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Good thing your competent chemistry student is a freshman.

So I decide he needs to learn a bit, and visit his lab. I let him have the finest, most accurate, and most recently calibrated instruments available.

My challenge? I will give him two samples, and bet him a night with his hot GF that he cannot tell me if they are the same element or not.

He accepts the challenge.

Later that night, he’s in the library, and I’m in bed with aforementioned hot GF snoring next to me, he was wrong.

One sample came in at around 2.2, the other about 3.4, he loudly and confidently declared they were not the same element.

One was (and here I am assuming these are the purest samples available)... one was graphite and the other was diamond, both were nothing but carbon. Just carbon. Simple carbon.

He would have had a nervous breakdown if I gave him buckyballs.

The difference, of course, is the crystalline structure.
The diamond tetrahedron structure packing much more densely than the hexagonal, planar structure of the graphite.

So now our student had learned an important lesson. Nature can be complex, and sometimes elements are not so “elemental”.

Next day I go back and hand him the purest piece of tungsten in the universe.

He measures it, comes up with a number of 19.17, asks what it is. I tell him it’s tungsten and he argues and argues and says “That can’t be! Wiki says 19.3”

So I ask him “Does that mean 19.3 exactly, or is it 19.27 rounded up or 19.34 rounded down?”

He’s not sure.

So I ask him “Who told Wiki it was 19.3?”

He thinks about the question and then decides the number probably came from one of the scientific/engineering committees that deal with that sort of stuff. Committees like ANSI, CODATA, heck, Underwriters Labs.

And he then realizes that the number on Wiki is sort of a best-guess estimate. Lots and lots of measurements were taken. They varied, some bigger, some smaller.
The number we get if we ask “What is the density of tungsten” is a simple answer to a question that can’t be answered simply, because nature is more complex than that. Tungsten is one of the transition elements, with bunches of valences, and a wide variety of stable quantum states, chem student could earn his PhD mapping out the various crystalline states of tungsten using x-ray diffraction. Each and every of the various crystalline states having different densities, forms, and even chemical and physical properties.

But, believe it or not, I have confidence in our freshman chem student.
He will learn.

As long as he does not assume too much!