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GCSE · Physics

P4a Atomic Structure and the Development of the Model

AQA 8464 · Calculator allowed · about 92 minutes

Total Marks

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Answer ALL questions. Show all your working.

A Model Under Construction

Original text written for Revision Library.

For most of the 19th century, scientists pictured the atom as a tiny, solid sphere that could not be divided into anything smaller. This changed in 1897, when J.J. Thomson discovered the electron, a negatively charged particle far lighter than a whole atom. Since atoms themselves have no overall charge, Thomson reasoned that the negative electrons had to be balanced by some positive charge, and he proposed the 'plum pudding' model: a ball of positive charge with tiny negative electrons scattered through it. This idea held for over a decade, until Ernest Rutherford asked two of his students, Hans Geiger and Ernest Marsden, to fire alpha particles at a very thin sheet of gold foil. Almost all of the alpha particles passed straight through, as expected, but a small number were deflected through large angles, and a very small number bounced almost straight back. Rutherford later said it was 'as if you had fired a fifteen-inch shell at a piece of tissue paper and it came back and hit you.' The plum pudding model could not explain this, so Rutherford proposed a new nuclear model: almost all of an atom's mass and all of its positive charge are concentrated in a tiny, dense nucleus at the centre, surrounded by mostly empty space. Niels Bohr then adapted this model further, suggesting that electrons do not orbit at random but occupy fixed shells, or energy levels, at set distances from the nucleus. Finally, in 1932, James Chadwick provided the evidence needed to show that the nucleus also contains neutral particles called neutrons, alongside positively charged protons. This gave scientists the model of the atom still used today, and explained why atoms of the same element can have different masses: isotopes.

This question is about the particles that make up an atom.

(a)The table shows the relative charge and relative mass of the three particles found in atoms. The row for the proton has been completed for you. Complete the two blank rows.

Particle	Relative charge	Relative mass
Proton	+1	1
Neutron	...	...
Electron	...	...

(4)

(b)Name the two types of particle found in the nucleus of an atom.(2)

(c)An atom of sodium has 11 protons and 11 electrons. Explain why the sodium atom has no overall electrical charge.(2)

(Total for Question 1 is 8 marks)

The diagram below (not drawn to scale) represents a simple model of an atom. It shows a small circle at the centre, labelled A. Two smaller particles are shown moving on circular paths around it: particle B, on a path close to the centre, and particle C, on a path further from the centre.

(a)What does label A represent?(1)

(b)What do labels B and C represent?(1)

(c)What term is used to describe the fixed circular paths followed by the electrons around the nucleus?(1)

(Total for Question 2 is 3 marks)

The radius of a typical atom is about 1 x 10^-10 m. The radius of the nucleus of that atom is about 1 x 10^-14 m.

(a)Calculate how many times smaller the radius of the nucleus is than the radius of the atom.(1)

(b)What does your answer to part (a) tell you about the structure of an atom?(2)

(Total for Question 3 is 3 marks)

Four scientists made important contributions to the development of the model of the atom. Match each scientist to the numbered contribution below.

Contributions:
1) Proposed that atoms are mostly empty space, with a tiny, dense, positively charged nucleus at the centre, based on the results of the α particle scattering experiment.
2) Discovered the electron and proposed the 'plum pudding' model, in which negative electrons are spread through a ball of positive charge.
3) Proposed that electrons orbit the nucleus at fixed distances, called shells or energy levels.
4) Provided evidence for a neutral particle in the nucleus (the neutron), helping to explain why atoms of the same element can have different masses.

(a)Which numbered contribution belongs to J.J. Thomson?(1)

(b)Which numbered contribution belongs to Ernest Rutherford?(1)

(c)Which numbered contribution belongs to Niels Bohr?(1)

(d)Which numbered contribution belongs to James Chadwick?(1)

(Total for Question 4 is 4 marks)

In 1904, before the nucleus had been discovered, J.J. Thomson proposed the 'plum pudding' model of the atom.

(a)Describe the plum pudding model of the atom.(2)

(b)Give one way in which the plum pudding model is different from the nuclear model of the atom accepted today.(2)

(Total for Question 5 is 4 marks)

Use the information in the source text above, and the data below, to answer this question about the α particle scattering experiment (required practical style).

A student repeats a version of the experiment, firing α particles at a thin gold foil and counting how many are detected at different angles of deflection, using a fixed counting time.

Angle of deflection	Number of α particles detected
0 to 10 degrees (undeflected, almost straight through)	8200
10 to 90 degrees (small/moderate deflection)	140
More than 90 degrees (large deflection, some almost straight back)	6

Total α particles detected = 8346

(a)Describe the set-up of this experiment.(2)

(b)Using the data above, calculate the percentage of α particles that were deflected through more than 90 degrees. Give your answer to 2 significant figures.(3)

(c)The plum pudding model predicted that α particles fired at the foil would all pass through with little or no deflection. Use the data above to explain why the results did not fully support this prediction.(3)

(d)Explain what the results suggest about (i) the size of the nucleus compared with the size of the atom, and (ii) how the positive charge and mass of the atom are distributed.(3)

(e)Suggest one improvement to this experimental method that would increase the reliability of the results.(1)

(f)State one variable that should be controlled (kept the same) throughout this experiment, and explain why.(1)

(Total for Question 6 is 13 marks)

After Rutherford's nuclear model was accepted, Niels Bohr adapted the model further.

(a)Describe how Bohr adapted the nuclear model of the atom.(2)

(b)State one reason why Bohr's adapted model was more widely accepted by scientists than Rutherford's original nuclear model.(1)

(Total for Question 7 is 3 marks)

Before 1932, the nucleus of the atom was thought to contain only protons.

(a)State what James Chadwick discovered in 1932, and the property of this particle that made it difficult to detect.(2)

(b)Explain how the discovery of the neutron accounted for atoms of the same element sometimes having different masses.(1)

(Total for Question 8 is 3 marks)

The table gives the atomic number and mass number of a sodium atom and a sodium ion.

Species	Atomic number	Mass number	Protons	Neutrons	Electrons
Sodium atom (Na)	11	23	...	...	...
Sodium ion (Na+)	11	23	...	...	...

(a)Complete the row for the sodium atom (Na).(3)

(b)Complete the row for the sodium ion (Na+), which has lost one electron.(3)

(Total for Question 9 is 6 marks)

Chlorine has two common isotopes: chlorine-35 and chlorine-37. The atomic number of chlorine is 17.

(a)Define the term 'isotope'.(2)

(b)State the number of neutrons in an atom of chlorine-35.(1)

(c)State the number of neutrons in an atom of chlorine-37.(1)

(d)Explain why chlorine-35 and chlorine-37 have the same chemical properties.(2)

(Total for Question 10 is 6 marks)

A neutral atom of magnesium-24 has 12 protons, 12 neutrons and 12 electrons. A neutral atom of fluorine-19 has 9 protons, 10 neutrons and 9 electrons.

(a)A magnesium atom loses 2 electrons to form a magnesium ion. State the number of protons, the number of neutrons and the number of electrons in this ion, and its overall charge.(3)

(b)A fluorine atom gains 1 electron to form a fluoride ion. State the number of electrons in this ion and its overall charge.(2)

(Total for Question 11 is 5 marks)

The table shows the approximate atomic radius of three elements.

Element	Number of protons	Atomic radius (m)
Hydrogen	1	5.3 x 10^-11
Carbon	6	7.0 x 10^-11
Uranium	92	1.75 x 10^-10

(a)State the order of magnitude of the radius, in metres, that is common to all three atoms.(1)

(b)Uranium atoms contain far more protons, neutrons and electrons than hydrogen atoms. Use the data in the table to explain what this suggests about the size of atoms in general.(2)

(Total for Question 12 is 3 marks)

A gold atom has a radius of about 1.4 x 10^-10 m. The nucleus of a gold atom has a radius of about 7.0 x 10^-15 m.

(a)Calculate how many times bigger the radius of a gold atom is than the radius of its nucleus. Give your answer in standard form.(3)

(Total for Question 13 is 3 marks)

A model of a gold atom is made using a small pea, of radius 3.5 mm, to represent the nucleus.

(a)Using a ratio of 2 x 10⁴ (or your answer to Question 13), calculate the radius that the model atom would need to be, in metres. Give your answer in standard form.(3)

(b)A football pitch is about 100 m long. Comment on what your answer to part (a) tells you about how much empty space there is inside an atom.(1)

(Total for Question 14 is 4 marks)

Higher tier only. An iron-56 nucleus contains 56 nucleons (protons and neutrons), each with a mass of approximately 1.67 x 10^-27 kg. Electron mass can be ignored, as it is very small compared with a nucleon's mass.

(a)Show that the mass of one iron-56 atom is approximately 9.4 x 10^-26 kg.(2)

(b)An iron nail contains approximately 5.4 x 10²² atoms of iron-56. Using the mass from part (a), 9.4 x 10^-26 kg, calculate the total mass of the nail. Give your answer in kg, in standard form, to 2 significant figures.(3)

(c)A typical steel nail has a mass of about 5 g. Comment on whether your answer to part (b) is a reasonable value for the mass of an iron nail.(1)

(Total for Question 15 is 6 marks)

The plum pudding model predicted that, because the positive charge of an atom was spread evenly throughout it, α particles fired at a thin gold foil should all pass through with little or no deflection.

(a)Evaluate why the results of the α particle scattering experiment led scientists to reject the plum pudding model in favour of the nuclear model.(4)

(Total for Question 16 is 4 marks)

This question draws together the whole history of the atomic model, from the plum pudding model to the model accepted today.

Describe how and why scientists' ideas about the structure of the atom have changed since the start of the 20th century. Your answer should refer to the contributions of at least three scientists.

(Total for Question 17 is 6 marks)

Mark scheme · P4a Atomic Structure and the Development of the Model

Question 1

(a) B1 neutron relative charge = 0
(a) B1 neutron relative mass = 1 (oe same as proton)
(a) B1 electron relative charge = -1
(a) B1 electron relative mass = very small (oe negligible / about 1/2000 of a proton); accept any answer indicating a mass much less than 1
(a) Answer: Neutron: charge 0, mass 1. Electron: charge -1, mass very small (about 1/2000 of a proton).
(b) B1 protons
(b) B1 neutrons
(b) Answer: Protons and neutrons.
(c) B1 the number of protons equals the number of electrons (11 = 11), oe equal numbers of positive and negative charges
(c) B1 the positive charge of the protons and negative charge of the electrons cancel out (balance), so there is no overall/net charge, oe
(c) Answer: The atom has equal numbers of protons (+) and electrons (-), 11 = 11, so their charges cancel out and there is no overall charge.

Question 2

(a) B1 the nucleus
(a) Answer: The nucleus.
(b) B1 electrons
(b) Answer: Electrons.
(c) B1 shells (oe energy levels)
(c) Answer: Shells (energy levels).

Question 3

(a) B1 10 000 (1 x 10⁴) times smaller, cao
(a) Answer: 10 000 times (1 x 10⁴)
(b) B1 the nucleus takes up only a tiny fraction of the total volume of the atom, oe
(b) B1 most of the atom (the space around the nucleus, where the electrons are found) is empty space, oe
(b) Answer: An atom is mostly empty space; the nucleus occupies only a tiny fraction of the atom's total volume.

Question 4

(a) B1 2
(a) Answer: 2
(b) B1 1
(b) Answer: 1
(c) B1 3
(c) Answer: 3
(d) B1 4
(d) Answer: 4

Question 5

(a) B1 a ball (sphere) of positive charge, spread evenly throughout the atom, oe
(a) B1 negative electrons are embedded/spread through the positive charge, like plums in a pudding, oe
(a) Answer: A ball of positive charge, spread evenly throughout the atom, with negative electrons embedded through it.
(b) B1 correctly describes one feature of the plum pudding model, e.g. positive charge is spread throughout the whole atom
(b) B1 correctly contrasts this with the corresponding feature of the nuclear model, e.g. the nuclear model has a small, dense, positively charged nucleus at the centre, with electrons in shells around it; accept any other valid, correctly contrasted difference
(b) Answer: E.g. in the plum pudding model, positive charge is spread through the whole atom; in the nuclear model, it is concentrated in a small, dense nucleus at the centre.

Question 6

(a) B1 a source of α particles fires (emits) a narrow beam of α particles at a very thin sheet of gold foil, oe
(a) B1 a detector (e.g. a movable fluorescent/scintillation screen) placed around the foil records the number of α particles scattered at each angle, usually inside a vacuum chamber so the α particles are not absorbed by air, oe
(a) Answer: An α source directs a narrow beam of α particles at thin gold foil; a movable detector around the foil (in a vacuum) records how many α particles are scattered at each angle.
(b) M1 total number of α particles detected = 8200 + 140 + 6 (= 8346)
(b) M1 percentage = (6 / 8346) x 100, oe
(b) A1 = 0.072% (2 s.f.), awrt 0.072%
(b) Answer: 0.072% (2 s.f.)
(c) B1 most α particles (8200 out of 8346) did pass straight through with little deflection, which was consistent with the prediction
(c) B1 however, a small number (140) were deflected by a small to moderate angle, and a very small number (6) were deflected by more than 90 degrees, some almost straight back
(c) B1 this large-angle deflection was not predicted by the plum pudding model, since spread-out (dilute) positive charge could not repel an α particle strongly enough to reverse its direction, oe
(c) Answer: Most particles passed straight through, supporting the idea of mostly empty space, but a very small number were deflected by more than 90 degrees, which spread-out positive charge (as in the plum pudding model) could not explain.
(d) B1 the nucleus must be very small compared with the whole atom, since most α particles pass through empty space without being deflected
(d) B1 the positive charge of the atom must be concentrated in a small volume (the nucleus), because only a concentrated charge could repel/deflect an α particle through a large angle
(d) B1 most of the mass of the atom must be concentrated in the nucleus, because only a very massive object could reverse the direction of a fast-moving α particle, oe
(d) Answer: The nucleus is very small compared with the atom, but concentrates almost all of the atom's positive charge and most of its mass.
(e) B1 repeat the count at each angle (for the same length of time) and calculate a mean, oe e.g. use a longer counting time, or more than one detector
(e) Answer: Repeat the count at each angle (for the same counting time) and take a mean, to reduce the effect of the random nature of radioactive emission.
(f) B1 any valid control variable with a correct reason, e.g. the thickness of the gold foil (or the activity/energy of the α source, or the counting time) should be kept constant, because changing it would also change the pattern of deflection, making it hard to link the results only to the structure of the atom, oe
(f) Answer: E.g. the thickness of the gold foil must be kept the same, otherwise a thicker patch could deflect more particles for a reason unrelated to atomic structure.

Question 7

(a) B1 Bohr proposed that electrons orbit the nucleus only at specific, fixed distances, oe
(a) B1 these fixed distances are called shells (or energy levels)
(a) Answer: Bohr proposed that electrons occupy fixed shells (energy levels) at set distances from the nucleus, rather than orbiting at any distance.
(b) B1 Bohr's theoretical calculations agreed with (matched) the results of experimental observations, oe
(b) Answer: Bohr's calculations agreed with experimental observations.

Question 8

(a) B1 Chadwick discovered the neutron, oe provided evidence for a neutral particle in the nucleus
(a) B1 the neutron has no electrical charge (is neutral), which made it hard to detect using methods that relied on charged particles, oe
(a) Answer: Chadwick discovered the neutron; it has no electrical charge, which made it difficult to detect.
(b) B1 atoms of the same element have the same number of protons but can have different numbers of neutrons, giving different mass numbers (isotopes), oe
(b) Answer: Atoms of the same element always have the same number of protons but can have different numbers of neutrons, giving different masses (isotopes).

Question 9

(a) B1 protons = 11
(a) B1 neutrons = 12 (23 - 11)
(a) B1 electrons = 11 (equal to protons, since the atom is neutral)
(a) Answer: Protons = 11, neutrons = 12, electrons = 11.
(b) B1 protons = 11 (unchanged)
(b) B1 neutrons = 12 (unchanged)
(b) B1 electrons = 10 (11 - 1)
(b) Answer: Protons = 11, neutrons = 12, electrons = 10.

Question 10

(a) B1 atoms of the same element (same number of protons/same atomic number), oe
(a) B1 with a different number of neutrons (different mass number), oe
(a) Answer: Atoms of the same element (same number of protons) with different numbers of neutrons.
(b) B1 18 (35 - 17), cao
(b) Answer: 18
(c) B1 20 (37 - 17), cao
(c) Answer: 20
(d) B1 they have the same number of protons, and therefore the same number of electrons (the same electron arrangement), oe
(d) B1 chemical properties depend on the number and arrangement of electrons, not on the number of neutrons, so both isotopes react in the same way, oe
(d) Answer: Both isotopes have the same number of protons and electrons, and chemical properties depend on electron arrangement, not neutron number, so they react in the same way.

Question 11

(a) B1 protons = 12, neutrons = 12 (both unchanged)
(a) B1 electrons = 10 (12 - 2)
(a) B1 overall charge = 2+ (Mg2+), because there are 2 more protons than electrons, oe
(a) Answer: Protons = 12, neutrons = 12, electrons = 10, overall charge = 2+ (Mg2+).
(b) B1 electrons = 10 (9 + 1)
(b) B1 overall charge = 1- (F-), because there is 1 more electron than protons, oe
(b) Answer: Electrons = 10, overall charge = 1- (F-).

Question 12

(a) B1 10^-10 m (1 x 10^-10 m), oe
(a) Answer: 10^-10 m (about 1 x 10^-10 m).
(b) B1 despite having very different numbers of particles (e.g. 1 proton compared with 92 protons), all three atoms have a radius of the same order of magnitude, oe
(b) B1 this suggests that (nearly) all atoms are approximately the same tiny size, regardless of which element they are, oe
(b) Answer: Although the number of particles varies hugely between elements, atomic radius stays roughly the same order of magnitude, so almost all atoms are approximately the same tiny size.

Question 13

(a) M1 (1.4 x 10^-10) / (7.0 x 10^-15), oe
(a) M1 (1.4 / 7.0) x 10^{-10 - (-15)} = 0.2 x 10⁵, oe correct handling of the power of ten
(a) A1 = 2 x 10⁴ (20 000) times bigger, cao, awrt 2 x 10⁴
(a) Answer: 2 x 10⁴ (20 000) times bigger.

Question 14

(a) M1 converts 3.5 mm to metres: 3.5 x 10^-3 m
(a) M1 (3.5 x 10^-3) x (2 x 10⁴), oe; follow through from candidate's Question 13 answer if used consistently
(a) A1 = 7 x 10¹ m = 70 m, cao, awrt 70 m
(a) Answer: 70 m (7 x 10¹ m).
(b) B1 the model atom (about 70 m across) is nearly as long as a football pitch, even though the model nucleus is only pea-sized; this shows that almost all of the volume of a real atom is empty space, oe
(b) Answer: The model atom is almost as long as a football pitch despite the nucleus being pea-sized, showing that a real atom is almost entirely empty space.

Question 15

(a) M1 56 x (1.67 x 10^-27), oe
(a) A1 = 9.352 x 10^-26 kg, which rounds to 9.4 x 10^-26 kg (2 s.f.), confirming the given value
(a) Answer: 9.4 x 10^-26 kg (2 s.f.), shown from 56 x 1.67 x 10^-27 = 9.352 x 10^-26 kg.
(b) M1 total mass = number of atoms x mass of one atom, oe
(b) M1 (5.4 x 10²²) x (9.4 x 10^-26), oe correct handling of the powers of ten
(b) A1 = 5.1 x 10^-3 kg (2 s.f.), cao, awrt 5.1 x 10^-3 kg (allow 5.1 g)
(b) Answer: 5.1 x 10^-3 kg (2 s.f.), equivalent to about 5.1 g.
(c) B1 yes, the answer to part (b) is 5.1 x 10^-3 kg, which is 5.1 g; this is close to the mass of a typical nail (about 5 g), so the answer is reasonable, oe
(c) Answer: Yes, 5.1 x 10^-3 kg equals 5.1 g, which is close to the mass of a typical nail (about 5 g), so it is a reasonable value.

Question 16

(a) B1 the plum pudding model predicted little or no deflection, and most α particles observed did pass straight through with little deflection, broadly matching this part of the prediction
(a) B1 however, a small number of α particles were deflected through large angles, and a very small number bounced back almost the way they had come, which was not predicted
(a) B1 spread-out (dilute) positive charge, as in the plum pudding model, could not repel an α particle strongly enough to cause this large-angle deflection or reversal
(a) B1 this large-angle scattering could only be explained by a concentrated, dense region of positive charge (a small nucleus), so the plum pudding model was rejected in favour of the nuclear model, oe
(a) Answer: The unexpected large-angle deflections (and rare backward bounces) of some α particles could not be explained by evenly spread-out positive charge; they could only be explained by a small, concentrated, dense positive nucleus, so the nuclear model replaced the plum pudding model.

Question 17

Level 3 (5-6): A detailed, coherent description is given of how the atomic model changed over time, correctly sequencing the contributions of at least three scientists and linking each change to the evidence or reasoning that caused it. The answer is well organised throughout, using correct scientific vocabulary.
Level 2 (3-4): A description is given of how the atomic model changed, referring to the contributions of at least two scientists, with some links made between evidence and the resulting changes to the model. The answer has some structure, with occasional lapses in scientific vocabulary.
Level 1 (1-2): Simple, isolated statements are made about one or two scientists or changes to the atomic model, with little or no linking between the evidence and the changes made. The answer lacks clear structure.
Level 0 (0): No relevant content.
Indicative content:
- J.J. Thomson discovered the electron and proposed the plum pudding model: an atom is a ball of positive charge with negative electrons embedded/scattered throughout it.
- Ernest Rutherford, with Hans Geiger and Ernest Marsden, carried out the alpha particle scattering experiment, firing alpha particles at thin gold foil.
- Most alpha particles passed straight through the foil, showing that atoms are mostly empty space.
- A small number of alpha particles were deflected through large angles, and a very small number bounced almost straight back.
- This showed that most of the mass and all of the positive charge of an atom must be concentrated in a tiny, dense region: the nucleus.
- The plum pudding model could not explain this large-angle scattering, so it was rejected in favour of Rutherford's nuclear model.
- Niels Bohr adapted the nuclear model, proposing that electrons orbit the nucleus at specific fixed distances, called shells or energy levels, rather than at any distance.
- Bohr's theoretical calculations agreed with experimental observations, which made the nuclear model more widely accepted by scientists.
- In 1932, James Chadwick provided evidence for a neutral particle in the nucleus, the neutron, explaining why atoms of the same element (same number of protons) can have different masses (isotopes).
- This led to the currently accepted model of the atom: a small, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons arranged in shells/energy levels.

Atomic Structure and the Development of the Model - Worksheets, Questions and Revision