MediumChemistryHigh School (11–12)Structure of the Atom › Subatomic Particles and Atomic ModelsConceptualBoard ExamA-LevelIB
Suppose Thomson's model were true, with the positive charge spread evenly through a gold atom…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Suppose Thomson's model were true, with the positive charge spread evenly through a gold atom. What would Geiger and Marsden have seen? Why did they see something else?
Need a reminder? The key ideaShow
A small, dense, positive nucleus turns back the few alpha particles that come close. Closest approach: . Nuclear radius: . An atom is about times wider than its nucleus.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Prediction. A thin spread of charge gives only weak forces. Every alpha particle would pass through, turned by at most a tiny angle.
Step 2. Observation. About in was turned back.
Step 3. Reason. Only a small, heavy, concentrated charge can push that hard. The positive charge sits in a tiny nucleus.
Answer: no large deflections at all; the ones they saw prove a concentrated nucleus.
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