AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2020: Question 5

✅ Correct Answers (Any 3)

• Empty space: The nuclear model is mostly empty space (plum pudding model has no empty space / is solid).
• Distribution of charge: In the nuclear model, the positive charge is concentrated in the nucleus (plum pudding is a single sphere of diffuse positive charge).
• Distribution of mass: In the nuclear model, mass is concentrated in the nucleus (plum pudding mass is spread out evenly).
• Position of electrons: In the nuclear model, electrons orbit outside the nucleus (in the plum pudding model, electrons are embedded inside the sphere of charge).

🧠 Exam Technique

The question says: "The plum pudding model did not have a nucleus. Describe three other differences..."

• Never state: "The nuclear model has a nucleus" — the question explicitly excluded this point!
• Structure each point by comparing space, mass, or charge location.

❌ Common Errors & Historical Traps

• Naming subatomic particles too early: Do not mention protons or neutrons . As shown on the timeline, protons (1920) and neutrons (1932) were discovered after the nuclear model (1911).
• Only describing one model without indicating a comparative difference.

💡 Visualizing the Two Models

Plum Pudding (Thomson, 1897): Solid ball of positive charge with negatively charged electrons embedded like raisins in a pudding.

Nuclear Model (Rutherford, 1911): Tiny, dense, positive central nucleus holding nearly all mass, with electrons moving in mostly empty surrounding space.

✅ Correct Answer

• Electrons orbit the nucleus / are in shells / are in energy levels around the nucleus. [1 mark]
• Electrons are at specific distances (fixed orbits/energy levels) from the nucleus. [1 mark]

🧠 Exam Technique: Two-Mark Breakdown

Many students only write "he added shells" and gain just 1 mark. To secure both marks:

1. State where the electrons are (orbiting in shells/energy levels).
2. State that these levels are at fixed / specific distances.

❌ Common Misconceptions

• Do not write that Bohr discovered protons or neutrons.
• Do not confuse Bohr's shells with Rutherford's random orbits. Bohr's key insight was quantised energy levels (fixed distances).

✅ Correct Answer

• Atomic number is the number of protons. [1 mark]
• Protons were not discovered until much later (around 1920, well after 1869). [1 mark]

🧠 Exam Technique: Using Timeline Evidence

When an exam question says "Use Figure 2", actively check the dates:

• Mendeleev published: 1869 (stated in the question stem).
• Figure 2 shows protons discovered: c. 1920.
• Therefore, Mendeleev could not possibly know about atomic numbers because protons didn't exist in science yet!

❌ Common Errors

• Saying "electrons / neutrons were not discovered yet" does not get credit. Atomic number specifically depends on protons.
• Failing to link the term "atomic number" to the definition "number of protons".

✅ Correct Answer

So that their properties matched the rest of the group / to fit the pattern of chemical reactions and properties. [1 mark]

💡 Famous Example: Iodine and Tellurium

Tellurium (atomic weight 128) has a higher atomic weight than Iodine (atomic weight 127). If placed purely by atomic weight, Iodine would sit in Group 6 and Tellurium in Group 7.

Mendeleev swapped them so that iodine lined up with fluorine, chlorine, and bromine (Group 7 halogens), matching their chemical properties!

🧠 Top Tip

Always mention properties and groups together. For example: "To place elements with similar chemical properties in the same vertical column (group)."