The skin is cooled through evaporation.
The nucleus will have the greatest mass. Among the 3 kinds of subatomic particles, electrons are the lightest (by several orders of magnitude), protons and neutrons are much closer is magnitude, though neutrons are slightly heavier. However, in the diagram, the nucleus contains both the neutrons and the protons, so its combined mass would be heavier than either the neutrons alone or the protons alone.
Answer: population
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