The smallest unit that can exist as an element and still have the properties of that element is a(n): electron proton neutron atom

Atom

Explanation:

An atom is the smallest quantity of any given element.

An atom includes a nucleus (which includes protons and neutrons) around which are located the electrons.  That's that specific assembly of nucleus and surrounding electrons that form an atom and define which element it is.

So,

electron: negative charge in orbit of the nucleus

proton: positively-charged particle inside the atomic nucleus

neutron: neutral particle inside the atomic nucleus

atom: ensemble of the nucleus and electrons, defining a given element.

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c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.