That is, two distinct species of the pure element combine to make an actual compound. What is the element and what is the compound?
I am not looking for molecules, I wanted to know of a specific compound, that is formed from two distinct species of the pure element combine to make an actual compound. What is the element and what is the compound?
Here’s one: Ozone, O3, has very different properties from molecular oxygen, O2. Of course O2 is different from atomic oxygen, O.
There are, of course, all of the "diatomic elements", H2, N2, O2, F2, Cl2, Br2, I2, as well as phosphorous, P4, and sulfur, S8.
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Since when did molecules cease to be compounds? That is simply ludicrous. Of course compounds exist as molecules. "True compounds" as Merlin seems to imply are not just "ionic". There are thousands and thousands of molecular compounds.
Next we get to compounds made up of the same element. That’s the crux of this question. For instance, ozone is not simply another allotrope of oxygen, it is recognized as a separate compound from oxygen. Ozone has different chemical and physical properties form oxygen. It even has a charge distribution within the molecule, which is polar. The fact that ozone has a net dipole moment means that there is a redistribution of charge within the molecule despite the electronegativity difference of zero.
Ozone (O3)
Melting point 80.7 K, −192.5 °C
Boiling point 161.3 K, −111.9 °C
Oxygen (O2)
Melting point 54.36 K, -218.79 °C
Boiling point 90.20 K, -182.95 °C
The "boron boride" is interesting and simply points out the bizarre nature of boron compounds. Entire books have been written on just the chemistry of boron. But just because different clusters of boron atoms have different charge distributions doesn’t mean that it is the "only" compound of the same element.
A quote from the article on boron boride says, "Electronegativities of the B12 icosahedra and B2 pairs are different, and this causes charge redistribution and the emergence of partial ionicity in this elemental structure."
This indicates that boron boride is not the simple case where one boron atom appears to have a positive charge and a second appears to have a negative charge. That would be a gross oversimplification.
And of course, we see from the phrase "partial ionicity" that, like all "ionic" compounds, that the bonds are not 100% ionic, but have a degree of covalency.
Let me reiterate, I think Merlin is going too far in saying that only "ionic" compounds are true compounds. He says, "The compound is called Boron boride and is the ONLY example of a true compound made up of one element." Nonsense.