Model of the atom, subatomic particles, isotopes & electronic structure – GCSE Chemistry exam questions with mark scheme

1Grade 1–33 marks

Atoms contain protons, neutrons and electrons.

Complete the table. [3]

Particle	Relative charge	Relative mass
proton	+1	…
neutron	…	1
electron	…	very small

proton relative mass = 1 ignore units(1)
neutron relative charge = 0 allow neutral / none(1)
electron relative charge = −1 allow negative / “−”; do not accept 1 or +1(1)
Answer: proton mass 1; neutron charge 0; electron charge −1
Examiner tip: Electrons have a negative charge: writing just “1” for the electron’s charge loses the mark.

2Grade 1–34 marks

An atom of chlorine is represented as $^{37}_{17}\text{Cl}$.

(a) Give the number of protons, neutrons and electrons in this atom. [3]

(b) Another atom of chlorine is $^{35}_{17}\text{Cl}$. What name is given to atoms like these? [1]

(a) protons = 17(1)
(a) neutrons = 20(1)
(a) electrons = 17(1)
(b) isotopes(1)
Answer: (a) 17 protons, 20 neutrons, 17 electrons (b) isotopes
Examiner tip: Neutrons = mass number − atomic number (37 − 17); do not give 37 as the number of neutrons.

3Grade 4–54 marks

Copper has two isotopes. A sample of copper contains 69.2% of copper-63 and 30.8% of copper-65.

(a) Calculate the relative atomic mass of copper. Give your answer to 1 decimal place. [2]

(b) The radius of an atom is about $1 \times 10^{-10}$ m. The radius of its nucleus is about $1 \times 10^{-14}$ m.

How many times larger is the radius of the atom than the radius of the nucleus? [1]

(a) $\dfrac{(69.2 \times 63) + (30.8 \times 65)}{100}$(1)
(a) 63.6 correct answer with no working scores 2 marks; an unrounded answer of 63.616 / 63.62 scores 1 mark(1)
(b) 10 000 allow $1 \times 10^{4}$(1)
(c) (same number of) electrons in the outer shell / same electronic structure allow same number of electrons; ignore same number of protons alone(1)
Answer: (a) 63.6 (b) 10 000 times (c) same electronic structure
Examiner tip: Divide by 100 at the end — leaving 6361.6 is a common slip; and round to the precision asked for.

4Grade 4–54 marks

Scientists once believed that the atom was a ball of positive charge with negative electrons embedded in it (the plum pudding model).

In the alpha particle scattering experiment, positive alpha particles were fired at a thin sheet of gold foil.

Explain how the results of this experiment led to the nuclear model of the atom. [4]

most alpha particles passed straight through (the foil)(1)
so most of the atom is empty space(1)
a few alpha particles were deflected / bounced back(1)
so the positive charge / mass is concentrated in a small central nucleus(1)
Answer: Most passed straight through → mostly empty space; a few deflected/bounced back → mass and positive charge concentrated in a tiny nucleus.
Examiner tip: “Explain” needs both the observation and the conclusion it supports — pair each result with what it shows.

5Grade 1–3Foundation4 marks

The figure shows the arrangement of electrons in an atom of an element.

(a) Give the electronic structure of this atom. Write your answer as numbers. [1]

(b) How many protons are in the nucleus of this atom? [1]

(d) Sulfur has the atomic number 16.

Give the electronic structure of a sulfur atom. [1]

(a) 2,8,3 allow 2.8.3 or 2:8:3; do not accept 3,8,2(1)
(b) 13 allow ecf from total number of electrons in (a)(1)
(c) (it has) the same number of protons and electrons (so the positive and negative charges cancel) allow numbers: 13 protons and 13 electrons; ignore references to neutrons; do not accept ‘the charges cancel’ without reference to equal numbers(1)
(d) 2,8,6 allow 2.8.6(1)
Answer: (a) 2,8,3 (b) 13 (c) equal numbers of protons (+) and electrons (−) (d) 2,8,6
Examiner tip: In (c), saying the charges ‘cancel out’ is not enough on its own: say there are the same number of protons and electrons.

6Grade 4–56 marks

The table shows the numbers of subatomic particles in five particles, A to E.

Particle	Number of protons	Number of neutrons	Number of electrons
A	11	12	11
B	12	12	10
C	17	18	18
D	17	20	17
E	12	14	12

(a) Which particle has a mass number of 37? [1]

(b) Which particle is a negative ion? Give a reason for your answer. [2]

(d) Particles B and E are particles of the same element but they have different mass numbers.

Explain why. Use data from the table. [2]

(a) D(1)
(b) C(1)
(b) (because it has) more electrons than protons allow 18 electrons and 17 protons; allow it has gained an electron(1)
(c) 2+ allow +2; do not accept 2 or −2(1)
(d) (both have) the same number of protons (12) / same atomic number ignore same number of electrons(1)
(d) different numbers of neutrons (12 and 14) allow E has 2 more neutrons; do not accept different numbers of protons or electrons(1)
Answer: (a) D (b) C: 18 electrons but only 17 protons (c) 2+ (d) both have 12 protons; B has 12 neutrons, E has 14
Examiner tip: Mass number counts protons and neutrons only — students often add the electrons and get the wrong particle in (a).

7Grade 6–75 marks

In one experiment, a sheet of gold foil was $4.0 \times 10^{-7}$ m thick.

The diameter of a gold atom is 0.28 nm. (1 nm = $1 \times 10^{-9}$ m)

(a) Give the diameter of a gold atom in metres. Give your answer in standard form. [1]

(b) Calculate how many gold atoms thick the foil was. [2]

A museum builds a scale model of an atom. The nucleus in the model is a ball with a radius of 5 mm.

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

(a) $2.8 \times 10^{-10}$ (m)(1)
(b) $\dfrac{4.0 \times 10^{-7}}{2.8 \times 10^{-10}}$ allow ecf from (a)(1)
(b) = 1400 (atoms) allow 1428 or 1429 or 1430 / any answer in range 1400–1430; ignore extra figures(1)
(c) 5 × 10 000 = 50 000 mm allow 0.005 m × 10 000; allow 5 × 10⁴ mm(1)
(c) = 50 m correct answer in m scores 2 marks; 50 000 with no unit conversion scores 1 mark only(1)
Answer: (a) $2.8 \times 10^{-10}$ m (b) about 1400 atoms (c) 50 m
Examiner tip: In (c), convert mm to m at the end — 50 000 is the right number of millimetres but the question asked for metres.

8Grade 6–7Higher6 marks

The figure shows the percentage abundance of each isotope in a sample of magnesium.

(a) Calculate the relative atomic mass ($A_r$) of magnesium in this sample. Give your answer to 1 decimal place. [2]

(b) Boron has two isotopes, boron-10 and boron-11.

The relative atomic mass of boron is 10.8.

Calculate the percentage abundance of boron-11. [3]

(a) $\dfrac{(24 \times 79.0) + (25 \times 10.0) + (26 \times 11.0)}{100}$(1)
(a) = 24.3 correct answer with no working scores 2; 24.32 (not given to 1 decimal place) scores 1 mark only(1)
(b) let abundance of boron-11 = $x$ (%), so boron-10 = $(100 - x)$ allow equivalent, e.g. fractions $y$ and $1-y$(1)
(b) $\dfrac{10(100 - x) + 11x}{100} = 10.8$ allow 1000 + x = 1080(1)
(b) $x$ = 80 (%) correct answer with no working scores 3; allow trial and improvement that reaches 80%(1)
(c) it is a (weighted) mean / average of the masses of the isotopes (taking account of their abundance) allow because there is a mixture of isotopes with different masses; ignore ‘because of isotopes’ alone(1)
Answer: (a) 24.3 (b) 80% (c) $A_r$ is an average of the isotope masses weighted by abundance
Examiner tip: In (b) set up one unknown for both isotopes ($x$ and $100 - x$) — guessing 50:50 or using 10.8 directly as a percentage loses all the method marks.

9Grade 6–76 marks

The model of the atom has changed over time.

In the plum pudding model, the atom was a ball of positive charge with electrons spread through it.

The model used today is the nuclear model, which has been developed further by scientists including Niels Bohr and James Chadwick.

Compare the plum pudding model with the nuclear model of the atom used today.

In your answer you should describe how the work of Bohr and Chadwick changed the nuclear model. [6]

Plum pudding: positive charge spread throughout the whole atom(1)
Plum pudding: electrons embedded / scattered through the positive charge; no nucleus(1)
Plum pudding: mass spread evenly through the atom; atom is solid / not mostly empty space(1)
Nuclear: positive charge concentrated in a small central nucleus(1)
Nuclear: almost all of the mass is in the nucleus(1)
Nuclear: most of the atom is empty space; nucleus radius about 1/10 000 of the atom(1)
Nuclear: electrons are outside the nucleus / orbit the nucleus(1)
Similarity: both models contain (negative) electrons; both atoms are neutral overall(1)
Bohr: electrons are in shells / energy levels at specific distances from the nucleus(1)
Bohr’s calculations agreed with experimental observations(1)
Later: the positive charge of the nucleus is due to protons (smaller positive particles)(1)
Chadwick: showed that neutrons (neutral particles) exist in the nucleus(1)
Chadwick’s work came about 20 years after the nuclear model was accepted(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed, coherent comparison with several direct linked differences (and possibly a similarity) between the two models, plus a clear account of what Bohr and Chadwick each added (shells at fixed distances; neutrons in the nucleus).
Level 2	3–4	Some correct features of both models with at least one direct comparison, and a relevant contribution from Bohr or Chadwick; the logic may be incomplete.
Level 1	1–2	Simple, separate statements about one or both models, or about Bohr or Chadwick, with little or no comparison.
No relevant content	0	Nothing worthy of credit.

Answer: Plum pudding: diffuse positive charge with embedded electrons, mass evenly spread, no nucleus. Nuclear: tiny dense positive nucleus holds almost all the mass, mostly empty space, electrons outside. Bohr put electrons in shells at fixed distances; protons explained the nuclear charge; Chadwick discovered neutrons in the nucleus.
Examiner tip: A ‘compare’ answer needs linked statements (e.g. ‘in plum pudding the positive charge is spread out, but in the nuclear model it is concentrated in the nucleus’), not two separate descriptions.

10Grade 8–9Higher6 marks

Argon-40 and calcium-40 are atoms of different elements.

The atomic number of argon is 18. The atomic number of calcium is 20.

(a) Explain how atoms of two different elements can both have a mass number of 40. Use numbers of subatomic particles in your answer. [2]

(b) A particle of calcium has the electronic structure 2,8,8.

Deduce the charge on this particle. Explain your answer. [2]

Explain why the 19th and 20th electrons in a calcium atom are in the fourth shell. [2]

(a) argon has 18 protons and 22 neutrons and calcium has 20 protons and 20 neutrons allow argon has 2 more neutrons but 2 fewer protons than calcium(1)
(a) mass number is the total number of protons and neutrons so both add up to 40 ignore references to electrons(1)
(b) 2+ allow +2(1)
(b) (it has) 20 protons but (only) 18 electrons / it has lost 2 electrons do not accept lost protons or gained 2 electrons(1)
(c) electrons occupy the lowest available energy levels / shells first(1)
(c) the first three shells are full (2,8,8) allow the third shell can only hold 8 electrons (at GCSE) / no more space in the lower shells(1)
Answer: (a) Ar: 18p + 22n = 40; Ca: 20p + 20n = 40 (b) 2+ (20 protons, 18 electrons) (c) lowest energy levels fill first and the first three shells (2,8,8) are already full
Examiner tip: In (b) the charge depends on protons minus electrons — students who forget calcium still has 20 protons often write 2− or ‘0’.

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