Fundamental particles — AQA A-Level Chemistry
Test yourself on Fundamental particles with AQA A-Level practice questions.
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Fundamental particles explained
The atom has been rebuilt every time an experiment refused to fit the old picture.
Read the full explanation
Dalton treated atoms as indivisible spheres; Thomson's discovery of the electron forced a model with negative particles embedded in positive charge. Rutherford fired alpha particles at gold foil and found a few bounced back, which a diffuse positive charge could not do, so he placed almost all the mass and all the positive charge in a tiny central nucleus. Bohr answered why orbiting electrons do not spiral inwards by restricting them to fixed energy levels, and Chadwick added the neutron to explain the missing mass. The modern model replaces orbits with orbitals, regions of probability. The pattern to learn is that a model survives only until evidence contradicts it.
Protons, neutrons and electrons: relative charge and relative mass.
Relative charge and relative mass are comparisons, not measurements in coulombs or kilograms. On the relative scale the proton has charge +1 and mass 1, the neutron has charge 0 and mass 1, and the electron has charge −1 and mass 1/1840, usually quoted as 0.0005. That tiny electron mass is why almost all the mass of an atom sits in the nucleus while almost all its volume is electron cloud. The equal and opposite charges on the proton and electron are why a neutral atom must contain equal numbers of each, and why removing one electron leaves a 1⁺ ion. Marks are given for the relative values in a table, so learn them as a set rather than trying to recall absolute masses such as 1.67 × 10⁻²⁷ kg.
An atom consists of a nucleus containing protons and neutrons surrounded by electrons.
An atom is mostly empty space. The nucleus holds the protons and neutrons and is around 10⁻¹⁵ m across, while the whole atom is about 10⁻¹⁰ m, so the nucleus is roughly one hundred-thousandth of the diameter yet carries almost all the mass. The electrons occupy shells around it, held by electrostatic attraction between their negative charge and the positive nucleus. Because the proton and electron charges are equal and opposite, a neutral atom contains equal numbers of each; gaining or losing electrons, never protons, is what produces ions. Chemical behaviour is decided by the outer electrons, which is why isotopes of one element react identically while the nucleus differs.
Your focus
- Describe the gold foil observation of alpha particles deflected through large angles and deduce from it that the nucleus is tiny, dense and positively charged.
- Explain which observation the plum pudding model failed to account for and what Rutherford changed as a result.
- State which particle Chadwick added and why the mass of an atom could not be explained by protons and electrons alone.
Show all 9 objectives
- Complete a table of relative charge and relative mass for the proton, neutron and electron without reference to data.
- Explain why the mass of an atom can be treated as the sum of its protons and neutrons alone.
- Deduce the overall charge on a particle from the numbers of protons and electrons it contains.
- Compare the diameter of the nucleus with that of the whole atom and use the comparison to explain where the mass is concentrated.
- Explain in terms of electrostatic attraction why electrons remain bound to the nucleus.
- Deduce the charge of a species from the numbers of protons and electrons given for it.
Fundamental particles exam tips
Marking Points
- one mark for naming the experiment or observation, such as alpha particles deflected back from gold foil
- one mark for saying what the earlier model could not explain
- one mark for the specific change made to the model, such as placing the mass and positive charge in a small nucleus
- one mark for stating that scientific models are revised when new experimental evidence conflicts with them
- one mark for the relative charges +1 for the proton, 0 for the neutron and −1 for the electron
- one mark for the relative masses 1 for both the proton and the neutron
- one mark for the electron mass given as 1/1840 or 0.0005 on the same scale
- one mark for stating that the electron mass is negligible so the nuclear particles account for the mass of the atom
- one mark for protons and neutrons located in the nucleus
- one mark for electrons arranged in shells or energy levels around the nucleus
- one mark for stating that the nucleus is very small compared with the atom but contains almost all the mass
- one mark for equal numbers of protons and electrons making the atom neutral overall
Examiner Tips
- 💡Pair every model change with the evidence that forced it; a name on its own rarely scores.
- 💡If asked why a model changed, say what the old model predicted and what was actually observed.
- 💡Keep answers about the nucleus quantitative where you can: nearly all the mass, almost none of the volume.
- 💡Draw the three-by-three table (particle, relative charge, relative mass) from memory before you start the question.
- 💡If the question gives you masses in kilograms, divide by the proton mass to get back to the relative scale.
- 💡Use the electron's negligible mass explicitly when explaining why mass number counts only protons and neutrons.
- 💡Quote the two order-of-magnitude sizes when asked to compare the nucleus with the atom; a comparison scores better than an adjective.
- 💡Say electrostatic attraction, not just attraction, whenever you explain what holds electrons to the nucleus.
- 💡Check any ion answer by confirming that only the electron count has changed.
Common Mistakes
- saying earlier models were simply wrong rather than superseded when new evidence appeared
- crediting Rutherford with discovering the neutron instead of Chadwick
- describing electrons as orbiting like planets in the current accepted model rather than occupying orbitals
- giving a list of dates and names with no experimental evidence attached to any of them
- quoting absolute masses in kilograms when the question asks for relative mass
- writing the electron's relative mass as zero rather than 1/1840 or 0.0005
- giving the neutron a small charge because the name sounds like it should be slightly negative
- leaving the sign off the electron charge so it reads 1 rather than −1
- placing electrons inside the nucleus or neutrons in the shells
- saying an atom becomes an ion by gaining or losing protons
- claiming the nucleus takes up most of the volume because it holds most of the mass
- describing the electrons as held by gravity rather than electrostatic attraction