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Name: Marcel
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Question:
I have been teaching my students that an atom (or molecule) is the smallest constituent that you can divide a substance into while maintaining its physical properties. I now read that some materials on the micron or smaller scales have dramatically different properties such as color, boiling point, melting point, etc. I am trying to keep current with the advances in materials science to deliver correct information in my classroom. As we incorporate more nanoscience, I am becoming a bit worried about my understanding of basic definitions.



Replies:
Marcel,

I think that the "original definition", that is, "an atom is the smallest constituent that you can divide a substance into while maintaining its physical properties", was really never a particularly good one. Exceptions easily come to mind. As an example of color, it has been known for centuries that if one rolls gold into thin sheets, its color becomes green. Carbon, as diamond and graphite has two entirely different colors (clear and black). If one considers a single atom, "melting point" and "boiling point" have never had any meaning. As for general properties, many physical properties depend on the way individual atoms are grouped together, into (for example) crystals. But this is complicated by some elements such as carbon or sulphur that have several radially different crystal structures, so not only do we often have conflicting physical properties for the same element, we have the problem that a crystal's properties depend on there being more than one atom of the element.

A better definition is that an atom is the smallest component of an element that not only can maintain that element's basic chemical properties, it is the smallest component of an element that has any chemical properties at all! Remember that all of an element's chemical and most of its physical properties are determined only by its electron shells. Without the electrons, the remaining nucleus has no chemical properties at all.

Regards,

Robert Wilson



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