nuclear Atom

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The Nuclear Atom — Cambridge IGCSE Coordinated Science 0654

The atom has a tiny, dense, positively charged nucleus containing protons and neutrons, with electrons orbiting in shells. Cambridge tests atomic structure, isotopes, nuclide notation, and the evidence from the Rutherford scattering experiment.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

P8.1a — Describe the structure of the atom and the properties of protons, neutrons, and electrons.
P8.1b — Use atomic/mass number notation; define and give examples of isotopes.
P8.1c — Describe the Rutherford scattering experiment and what it shows.

Atomic Structure

The atom is mostly empty space — a dense positive nucleus surrounded by orbiting electrons.

Structure of the atom:

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Nucleus
Neutron	1	0	Nucleus
Electron	1/1836 (≈ 0)	−1	Shells around nucleus

Key numbers:

Atomic number (Z) = number of protons in nucleus (defines the element)
Mass number (A) = number of protons + number of neutrons
Number of neutrons = A − Z
In a neutral atom: number of electrons = number of protons (= Z)

Nuclide notation:

ᴬ_Z X

Top number (A) = mass number
Bottom number (Z) = atomic number
Example: ¹⁴_₆C = carbon-14; 6 protons, 8 neutrons, 6 electrons

Isotopes:

Atoms of the same element with the same number of protons but different numbers of neutrons
Same atomic number Z; different mass number A
Same chemical properties (same number of electrons → same electron configuration → same reactions)
Different physical properties (different mass → different density, melting point, boiling point)
Example: Carbon-12 (⁶ protons, ⁶ neutrons) and Carbon-14 (⁶ protons, ⁸ neutrons)
Example: Uranium-235 and Uranium-238 (both 92 protons; 143 or 146 neutrons)

Ions:

Atom that has gained or lost electrons → charged particle
If atom loses electrons → positive ion (cation): more protons than electrons
If atom gains electrons → negative ion (anion): more electrons than protons
Example: Na⁺ has 11 protons and 10 electrons

Proton: +1, mass 1. Neutron: 0, mass 1. Electron: −1, mass ≈ 0.
Z = protons; A = protons + neutrons; neutrons = A − Z.
Isotopes: same Z, different A, different neutron number. Same chemistry.

Rutherford Scattering — Evidence for the Nuclear Atom

The Geiger-Marsden experiment disproved the plum pudding model and led to the nuclear atom model.

The plum pudding model (Thomson, 1904):

Atom = uniform sphere of positive charge with electrons embedded in it (like raisins in a pudding)

The Rutherford scattering experiment (Geiger and Marsden, ~1909):

Setup:

Alpha particles (positive, massive) fired at thin gold foil
Zinc sulfide screen surrounding the foil → flashes when alpha particles hit (used to detect particles at different angles)

Results:

Most alpha particles passed straight through — experienced little or no deflection
Some were deflected slightly
A very small number (about 1 in 8000) bounced back (deflected by more than 90°)

Conclusions (Rutherford's interpretation):

Most particles pass straight through → atom is mostly empty space
Some deflected → there is a concentration of positive charge
Some bounce straight back → the positive charge is concentrated in a very small, dense region (the nucleus); nucleus must be much smaller than the atom

The nuclear model:

Tiny, dense, positively charged nucleus at the centre
Electrons orbit at relatively large distances (the atom is mostly empty space)
The nucleus contains all the positive charge and most of the mass

Scale: If an atom were the size of a football stadium, the nucleus would be the size of a grain of rice.

Most α particles: straight through → atom mostly empty space.
Few bounced back → tiny, dense, positive nucleus.
Rutherford replaced Thomson's plum pudding with the nuclear model.

How it’s examined

Paper 4: 'How many neutrons are in ⁵⁶₂₆Fe (iron-56)?' (1 mark — 56 − 26 = 30 neutrons). 'State what is meant by isotopes of an element' (2 marks — atoms with the same number of protons/atomic number but different number of neutrons/mass number). 'In the Rutherford scattering experiment, most alpha particles pass through the gold foil with little deflection. What does this tell us about the structure of the gold atom?' (2 marks — atom is mostly empty space; nucleus is very small). 'A small fraction of alpha particles are deflected by more than 90°. Explain this observation' (2 marks — nucleus is very small but dense and positively charged; alpha particles that pass very close to the nucleus are repelled by the strong positive charge).

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (P8); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — The Nuclear Atom

Step-by-step solutions to past-paper-style questions on the nuclear atom, written exactly the way a tutor would explain them at the board.

1Interpreting nuclide notation
Core• nuclide notation, atomic number, mass number, neutrons
Question
A nuclide is written as $^{56}_{26}\text{Fe}$ . State (a) the atomic number, (b) the mass number, (c) the number of protons, (d) the number of neutrons, and (e) the number of electrons in a neutral atom.
Step-by-step solution
1. Step 1
  The atomic number $Z = 26$ (bottom number). This equals the number of protons.
2. Step 2
  The mass number $A = 56$ (top number). This equals the total number of nucleons (protons + neutrons).
3. Step 3
  Number of neutrons: $N = A - Z = 56 - 26 = 30$ .
  $N = 56 - 26 = 30$
4. Step 4
  In a neutral atom, number of electrons = number of protons = 26.
Answer
$Z = 26$ (protons); $A = 56$ ; neutrons $= 30$ ; electrons $= 26$ .
2Rutherford scattering experiment
Extended• Adapted from 0654/42 Oct/Nov 2023 Q7• Rutherford, alpha scattering, nuclear model
Question
Describe the Geiger-Marsden (Rutherford) alpha-scattering experiment and explain what each observation revealed about atomic structure.
Step-by-step solution
1. Step 1
  Setup: a beam of alpha particles was directed at a thin gold foil. A detector (zinc sulfide screen) surrounded the foil to detect alpha particles at all angles.
2. Step 2
  Observation 1: Most alpha particles passed straight through without deflection. Conclusion: the atom is mostly empty space — most of the volume contains no significant matter.
3. Step 3
  Observation 2: A small fraction of alpha particles were deflected through large angles (more than 90°), some almost straight back. Conclusion: there is a tiny, dense, positively charged nucleus at the centre. The positive charge of the nucleus repels the positive alpha particles.
4. Step 4
  Conclusion: Atoms consist of a tiny, dense, positively charged nucleus surrounded by a large volume of empty space in which electrons orbit.
Answer
Most alphas: pass straight through (atom is mainly empty space). Some large deflections: tiny dense positive nucleus exists. Very few bounce back: nuclear diameter $\ll$ atomic diameter.
Examiner tip
Examiners reward explicit links between each observation and its conclusion. State both the observation AND what it proves.
3Isotopes — same element, different mass
Challenge• isotopes, atomic number, neutron number
Question
Carbon-12 ( $^{12}_{6}\text{C}$ ) and carbon-14 ( $^{14}_{6}\text{C}$ ) are isotopes. Explain what isotopes are and state how these two atoms are similar and how they differ.
Step-by-step solution
1. Step 1
  Isotopes are atoms of the same element that have the same number of protons (atomic number) but different numbers of neutrons (and therefore different mass numbers).
2. Step 2
  Similarities: both have 6 protons and 6 electrons. They have the same chemical properties because chemical behaviour is determined by the number of electrons (and hence protons).
3. Step 3
  $^{12}\text{C}$ : 6 protons, 6 neutrons. $^{14}\text{C}$ : 6 protons, 8 neutrons.
4. Step 4
  Difference: different mass numbers ( $A = 12$ vs $A = 14$ ) and different numbers of neutrons. They may also differ in nuclear stability — $^{14}\text{C}$ is radioactive.
Answer
Isotopes: same $Z$ (protons), different $N$ (neutrons). Same chemical properties, different mass numbers. $^{12}\text{C}$ : 6n; $^{14}\text{C}$ : 8n.

Key Formulae — The Nuclear Atom

The formulae you need to memorise for the nuclear atom on the Cambridge IGCSE paper, with every variable defined in plain English and a note on when to use it.

Mass number
$A = Z + N$
$A$
mass number (total number of nucleons)
$Z$
atomic number (number of protons)
$N$
number of neutrons
When to use
Finding the number of neutrons when atomic number and mass number are given.
Nuclide notation
$^{A}_{Z}\text{X}$
$A$
mass number (top)
$Z$
atomic number (bottom)
$\text{X}$
chemical symbol of the element
When to use
Writing and interpreting the standard notation for any nuclide.

Key Definitions and Keywords — The Nuclear Atom

Definitions to memorise and the exact keywords mark schemes credit for the nuclear atom answers — sharpened from recent examiner reports for the 2026 Cambridge IGCSE sitting.

Proton
Examiner keyword
A positively charged particle in the nucleus. Relative charge: $+1$ ; relative mass: 1. The number of protons (atomic number $Z$ ) defines which element the atom is.
Neutron
Examiner keyword
An uncharged particle in the nucleus. Relative charge: 0; relative mass: 1. The number of neutrons can vary between atoms of the same element (isotopes).
Electron
Examiner keyword
A negatively charged particle orbiting the nucleus. Relative charge: $-1$ ; relative mass: $\approx 1/1836$ (negligible). In a neutral atom, the number of electrons equals the number of protons.
Isotope
Examiner keyword
Atoms of the same element (same atomic number $Z$ ) with different numbers of neutrons (different mass numbers $A$ ). Isotopes have identical chemical properties.
Example
$^{235}_{92}\text{U}$ and $^{238}_{92}\text{U}$ are isotopes of uranium.
Nucleon
A collective term for protons and neutrons — the particles found in the nucleus. The mass number equals the total number of nucleons.

Common Mistakes and Misconceptions — The Nuclear Atom

The traps other students keep falling into on the nuclear atom questions — taken from recent Cambridge IGCSE examiner reports and mark schemes — and how to avoid them.

✕Confusing mass number with atomic number
Why it happens
Both are numbers associated with an atom; students do not remember which is on top and which is on the bottom in nuclide notation.
How to avoid it
In $^{A}_{Z}\text{X}$ : $A$ (top) = mass number (total nucleons); $Z$ (bottom) = atomic number (protons). Neutrons $= A - Z$ .
✕Saying isotopes have different numbers of electrons (in neutral atoms)
Why it happens
Students think 'different mass number → different everything'.
How to avoid it
Neutral atoms always have the same number of electrons as protons. Isotopes differ only in neutron number. Electron number is unchanged between isotopes of the same element.
✕Describing the pre-Rutherford model correctly but attributing it to Rutherford
Why it happens
Students mix up Thomson's plum-pudding model with Rutherford's nuclear model.
How to avoid it
Rutherford's experiment disproved the plum-pudding model. The nuclear model (tiny dense nucleus) was Rutherford's conclusion, not Thomson's.

Practice questions

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Past paper style quiz

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The Nuclear Atom — frequently asked questions

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The Nuclear Atom

Short Study Notes in the form of Slides

Short Study Notes — The Nuclear Atom

Detailed Notes

What you’ll learn

How it’s examined

1Interpreting nuclide notation

2Rutherford scattering experiment

3Isotopes — same element, different mass

Mass number

Nuclide notation

Proton

Neutron

Electron

Isotope

Nucleon

✕Confusing mass number with atomic number

✕Saying isotopes have different numbers of electrons (in neutral atoms)

✕Describing the pre-Rutherford model correctly but attributing it to Rutherford

Practice questions

Past paper style quiz

Assess your understanding

The Nuclear Atom — frequently asked questions