I have a significant problem with the following excerpt from a general chemistry text:
Note that ON stands for oxidation number (state). The excerpt is with regard to polyatomic anions - specifically, chloride ion and its related polyatomic anions.
It now follows that as oxygen-atom content increases by one, ON for the atom of variable ON increases by two because each added O has an ON of -2 and O is more EN than the atom of variable ON - thus, bonding O to this atom makes the atom more electropositive.
If electropositivity is defined as the ability to stabilize positive charge, how does a highly partially positive central chlorine atom have this ability? How can EP increase?
If electropositivity is more formally defined as the ability for an atom to give up electrons within a covalent framework (note that this is the converse of the definition of electronegativity), how exactly does a highly oxidized chlorine atom, such as the chlorine atom in the perchloric ion, have the ability to give up any more electrons? It already has no valence electrons to give up if we stick with an ionic model to assign oxidation states!
If we look at this intuitively, and agree that more electropositive elements are more likely to bear positive charges - i.e. sodium (Na) is highly electropositive and it is very easy for Na to be stripped of its one electron to become isoelectronic with a noble gas - then why would chlorine become more electropositive as it is further oxidized?
If we consider the successive ionization energies of chlorine, they become more and more massive! So how in the world would chlorine be more electropositive (willing to give up electrons within a covalent framework) as it is further oxidized?
(I do recognize that ionization energy of an electron is not a measure of electronegativity but still it is highly correlated with EN).
My professor, respectfully, didn't offer much a defense. He said something about how positive charge doesn't exist because it's the lack of valence electrons rather than the presence of something material. This, to me, is sidestepping the argument.
He also asked me which would be more likely to bear a positive charge - Cl or O? Obviously Cl; it has electrons way further from the nucleus than oxygen, so of course Cl would more easily be stripped of an electron than O. However, this again, I believe, is a mistake, because we cannot consider electronegativities of elements and expect them to hold within ions and molecules. Carbon, for example, isn't very electronegative at all. But no one doubts the electron-withdrawing powers of a carbonyl carbon, or better yet, the electron-stabilizing powers of a ... CARBOCATION.
Further, last semester, I distinctly remember correcting him with regard to this electronegativity business; he said that a more negative ion would be more electronegative. I had to correct that statement; a more negative ion would be less able to stabilize valence electron density and less able to attract valence electrons if bound within a covalent framework, so the correct term would be instead more electropositive.
Does anyone here agree with me? Sometimes I feel that it's not the argument that matters as much as your tenacity and ability to present the argument. I lacked both tenacity and presentation skills when presenting the above argument.
I came close to telling him that his entire take of electronegativity was fundamentally flawed when I told him that "I don't associate a positive charge with electropositivity and a negative charge with electronegativity" but that was met with nothing to the contrary (which would have been welcome!) - because no, positive charge has nothing to do with electropositivity; in fact, it suggests the opposite of electropositivity!