IGCSE Physics Atom Questions and Worked Solutions (5.1.1) | 0625

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IGCSE Physics 0625 · Topic 5.1.1

IGCSE Physics Atom Questions and Worked Solutions

50 topic-wise practice questions on The Atom (Unit 5: Nuclear Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions21 Core19 Extended10 Challenge119 marksTextbook §5.1 · pp. 252–259
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: The Atom

An atom has a tiny, positively charged nucleus surrounded by negatively charged electrons. Atoms become ions by losing or gaining electrons. (Extended) The alpha-particle scattering experiment provided the evidence for the nuclear model.

Key formulas

ion charge = number of protons − number of electronsin units of eCore

Prerequisite knowledge

  • Charge (4.2.1)
  • Basic chemistry: protons, neutrons, electrons

Exam tips & common mistakes

  • The nucleus does not change when an ion forms
  • Match each alpha-scattering observation to its conclusion
  • Use 'repelled' for alpha deflection, not 'attracted'

Key facts from the syllabus (5.1.1)

  • Atom: positive nucleus with electrons orbiting around it
  • Positive ions form by losing electrons; negative ions by gaining electrons
  • The atom is neutral when protons = electrons
  • Extended alpha particles fired at thin gold foil — most pass straight through (mostly empty space)
  • Extended a few are deflected through large angles (small, positive nucleus)
  • Extended very few bounce back (nucleus contains most of the mass)

Textbook: §5.1 The nuclear model of the atom, pp. 252–259

1
Core[2 marks]

Describe the structure of an atom.

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Method: Nucleus and electrons.

A small, positively charged nucleus with negatively charged electrons orbiting around it
2
Core[1 mark]

State the charge on the nucleus of an atom.

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Method: Protons are positive.

Positive
3
Core[1 mark]

State why an atom is electrically neutral.

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Method: Equal protons and electrons.

It has equal numbers of protons and electrons
4
Core[2 marks]

Describe how an atom forms a positive ion.

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Method: Loses electrons.

It loses one or more electrons
5
Core[2 marks]

Describe how an atom forms a negative ion.

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Method: Gains electrons.

It gains one or more electrons
6
Core[2 marks]

A sodium atom has 11 protons and 11 electrons. It loses one electron. State the charge on the ion formed.

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Method: 11 + and 10 −.

+1 (a positive ion)
7
Core[2 marks]

A chlorine atom (17 protons) gains one electron. State the number of electrons and the charge on the ion.

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Method: 17 + 1.

18 electrons; charge −1
8
Core[2 marks]

An ion has 12 protons and 10 electrons. State its charge.

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Method: 12 − 10.

+2
9
Core[1 mark]

State where most of the mass of an atom is found.

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Method: Nucleus.

In the nucleus
10
Core[1 mark]

State which particles orbit the nucleus.

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Method: Electrons.

Electrons
11
Core[2 marks]

Explain why the electrons do not fly away from the nucleus.

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Method: Electrostatic attraction.

They are attracted to the positively charged nucleus (opposite charges attract)
12
Core[2 marks]

State the charges of the nucleus and of the electrons in an atom.

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Method: Positive and negative.

Nucleus: positive; electrons: negative
13
Core[2 marks]

An atom has 8 protons. State the number of electrons in the neutral atom.

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Method: Equal.

8
14
Core[2 marks]

An oxygen atom (8 protons) gains two electrons. State the charge on the ion.

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Method: −2.

−2
15
Core[2 marks]

An aluminium atom (13 protons) loses three electrons. State the number of electrons left and the charge.

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Method: 13 − 3.

10 electrons; +3
16
Core[2 marks]

An ion has 20 protons and 18 electrons. State its charge and whether it is positive or negative.

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Method: 20 − 18.

+2, a positive ion
17
Core[2 marks]

An ion has 9 protons and 10 electrons. State its charge.

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Method: 9 − 10.

−1
18
Core[2 marks]

State what happens to the nucleus when an atom becomes an ion.

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Method: Unchanged.

Nothing; only the number of electrons changes
19
Core[1 mark]

State the name of the process of forming ions.

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Method: Ionisation.

Ionisation
20
Core[2 marks]

Explain why the size of an atom is much larger than the size of its nucleus.

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Method: Electrons orbit far out.

The electrons orbit at large distances from the tiny nucleus
21
Core[2 marks]

State what keeps electrons in orbit around the nucleus.

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Method: Electrostatic attraction.

The electrostatic attraction between the negative electrons and positive nucleus
22
Extended[3 marks]

Describe the alpha-particle scattering experiment.

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Method: Gold foil, detector.

Alpha particles are fired at a thin sheet of gold foil in a vacuum; a detector measures how many are scattered at different angles
23
Extended[2 marks]

Most alpha particles passed straight through the foil. State what this shows.

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Method: Mostly empty space.

The atom is mostly empty space
24
Extended[2 marks]

A few alpha particles were deflected through large angles. State what this shows.

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Method: Small concentrated charge.

There is a small, concentrated, positively charged nucleus
25
Extended[2 marks]

A very small number of alpha particles bounced straight back. State what this shows.

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Method: Massive nucleus.

The nucleus contains most of the mass of the atom
26
Extended[2 marks]

Explain why the deflection of alpha particles shows the nucleus is positively charged.

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Method: Repulsion.

Alpha particles are positive and are repelled by the nucleus, so the nucleus must be positive
27
Extended[2 marks]

Explain why the experiment needed a very thin foil.

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Method: Pass-through.

So alpha particles could pass through and only interact with a few atoms (alpha has low penetration)
28
Extended[2 marks]

Explain why the experiment was carried out in a vacuum.

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Method: Alpha absorbed by air.

Alpha particles are absorbed by a few cm of air
29
Extended[3 marks]

Explain how the scattering results contradicted the idea of a uniform 'plum pudding' atom.

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Method: Large deflections.

A uniform spread of positive charge could not deflect alpha particles through large angles; only a small, dense, charged nucleus could
30
Extended[2 marks]

Suggest why only very few alpha particles were deflected through large angles.

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Method: Nucleus tiny.

The nucleus is very small, so few alpha particles pass close enough to it
31
Extended[2 marks]

State the target material used in the alpha-scattering experiment.

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Method: Gold foil.

Thin gold foil
32
Extended[2 marks]

State the charge on an alpha particle.

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Method: +2.

+2
33
Extended[3 marks]

Explain why the alpha-scattering results show that most of the atom's mass is in the nucleus.

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Method: Heavy alpha bounced back.

Some alpha particles (which are relatively heavy) bounced straight back, which needs a very massive object
34
Extended[3 marks]

Explain why only a tiny fraction of alpha particles are scattered through large angles.

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Method: Nucleus tiny.

The nucleus is extremely small compared with the atom, so few alpha particles pass close to it
35
Extended[2 marks]

Describe the path of an alpha particle that passes close to, but not directly at, a nucleus.

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Method: Deflected away.

It is deflected through an angle, repelled by the positive nucleus
36
Extended[2 marks]

Describe the path of an alpha particle passing far from any nucleus.

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Method: Straight.

It passes straight through, almost undeflected
37
Extended[2 marks]

State the nuclear model's conclusion about the space in an atom.

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Method: Mostly empty.

An atom is mostly empty space
38
Extended[3 marks]

Suggest what would be observed if the nucleus were negatively charged.

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Method: Attraction.

Alpha particles would be attracted towards the nucleus, curving inwards rather than being repelled; none would bounce straight back
39
Extended[2 marks]

Explain why the electrons in the gold atoms do not deflect the alpha particles significantly.

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Method: Tiny mass.

Electrons have a very small mass compared with alpha particles
40
Extended[2 marks]

State the scientist who led the alpha-scattering experiments.

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Method: Rutherford.

Ernest Rutherford (with Geiger and Marsden)

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

An atom of magnesium has 12 protons. It forms an ion with 10 electrons. State the charge on the ion, explain how it formed and state whether the nucleus changed.

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Method: Lost electrons.

+2; it lost two electrons; the nucleus is unchanged
42
ChallengeCore[4 marks]

Describe the nuclear model of the atom and explain why the atom is neutral overall.

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Method: Model.

A tiny positive nucleus containing protons and neutrons, with negative electrons orbiting; equal numbers of protons and electrons make the charges cancel
43
ChallengeCore[3 marks]

Explain how ionising radiation can turn neutral atoms into ions.

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Method: Knock out electrons.

Radiation transfers energy to electrons, knocking them out of atoms, leaving positive ions (and free electrons that may attach to other atoms, forming negative ions)
44
ChallengeExtended[5 marks]

Describe the alpha-particle scattering experiment and state the three observations and the conclusion drawn from each.

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Method: Three observations.

Most passed straight through → atom mostly empty space; some deflected through small/large angles → small positive nucleus; very few bounced back → nucleus contains most of the mass
45
ChallengeExtended[4 marks]

Explain why the alpha-scattering results could not be explained by a model in which positive charge was spread evenly throughout the atom.

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Method: Spread charge too weak.

A spread-out positive charge would produce only small deflections; large-angle scattering requires the charge and mass to be concentrated in a tiny region
46
ChallengeExtended[4 marks]

Predict how the scattering results would change if a foil of a metal with a smaller nuclear charge were used, and explain.

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Method: Weaker repulsion.

Fewer large-angle deflections, because the smaller positive charge repels alpha particles less strongly
47
ChallengeExtended[4 marks]

Predict how the scattering results would change if faster alpha particles were used.

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Method: More KE.

Fewer would be deflected through large angles, because they have more kinetic energy and are deflected less by the same repulsion
48
ChallengeExtended[3 marks]

Explain why the gold foil had to be extremely thin.

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Method: Single scattering.

So each alpha particle interacts with only one nucleus at most and is not absorbed, allowing the scattering pattern to be interpreted
49
ChallengeExtended[4 marks]

The nucleus of an atom is about 10⁻¹⁵ m across and the atom about 10⁻¹⁰ m. Calculate the ratio of their diameters and explain how this supports the scattering results.

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Method: Ratio.

Atom is about 100 000 times wider; the nucleus is so tiny that most alpha particles miss it and pass straight through
50
ChallengeExtended[4 marks]

Explain why alpha particles that pass very close to a nucleus are deflected more than those passing further away.

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Method: Force increases closer.

The repulsive force is much stronger at smaller distances, producing a larger deflection
All IGCSE Physics topics← 4.5.6 The Transformer5.1.2 The Nucleus →

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