Atoms, isotopes and the development of the atomic model – GCSE Physics exam questions with mark scheme

1Grade 1–33 marks

An atom of carbon-14 can be written as $^{14}_{6}\text{C}$.

(a) State the number of protons in the nucleus of this atom. [1]

(b) Calculate the number of neutrons in the nucleus of this atom. [1]

(a) 6(1)
(b) 8 (14 − 6)(1)
(c) 6(1)
Answer: (a) 6 (b) 8 (c) 6
Examiner tip: The top number is the mass number (protons + neutrons), not the number of neutrons.

2Grade 4–54 marks

Before the nuclear model, scientists used the plum pudding model of the atom.

Describe the plum pudding model and explain how the results of the alpha particle scattering experiment led to the nuclear model of the atom. [4]

plum pudding model: a ball / sphere of positive charge with (negative) electrons embedded in it allow electrons spread through a positive cloud(1)
most alpha particles passed straight through the (gold) foil, so the atom is mostly empty space allow most of the atom is empty space / the nucleus is very small compared with the atom(1)
some alpha particles were deflected (repelled), so the centre / nucleus of the atom is positively charged allow positive charge is concentrated in the centre / a small nucleus(1)
a very small number bounced back / were deflected through large angles, so most of the mass is concentrated in a tiny nucleus (allow: nucleus is very small and dense) the conclusions for marks 3 and 4 may be credited from either deflection observation; a conclusion with no supporting observation does not score(1)
Answer: Plum pudding: positive ball with electrons embedded. Most alphas pass through (mostly empty space); some deflected (positive nucleus); a few bounce back (mass concentrated in tiny nucleus).
Examiner tip: Link each observation to its conclusion — a list of observations without conclusions scores very little.

3Grade 1–3Foundation6 marks

The diagram shows a model of an atom of lithium. The particles in the nucleus are labelled P and Q. The particles outside the nucleus are labelled R.

(a) Name particles P, Q and R. [2]

(b) Use the diagram to give:

(i) the atomic number of lithium [1]

(ii) the mass number of this atom of lithium. [1]

(d) Give the relative charge of particle R. [1]

(a) any two of: P = proton; Q = neutron; R = electron(1)
(a) all three correct(1)
(b)(i) 3 ignore units(1)
(b)(ii) 7(1)
(c) $^{7}_{3}\text{Li}$ allow ecf from (b); numbers must be in the correct positions (mass number on top)(1)
(d) −1 accept negative one; do not accept 'negative' alone(1)
Answer: (a) P proton, Q neutron, R electron (b)(i) 3 (ii) 7 (c) $^{7}_{3}\text{Li}$ (d) −1
Examiner tip: Mass number counts protons and neutrons – students who count every particle in the diagram (including the electrons) get 10, which scores nothing.

4Grade 6–75 marks

The radius of a gold atom is $1.4\times10^{-10}\,\text{m}$.

The radius of a gold nucleus is $7.0\times10^{-15}\,\text{m}$.

(a) Calculate how many times greater the radius of the atom is than the radius of the nucleus. [2]

(b) A teacher makes a scale model of a gold atom. The nucleus is represented by a bead of radius $2.5\,\text{mm}$.

Calculate the radius the model atom would need to have. Give your answer in metres. [2]

(a) $\dfrac{1.4\times10^{-10}}{7.0\times10^{-15}}$(1)
(a) 20 000 accept $2\times10^{4}$(1)
(b) $2.5\times10^{-3}\,(\text{m})\times 20\,000$ allow ecf from (a); allow 2.5 × 20 000 = 50 000 (mm) for this mark(1)
(b) 50 (m) allow ecf from (a); answer must be in metres for this mark(1)
(c) it shows that (most of) the atom is empty space / the nucleus is tiny compared with the atom ignore 'atoms are small'(1)
Answer: (a) 20 000 (b) 50 m (c) shows the atom is mostly empty space with a tiny nucleus
Examiner tip: Convert 2.5 mm to 0.0025 m before giving the final answer – '50 000' without a unit change loses the second mark.

5Grade 4–55 marks

The diagram shows some of the energy levels for the electrons in an atom. The arrows A, B and C show three changes that an electron makes, one after the other.

(a) Which arrow, A, B or C, shows the electron absorbing electromagnetic radiation? [1]

(b) What happens when the electron moves along arrow B? [1]

Give a reason for your answer. [2]

(d) An electron in the atom absorbs enough energy to leave the atom completely.

Name the particle that the atom becomes. [1]

(a) A(1)
(b) electromagnetic radiation is emitted (by the atom) accept light / a photon is given out; ignore 'energy is given out' alone(1)
(c) C(1)
(c) the gap (in energy) between the levels is bigger / it moves down a larger energy difference only award if C is chosen(1)
(d) (positive) ion(1)
Answer: (a) A (b) EM radiation is emitted (c) C – the energy gap between level 2 and level 1 is bigger (d) a positive ion
Examiner tip: Electrons move up by absorbing and down by emitting electromagnetic radiation – mixing these up costs marks in (a) and (b).

6Grade 4–5Foundation5 marks

Scientists' ideas about the atom have changed over time.

(a) Write the four models in the order in which they were suggested, starting with the earliest. [2]

electrons orbit the nucleus at specific distances
atoms are tiny spheres that cannot be divided
a ball of positive charge with electrons embedded in it
a small, positive nucleus surrounded by electrons

(b) Give the name of the model 'a ball of positive charge with electrons embedded in it'. [1]

Name the scientist whose work showed that neutrons exist. [1]

(d) Suggest why a scientific model is replaced by a new model. [1]

(a) tiny spheres first and 'ball of positive charge' before 'small positive nucleus'(1)
(a) fully correct order: tiny spheres → ball of positive charge → small positive nucleus → electrons orbit at specific distances(1)
(b) plum pudding (model)(1)
(c) (James) Chadwick allow phonetic spellings; do not accept Rutherford or Bohr(1)
(d) new (experimental) evidence / results that the old model cannot explain allow new experiments / observations disagree with the old model; ignore 'new technology' alone(1)
Answer: (a) tiny spheres → plum pudding → nuclear model → Bohr's orbits (b) plum pudding model (c) Chadwick (d) new evidence that the old model cannot explain
Examiner tip: Bohr's electron orbits came after the nuclear model – many students put it before the nucleus.

7Grade 8–9Higher6 marks

In the alpha particle scattering experiment, a beam of alpha particles was fired at a very thin sheet of gold foil. The number of alpha particles scattered through different angles was counted.

The table shows some results.

angle of deflection	number of alpha particles counted
less than 10°	47 700
10° to 90°	294
more than 90°	6

(a) Calculate the fraction of the alpha particles that were deflected through more than 90°.

Give your answer in the form '1 in n'. [2]

(b) Explain what the results for angles less than 10° show about the structure of the atom. [2]

Predict how the number of alpha particles deflected through more than 90° would change. Give a reason for your answer. [2]

(a) total = 48 000 and $\dfrac{6}{48\,000}$(1)
(a) 1 in 8000 $\frac{6}{48\,000}$ or 0.000125 not in the required form scores the first mark only(1)
(b) most (of the) alpha particles passed straight through / were (hardly) deflected(1)
(b) so most of the atom is empty space do not accept 'the atom is empty space' without 'most'(1)
(c) (about) half as many / fewer do not accept 'none'(1)
(c) (alpha particles pass) fewer gold atoms / nuclei, so fewer pass close (enough) to a nucleus to be repelled back allow fewer nuclei in the path of the beam(1)
Answer: (a) 1 in 8000 (b) most alpha particles passed straight through, so most of the atom is empty space (c) about half as many – there are half as many nuclei in the path for an alpha particle to pass close to
Examiner tip: Divide by the total number counted (48 000), not by the 47 700 that went almost straight through.

8Grade 6–76 marks

The plum pudding model of the atom was replaced by the nuclear model. The nuclear model was later developed further to give the model of the atom used today, with electrons in energy levels and a nucleus containing protons and neutrons.

Compare the plum pudding model of the atom with the model of the atom used today. [6]

both models: the atom contains (negative) electrons / the atom is neutral overall(1)
both models: the atom has positive charge (balancing the electrons' negative charge)(1)
plum pudding: positive charge is spread throughout the whole atom (a ball of positive charge)(1)
today: positive charge is concentrated in a tiny nucleus at the centre(1)
plum pudding: mass is spread (evenly) through the atom(1)
today: (almost) all the mass is concentrated in the nucleus(1)
plum pudding: no empty space – the atom is solid / continuous(1)
today: most of the atom is empty space; nuclear radius is less than 1/10 000 of the atom's radius(1)
plum pudding: electrons are embedded / scattered within the positive charge(1)
today: electrons are outside the nucleus, in energy levels / orbits at fixed distances(1)
plum pudding: no separate particles carry the positive charge / no neutrons(1)
today: the nucleus contains protons (positive) and neutrons (no charge)(1)

Level	Marks	What it looks like
Level 3	5–6	Makes several clear, direct comparisons (similarity and differences) covering where the mass and the positive charge are, the arrangement of the electrons and the particles present. Points are linked to each model, not just listed for one.
Level 2	3–4	Makes some comparisons, e.g. a correct statement about the positive charge or the mass in each model, with a description of the electrons in at least one model. Some comparisons may be implied rather than explicit.
Level 1	1–2	Simple statements about one or both models with little or no comparison, e.g. 'the nuclear model has a nucleus' or 'plum pudding has electrons in it'.
0	0	No relevant content.

Answer: Both have electrons and are neutral overall. In the plum pudding model the positive charge and mass are spread through a solid ball with electrons embedded in it; today the positive charge and almost all the mass are in a tiny nucleus of protons and neutrons, most of the atom is empty space, and electrons are in energy levels outside the nucleus.
Examiner tip: 'Compare' needs linked statements about both models (e.g. 'whereas…'); describing only the modern atom caps the answer at Level 1–2.

9Grade 8–9Higher6 marks

An atom of oxygen-16 contains 8 protons, 8 neutrons and 8 electrons.

mass of a proton = $1.67\times10^{-27}\,\text{kg}$
mass of a neutron = $1.67\times10^{-27}\,\text{kg}$
mass of an electron = $9.11\times10^{-31}\,\text{kg}$

(a) Calculate the mass of the nucleus of an oxygen-16 atom. [2]

(b) Calculate the percentage of the total mass of the atom that is in the nucleus.

Give your answer to 4 significant figures. [2]

The volume $V$ of a sphere of radius $r$ is given by $V=\frac{4}{3}\pi r^{3}$.

Determine the fraction of the volume of the atom that is taken up by the nucleus. [2]

(a) $16\times1.67\times10^{-27}$ allow $8\times1.67\times10^{-27}+8\times1.67\times10^{-27}$(1)
(a) $2.672\times10^{-26}$ (kg) accept $2.67\times10^{-26}$(1)
(b) $\dfrac{2.672\times10^{-26}}{2.672\times10^{-26}+8\times9.11\times10^{-31}}\times100$ allow ecf from (a); electron mass total = $7.288\times10^{-30}$ kg(1)
(b) 99.97 (%) allow ecf from (a); must be 4 s.f. – 99.973 or 100 does not score this mark(1)
(c) $\left(\frac{1}{10\,000}\right)^{3}$ / volume ∝ $r^{3}$ allow $\frac{\frac{4}{3}\pi (r/10\,000)^{3}}{\frac{4}{3}\pi r^{3}}$(1)
(c) $1\times10^{-12}$ accept 1 in 10¹² / 1 in a million million; ignore 'or less'(1)
Answer: (a) $2.67\times10^{-26}$ kg (b) 99.97 % (c) $1\times10^{-12}$
Examiner tip: In (c) the radius ratio must be cubed for volume – giving $\frac{1}{10\,000}$ again is the most common error.

10Grade 1–3Foundation5 marks

Potassium has two common stable isotopes, potassium-39 and potassium-41.

Every potassium atom has 19 protons.

(a) What is meant by the word isotopes? [1]

(b) Complete the table. [2]

	potassium-39	potassium-41
number of protons	19	19
number of neutrons
number of electrons	19

(i) Name the type of particle that is formed. [1]

(ii) Give the overall charge on this particle. [1]

(a) atoms of the same element / with the same number of protons and different numbers of neutrons both parts needed; allow same atomic number, different mass number(1)
(b) neutrons: 20 (potassium-39) and 22 (potassium-41)(1)
(b) electrons in potassium-41: 19(1)
(c)(i) (positive) ion accept cation; do not accept isotope(1)
(c)(ii) +1 accept 1+ / positive one; do not accept 'positive' alone or −1(1)
Answer: (a) same number of protons, different number of neutrons (b) neutrons 20 and 22; electrons 19 (c)(i) an ion (ii) +1
Examiner tip: A definition of isotopes needs both halves – 'same protons' alone or 'different neutrons' alone gets no mark.

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