AQA A-Level Chemistry: Fundamental particles (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.1.1.1 Fundamental particles of the AQA AS and A-level Chemistry specification (7404/7405), for AS and A-level exams from June 2016 onwards. Everything here is AS content and sits in Paper 1. Where a calculation needs a relative mass for the electron, the question gives you 1/1840.

Learn the content first in the Fundamental particles study guide. For wider atomic structure recall, use the Atomic Structure revision notes and the Atomic Structure practice set. The course hub is AQA A-Level Chemistry, the printable list is the AQA A-Level Chemistry checklist, and the free 10-minute diagnostics show where to start.

Questions

1. Give the relative mass and the relative charge for each of three particles: the proton, the neutron and the electron. [3]

2. Describe where the protons, neutrons and electrons are found in an atom. [2]

3. One particle has a relative charge of 0 and a relative mass of 1. Another has a relative charge of −1.

(a) Name the two particles. [2] (b) State roughly how many times heavier the first particle is than the second. [1]

4. An aluminium ion contains 13 protons, 14 neutrons and 10 electrons.

(a) Give the relative charge of the ion. [1] (b) Give the relative mass of its nucleus. [1] (c) A different ion has 16 protons and 18 electrons. State its relative charge. [1] (d) Explain why the number of neutrons does not affect the charge of a particle. [1]

5. A lithium atom has 3 protons, 4 neutrons and 3 electrons. Calculate the percentage of the mass of the atom that is due to its electrons. Use 1/1840 for the relative mass of an electron and give your answer to 3 significant figures. [3]

6. In an invented experiment, 20 000 alpha particles are fired at thin metal foil. 19 984 pass through with almost no change of direction, 14 are deflected slightly, and 2 bounce back towards the source.

(a) Calculate the percentage of the particles that bounce back. [1] (b) State what the 19 984 particles show about the structure of the atom. [1] (c) Explain what the 2 particles that bounce back show about the structure of the atom. [2]

7. Explain why an atom has no overall charge although its nucleus is positively charged. [3]

8. Before the nucleus was discovered, one model of the atom pictured a sphere of positive charge with the electrons scattered through it.

(a) State one observation from alpha particle scattering that this model could not explain. [1] (b) Explain why the model could not explain it. [2] (c) Describe how the nuclear model explains it. [1]

9. A helium atom has two protons and two electrons, but its mass is about four times that of a hydrogen atom, which has one proton. Explain how the discovery of the neutron resolved this problem. [3]

10. A magnesium atom has 12 protons, 12 neutrons and 12 electrons. It forms the ion Mg²⁺. Use 1/1840 for the relative mass of an electron.

(a) Calculate the relative mass of the atom, to 5 decimal places. [2] (b) Calculate the relative mass of the ion, to 5 decimal places. [1] (c) Calculate the difference between the two, and express it as a percentage of the mass of the atom, to 3 significant figures. [2] (d) State what your answer shows about using the mass of an atom for its ion. [1]

11. A student writes: “A copper atom has 29 electrons, so the electrons must make up a large part of its mass.” A copper atom in this question has 29 protons, 34 neutrons and 29 electrons.

(a) Calculate the percentage of the mass of the atom that is due to its electrons, to 3 significant figures. [3] (b) State how many electrons have the same total mass as one proton. [1] (c) Use your answers to evaluate the student’s statement. [2] (d) Describe what the rest of the volume of the atom contains. [2]

12. A calcium atom has 20 protons, 20 neutrons and 20 electrons.

(a) State the total relative charge of the nucleus. [1] (b) The atom loses two electrons. State the number of electrons in the ion, its relative charge and its relative mass to the nearest whole number. [3] (c) Many scientists repeated alpha scattering experiments before the nuclear model was accepted. Explain why this was needed. [2]

Answers

1. Proton has mass 1 and charge +1 [1]. Neutron has mass 1 and charge 0 [1]. Electron has mass 1/1840 and charge −1 [1]. [3] Examiner insight: each particle needs both values, and the sign on the electron’s charge is part of the answer. A mass of zero for the electron is a different claim from a very small mass.

2. The protons and neutrons are in the nucleus at the centre [1]. The electrons are outside the nucleus [1]. [2] Examiner insight: “in the shells” is a weaker answer than “outside the nucleus” unless you also say the protons and neutrons are in the nucleus; give a location for every particle.

3. (a) The first particle is a neutron [1] and the second is an electron [1]. [2] (b) About 1840 times [1]. [1] Examiner insight: a mass ratio needs a number. Say “about 1840 times”, not “much heavier”.

4. (a) Charge = 13 − 10 = +3 [1]. [1] (b) Relative mass of the nucleus = 13 + 14 = 27 [1]. [1] (c) Charge = 16 − 18 = 2− [1]. [1] (d) Neutrons carry no charge, so they add mass but nothing to the total charge [1]. [1] Examiner insight: write the charge as a number with its sign (3+ or +3). “3” alone loses the information that the ion is positive. Show protons minus electrons so the method is visible.

5. Mass of 3 electrons = 3 ÷ 1840 = 0.00163 [1]. Mass of atom = 7 + 0.00163 = 7.00163 [1]. Percentage = 0.00163 ÷ 7.00163 × 100 = 0.0233% [1]. [3] Examiner insight: the nucleus mass is the protons plus the neutrons (3 + 4 = 7), and it is easy to forget the neutrons. Keep spare digits in the middle of a calculation and round only at the end.

6. (a) 2 ÷ 20 000 × 100 = 0.01% [1]. [1] (b) Most of the atom is empty space [1]. [1] (c) Alpha particles are positive, so something positive repelled them [1]. Only a small, dense, heavy region could turn a particle back, so the positive charge and nearly all the mass must be packed into a tiny nucleus [1]. [2] Examiner insight: link each observation to the conclusion it supports. Listing the three observations without saying what each shows describes the experiment and does not use it as evidence.

7. The nucleus contains protons, which are +1, and neutrons, which are 0, so it is positive overall [1]. The electrons outside the nucleus are −1 each [1]. In an atom the number of protons equals the number of electrons, so the charges cancel [1]. [3] Examiner insight: the statement “equal numbers of protons and electrons” is the key idea. Without saying that the charges are equal and opposite, the answer only restates the question.

8. (a) Some alpha particles were deflected through large angles or bounced back [1]. [1] (b) In the plum pudding model the positive charge is spread through the whole atom [1], so there is no concentrated charge to repel an alpha particle strongly enough to turn it back [1]. [2] (c) The nuclear model puts the positive charge and nearly all the mass inside a small nucleus, which can turn back a particle that meets it head-on [1]. [1] Examiner insight: “explain” asks for a reason. “The model was wrong” does not say why; state what the model predicted and how the observation differs.

9. Two protons account for a charge of +2 and a mass of only about 2 units [1]. The atom’s mass is about 4, so about 2 more mass units are needed without extra charge [1]. The neutron has a mass similar to the proton and no charge, so two neutrons in the nucleus supply the missing mass [1]. [3] Examiner insight: both properties are needed for the argument: no charge, so it does not change the +2, and a mass of 1, so it fills the gap. An answer that gives only one of the two usually leaves the argument incomplete.

10. (a) Nucleus = 12 + 12 = 24 [1]. Electrons = 12 ÷ 1840 = 0.00652, so the atom is 24.00652 [1]. [2] (b) The ion has 10 electrons: 10 ÷ 1840 = 0.00543, so the mass is 24.00543 [1]. [1] (c) Difference = 2 ÷ 1840 = 0.00109 [1]. Percentage = 0.00109 ÷ 24.00652 × 100 = 0.00453% [1]. [2] (d) The change is so small that the atom and ion can be treated as having the same mass [1]. [1] Examiner insight: the difference is two electrons, so check that you used 10 and 12 electrons and not the charge by mistake. Quote the number of decimal places the question asks for.

11. (a) Mass of 29 electrons = 29 ÷ 1840 = 0.0158 [1]. Mass of atom = 63 + 0.0158 = 63.0158 [1]. Percentage = 0.0158 ÷ 63.0158 × 100 = 0.0250% [1]. [3] (b) 1840 electrons [1]. [1] (c) The electrons contribute about 0.025% of the mass, so the student’s statement is not supported [1]. Number of electrons does not decide mass because each is so light, and nearly all the mass comes from the protons and neutrons [1]. [2] (d) The rest of the atom is mostly empty space [1], containing the moving electrons only [1]. [2] Examiner insight: an evaluation should give a verdict backed by your own number. Quoting 0.0250% and then saying why it matters is stronger than a general statement that electrons are light.

12. (a) Only protons carry positive charge, so the nucleus has a charge of +20 [1]. [1] (b) The ion has 18 electrons [1]. Its charge is 20 − 18 = 2+ [1]. The relative mass is 20 + 20 = 40 plus a negligible 18 ÷ 1840, so 40 to the nearest whole number [1]. [3] (c) New evidence may come from an error or a one-off result, so repeating by other scientists shows whether the result can be relied on [1]. A model is only accepted if it explains the repeated results and the earlier evidence [1]. [2] Examiner insight: the strongest answers to part (c) say what repetition achieves (reliability, independent confirmation) rather than saying only that scientists “check”. State the charge in part (b) with its sign.

Where marks are usually lost

• Quoting only one property of a particle when the question asks for charge and mass.
• Leaving the sign off the electron’s charge, or writing an ion’s charge without + or −.
• Writing the relative mass of the electron as 0, or as 1.
• Placing electrons in the nucleus.
• Forgetting the neutrons when calculating the mass of the nucleus.
• Rounding in the middle of a percentage calculation, then losing the answer.
• Describing the alpha scattering results but not stating what each result shows about the atom.
• Saying the old model was “wrong” without naming the evidence that did not fit.
• Writing “equal charges” instead of “equal and opposite charges” when explaining why atoms are neutral.
• Using neutron number to work out charge.

Frequently asked questions

Is fundamental particles AS content in AQA Chemistry? Yes. Section 3.1.1.1 Fundamental particles of the AQA 7404/7405 specification is AS content and sits in Paper 1. Where a calculation needs the relative mass of the electron, the question gives 1/1840.

How should you write the charge on an ion? Write the charge as a number with its sign, such as 3+ or +3, and show protons minus electrons so the method is visible. Writing “3” alone loses the information that the ion is positive.

What do examiners want when you use alpha scattering as evidence? Link each observation to the conclusion it supports about the atom. Listing the results without saying what each one shows describes the experiment but does not use it as evidence.

What is a common mistake when comparing the masses of particles? Saying an electron is “much lighter” without a number. Give the ratio, such as about 1840 times, and never write the electron’s relative mass as 0 or 1.

Why are atoms neutral overall? Write “equal and opposite charges”: the number of protons equals the number of electrons, and their charges cancel. “Equal charges” alone loses the mark.

Next steps

• Fundamental particles study guide
• Atomic Structure revision notes
• Atomic Structure study guide
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), for AS and A-level exams June 2016 onwards: section 3.1.1.1 Fundamental particles.