Final answer:
The main difference is the hardness of the metal.
Step-by-step explanation:
The main difference between a metal that scratches a human fingernail and a copper coin is the hardness of the metal. Hardness is a measure of a material's resistance to being scratched, and different metals have different levels of hardness. For example, copper is a relatively soft metal, with a hardness of about 3 on the Mohs scale, while metals that can scratch a human fingernail, such as steel or titanium, are much harder, typically with a hardness of 5 or higher.
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